The Hybrid Hypothesis A new theory of human origins

 http://www.macroevolution.net/hybrid-hypothesis-section-4.html

1: Human origins: Are we hybrids?

Die Stimme der Vernunft ist leise. (Reason speaks softly.) — Sigmund Freud

This section is a little different from others on this site, because it’s about the findings of my own research. I am a University of Georgia trained geneticist (M.S., Ph.D.) who worked in various genetics laboratories at the University of Georgia and conducted research there from 1989 to 2007 (see my Google+ profile). During those years I also taught biology and genetics at UGA.

Got a question? I would be happy to answer any specific questions you might have about this theory. Please just contact me here. —Gene McCarthy

My work focuses on hybrids and, particularly, the role of hybridization in the evolutionary process. Here, I report certain facts, which seem to indicate that human origins can be traced to hybridization, specifically to hybridization involving the chimpanzee (but not the kind of hybridization you might suppose!). You can access detailed and documented discussions supporting this claim from links on this page. But I’ll summarize the basic reasoning here, without a lot of citations and footnotes (see below).

Rebuttals. The following rebuttals address the most common objections raised against the hybrid theory of human origins: Rebuttal of Prothero's comments >>
Rebuttal of PZ Myers' comments >>

A comment from the website Skeptophilia:

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“All I can say is that I'm beginning to get very, very tired of so-called “scientists” and their groupies thinking and talking like religious fanatics, and of existing “acceptable” scientific theories increasingly taking on the stained patina of dogma. McCarthy appears to have the credentials; why don't those who disagree with his “outlandish” conclusions answer him with scientific rigor, rather than ad hominem?”

Will a wild pig uproot the Tree of Life?
A recent story in the Los Angeles Review of Books by longtime science journalist Greg Critser examines the hybrid theory of human origins. Read more…

External links. If you wish, you can read news articles about the hybrid theory of human origins: article #1, article #2, article #3, article #4.

My theory of human origins: Rationale

Always remember that it is impossible to speak in such a way that you cannot be misunderstood. — Karl Popper

McCarthy

So why do I think humans are hybrids? Well, first of all, I’ve had a different experience from most people. I’ve spent most of my life (the last thirty years) studying hybrids, particularly avian and mammalian hybrids. I’ve read thousands of reports describing them. And this experience has dispelled some mistaken ideas I once had about hybrids, notions that I think many other people continue to take for granted.
For example, one widespread, but erroneous, belief is that all hybrids are sterile. This idea keeps a lot of people from even considering the possibility that humans might be of hybrid origin. The reality, however, is something quite different. For instance, in reviewing the reports I collected for my book on hybridization in birds (Handbook of Avian Hybrids of the World, Oxford University Press, 2006), which documents some 4,000 different kinds of hybrid crosses among birds, I found that those crosses producing partially fertile hybrids are about eight times as common as crosses known to produce sterile ones. The usual result is a reduction in fertility, not absolute sterility. My current work documenting hybridization among mammals shows that partially fertile natural hybrids are common, too, in Class Mammalia. And yet, it seems most people base their ideas of hybrids on the common mule (horse x ass), which is an exceptionally sterile hybrid, and not at all representative of hybrids as a whole.

From a discussion of mules elsewhere on this website: Zirkle (1935, p. 7) says that the sterility of the mule made it the first animal whose hybrid origin was generally recognized because “the origin of fertile hybrids could easily be forgotten, particularly the origin of those which appeared before the dawn of history.” The mule, however, was so sterile that it was necessary to produce it with the original cross (Zirkle provides extensive information on the early history of mule). The fact that the hybrid origin of the mule has so long been known, together with its marked sterility, has no doubt greatly contributed to the widespread, but erroneous belief that all hybrids are sterile.
Read more about mules >>

I should, perhaps, also mention that differences in parental chromosome counts, even rather large ones, do not preclude the production of fertile hybrids. While differences of this sort do bode ill for the fertility of the resulting progeny, it is only a rule of thumb. For example, female geeps, the products of hybridization between sheep (2n=54) and goats (2n=60), can produce offspring in backcrosses. Likewise, female zeedonks (Burchell’s Zebra, 2n=44 x Ass, 2n=62) have also been fertile in backcrosses. There are many other examples of this sort among mammalian hybrids. Therefore, such differences between the parents in a cross do not in any way guarantee an absolute sterility in the hybrid offspring. (For those readers who do not know, backcross hybrids are produced when hybrids from a first cross mate with either of the two types of parents that produced them. When the resulting progeny mate again with the same parental type, the result is the second backcross generation, and so forth.)

An email from a physicist: Dear Gene,
Thank you again, I've read your profound considerations with the greatest interest. I think nowadays the situation is worse than it used to be at Darwin’s age. The case of physics (cosmology and high energy physics, above all) is emblematic: big science, massive investments involving thousands of researchers, industrial operations essentially leading nowhere. New ideas cannot filter through, they would too badly damage ongoing business. Even people iconized in the first part of their life like Fred Hoyle, Dirac, Einstein have been in the latter part considered kind of nuts since they didn't abjure independent thinking to adore any ‘standard model’ idol, like present-day believers.
Your proposal in this environment is very courageous, and the huge and admirable work you carried out to supplement and document it vigorously goes in the direction of re-establishing sensible science in one of the most relevant subjects for human society.
The aim surely deserves the effort.

A second so-called fact, which might make it seem impossible for humans to have had a hybrid origin, is the equally erroneous notion that hybrids, especially successful hybrids, do not occur in a state of nature. A third is the mistaken idea that only plants hybridize, and never animals. In fact, however, natural, viable, fertile animal hybrids are abundant. A wide variety of such hybrids occur on an ongoing basis (read a detailed discussion documenting these facts). For example, of the more than 4,000 different types of hybrid crosses listed in my book on hybridization in birds, approximately half are known to occur in a natural setting (download a PowerPoint presentation summarizing data on hybridization in birds). My current research indicates a comparable rate for mammals.

An email from a supporter: “Dear Dr. McCarthy, I admire your work as you described in your recent newsletter. I was surprised by the reaction of the chattering biologic community and was pleased by your calm reaction to it all. Now I know what Darwin went through. You have my support and if this old biologist/physician/urologist can help please let me know. I've directed all my bio friends to your site.”

Sequence data. And I must now emphasize a fact that I, as a geneticist, find somewhat disappointing: Though there are other ways of detecting them, with nucleotide sequence data, it can be very difficult to identify later-generation backcross hybrids derived from several repeated generations of backcrossing (and this would be especially true of any remote descendants of backcross hybrids produced in ancient times, which is what I'm proposing humans may actually be). To better understand why backcross hybrids are hard to analyze with sequence data, take a look at this diagram:

Genomic effects of repeated backcrossing:

Instead, as is the case with other later-generation backcross hybrids, the most revealing data is of an anatomical and/or physiological nature. And this is exactly the kind of hybrid that humans seem to be, that is, it appears that humans are the result of multiple generations of backcrossing to the chimpanzee.
The thing that makes backcross hybrids hard to analyze using genetic techniques is that, in terms of nucleotide sequences, they can differ very little from the parent to which backcrossing occurs. It’s important to realize, however, that a lack of such differences does not prevent them from differing anatomically. Sequence differences are not necessary for anatomical differences to be present. An obvious example of this phenomenon is Down’s syndrome. Individuals affected by Down’s regularly exhibit certain distinctive anatomical features, and yet in terms of their nucleotide sequences they do not differ in any way from other humans. To detect someone with Down’s syndrome, sequence data is completely useless. But with anatomical data, detecting affected individuals is easy. This issue is discussed in more detail in a subsequent section. The key fact is that with Down’s syndrome the differences that we see are due to differences in the number of genes present, that is, dosage differences, and not to differences in the nucleotide sequences of those genes. Dosage differences of this sort are exactly what hybridization typically produces.
Human infertility. Another observation that appears significant in connection with the theory of human origins under consideration is that it has been well known for decades that human sperm is abnormal in comparison with that of the typical mammal. Human spermatozoa are not of one uniform type as in the vast majority of all other types of animals. Moreover, human sperm is not merely abnormal in appearance — a high percentage of human spermatozoa are actually dysfunctional. These and other facts demonstrate that human fertility is low in comparison with that of other mammals (for detailed documentation of this fact see the article Evidence of Human Infertility). Infertility and sperm abnormalities are characteristic of hybrids. So this finding suggests that it's reasonable to suppose, at least for the sake of argument, that human origins can be traced to a hybrid cross. It is also consistent with the idea that the hybridization in question was between two rather distinct and genetically incompatible types of animals, that is, it was a distant cross.
Methodology. The chimpanzee is plausible in the role of one of the parents that crossed to produce the human race because they are generally recognized as being closest to humans in terms of their genetics (here, I use the term chimpanzee loosely to refer to either the common chimpanzee or to the bonobo, also known as the pygmy chimpanzee; the specific roles of these two rather similar apes within the context of the theory of human origins now under consideration will be explained in a subsequent section). But then the question arises: If an ancient cross between the chimpanzee and some parental form “X” produced the first humans, then what was that parent? Does it still exist? What was it like?
As the reader might imagine, if the assumption is correct that one of our parents is the chimpanzee, then it should be possible actually to identify the other parent as well. A hybrid combines traits otherwise seen only separately in the two parental forms from which it is derived, and it is typically intermediate to those parents with respect to a wide range of characters. Naturalists routinely use these facts to identify the parents of hybrids of unknown origin, even backcross hybrids.
First they posit a particular type of organism as similar to the putative hybrid (in the present case, this organism is the chimpanzee). They then list traits distinguishing the hybrid from the hypothesized parent, and this list of distinguishing traits will describe the second parent. A detailed analysis of such a triad will often establish the parentage of the hybrid. The traits in question in such studies are generally anatomical, not genetic. DNA evidence is used in only a very small percentage of such identifications (and even then, rarely in efforts to identify backcross hybrids), and yet firm conclusions can generally be reached.
So in the specific case of humans, if the two assumptions made thus far are correct (i.e., (1) that humans actually are hybrids, and (2) that the chimpanzee actually is one of our two parents), then a list of traits distinguishing human beings from chimpanzees should describe the other parent involved in the cross. And by applying this sort of methodology, I did in fact succeed in narrowing things down to a particular candidate. That is, I looked up every human distinction that I could find and, so long as it was cited by an expert (physical anthropologist, anatomist, etc), I put it on a list. And that list, which includes many traits (see the lengthy table on next page), consistently describes a particular animal. Keep reading and I’ll explain.

A reader's comment following a news story about this theory: “I possess a SUNY Stony Brook (DPAS) doctorate in anthropological sciences, and have followed the macroevolution web page cited above for more than a year. McCarthy's hybrid hypothesis is elegantly elaborated and carefully integrated into the state-of-the-art genetics of our times. But much better than that, his work exhibits fine craft, rigorous scientific method, rare synthetic applications, fair intellectual play, and above all, readability. Stodgy naysayers run the risk of looking like the very kind of fundamentalists they loathe.”

2: The Other Parent

Teiresias: To you, I am mad; but not to your parents. Oedipus: Wait! My parents? Who are my parents? —Sophocles, Oedipus Tyrannus

(Continued from the previous section)

And why might one suppose that humans are backcross hybrids of the sort just described? Well, the most obvious reason is that humans are highly similar to chimpanzees at the genetic level, closer than they are to any other animal. If we were descended from F₁ hybrids without any backcrossing we would be about halfway, genetically speaking, between chimpanzees and whatever organism was the other parent. But we’re not. Genetically, we’re close to chimpanzees, and yet we have many physical traits that distinguish us from chimpanzees. This exactly fits the backcross hypothesis.
Moreover, in mammalian hybrid crosses, the male hybrids are usually more sterile than are the females. In a commercial context, this fact means that livestock breeders typically backcross F₁ hybrids of the fertile sex back to one parent or the other. They do not, as a rule, produce new breeds by breeding the first cross hybrids among themselves. Often, even after a backcross, only the females are fertile among the resulting hybrids. So repeated backcrossing is typical. Commonly there are two or more generations of backcrossing before fertile hybrids of both sexes are obtained and the new breed can be maintained via matings among the hybrids themselves. More backcrossing tends to be necessary in cases where the parents participating in the original cross are more distantly related.

Genomic effects of backcrossing in a typical hybrid cross:

A reader’s comment: “Your conjecture is not unlike trying to reverse engineer a human being. Logically it all makes a good argument, down to the detailed level you’ve taken it to. I imagine that working with hybrids you HAVE to do that - even in cases where you may not think so. Logically your arguments make a lot of sense. And the corollaries and ramifications all seem to come true. I am impressed, frankly.”

Stephen Garcia
Mechanical Design Engineer
Guanajuato, Mexico

Traits distinguishing humans from other primates
Many characteristics that clearly distinguish humans from chimps have been noted by various authorities over the years. The task of preliminarily identifying a likely pair of parents, then, is straightforward: Make a list of all such characteristics and then see if it describes a particular animal. One fact, however, suggests the need for an open mind: as it turns out, many features that distinguish humans from chimpanzees also distinguish them from all other primates. Features found in human beings, but not in other primates, cannot be accounted for by hybridization of a primate with some other primate. If hybridization is to explain such features, the cross will have to be between a chimpanzee and a nonprimate — an unusual, distant cross to create an unusual creature.
The fact that even modern-day humans are relatively infertile may be significant in this connection. If a hybrid population does not die out altogether, it will tend to improve in fertility with each passing generation under the pressure of natural selection. Fossils indicate that we have had at least 200,000 years to recover our fertility since the time that the first modern humans (Homo sapiens) appeared. The earliest creatures generally recognized as human ancestors (Ardipithecus, Orrorin) date to about six million years ago. So our fertility has had a very long time to improve. If we have been recovering for thousands of generations and still show obvious symptoms of sterility (see previous section), then our earliest human ancestors, if they were hybrids, must have suffered from an infertility that was quite severe. This line of reasoning, too, suggests that the chimpanzee might have produced Homo sapiens by crossing with a genetically incompatible mate, possibly even one outside the primate order.
For the present, I ask the reader to reserve judgment concerning the plausibility of such a cross. I’m an expert on hybrids and I can assure you that our understanding of hybridization at the molecular level is still far too vague to rule out the idea of a chimpanzee crossing with a nonprimate. Anyone who speaks with certainty on this point speaks from prejudice, not knowledge. No systematic attempts to cross distantly related mammals have been reported. However, in the only animal class (Pisces) where distant crosses have been investigated scientifically, the results have been surprisingly successful (399.6, 399.7, 399.8). In fact, there seems to be absolutely nothing to support the idea that inter-ordinal crosses (such as a cross between a primate and a nonprimate) are impossible, except what Thomas Huxley termed “the general and natural belief that deliberate and reiterated assertions must have some foundation.” Besides, to deny that inter-ordinal mammalian crosses are possible would be to draw, at the outset of our investigation, a definite conclusion concerning the very hypothesis that we have chosen to investigate. Obviously, if humans were the product of such a cross, then such crosses would, in fact, be possible. We cannot tell, simply by supposing, whether such a thing is possible — we have to look at data.
The Other Parent

A list of traits distinguishing humans from other primates

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DERMAL FEATURES
Naked skin (sparse pelage)
Panniculus adiposus (layer of subcutaneous fat)
Panniculus carnosus only in face and neck
In “hairy skin” region:
 - Thick epidermis
 - Crisscrossing congenital lines on epidermis
 - Patterned epidermal-dermal junction
Large content of elastic fiber in skin
Thermoregulatory sweating
Richly vascularized dermis
Normal host for the human flea (Pulex irritans)
Dermal melanocytes absent
Melanocytes present in matrix of hair follicle
Epidermal lipids contain triglycerides and free fatty acids

FACIAL FEATURES
Lightly pigmented eyes common
Protruding, cartilaginous mucous nose
Narrow eye opening
Short, thick upper lip
Philtrum/cleft lip
Glabrous mucous membrane bordering lips
Eyebrows
Heavy eyelashes
Earlobes

FEATURES RELATING TO BIPEDALITY
Short, dorsal spines on first six cervical vertebrae
Seventh cervical vertebrae:
- long dorsal spine
- transverse foramens
Fewer floating and more non-floating ribs
More lumbar vertebrae
Fewer sacral vertebrae
More coccygeal vertebrae (long “tail bone”)
Centralized spine
Short pelvis relative to body length
Sides of pelvis turn forward
Sharp lumbo-sacral promontory
Massive gluteal muscles
Curved sacrum with short dorsal spines
Hind limbs longer than forelimbs
Femur:
- Condyles equal in size
- Knock-kneed
- Elliptical condyles
- Deep intercondylar notch at lower end of femur
- Deep patellar groove with high lateral lip
- Crescent-shaped lateral meniscus with two tibial insertions
Short malleolus medialis
Talus suited strictly for extension and flexion of the foot
Long calcaneus relative to foot (metatarsal) length
Short digits (relative to chimpanzee)
Terminal phalanges blunt (ungual tuberosities)
Narrow pelvic outlet

ORGANS
Diverticulum at cardiac end of stomach
Valves of Kerkring present in small intestines
Mesenteric arterial arcades
Multipyramidal kidneys
Heart auricles level
Tricuspid valve of heart
Laryngeal sacs absent
Vocal ligaments
Prostate encircles urethra
Bulbo-urethral glands present
Os penis (baculum) absent.
Hymen
Absence of periodic sexual swellings in female
Ischial callosities absent
Nipples low on chest
Bicornuate uterus (occasionally present in humans)
Labia majora

CRANIAL FEATURES
Brain lobes: frontal and temporal prominent
Thermoregulatory venous plexuses
Well-developed system of emissary veins
Enlarged nasal bones
Divergent eyes (interior of orbit visible from side)
Styloid process
Large occipital condyles
Primitive premolar
Large, blunt-cusped (bunodont) molars
Thick tooth enamel
Helical chewing

OTHER TRAITS
Nocturnal activity
Particular about place of defecation
Good swimmer, no fear of water
Extended male copulation time
Female orgasm
Short menstrual cycle
Snuggling
Tears
Alcoholism
Terrestrialism (Non-arboreal)
Able to exploit a wide range of environments and foods
Heart attack
Atherosclerosis
Cancer (melanoma)

Let’s begin, then, by considering the list in the sidebar at right, which is a condensed list of traits distinguishing humans from chimpanzees — and all other nonhuman primates. Take the time to read this list and to consider what creature — of any kind — it might describe. Most of the items listed are of such an obscure nature that the reader might be hard pressed to say what animal might have them (only a specialist would be familiar with many of the terms listed, but all the necessary jargon will be defined and explained). For example, consider multipyramidal kidneys. It’s a fact that humans have this trait, and that chimpanzees and other primates do not, but the average person on the street would probably have no idea what animals do have this feature.
Looking at a subset of the listed traits, however, it’s clear that the other parent in this hypothetical cross that produced the first human would be an intelligent animal with a protrusive, cartilaginous nose, a thick layer of subcutaneous fat, short digits, and a naked skin. It would be terrestrial, not arboreal, and adaptable to a wide range of foods and environments. These traits may bring a particular creature to mind. In fact, a particular nonprimate does have, not only each of the few traits just mentioned, but every one of the many traits listed in the sidebar. Ask yourself: Is it likely that an animal unrelated to humans would possess so many of the “human” characteristics that distinguish us from primates? That is, could it be a mere coincidence? It’s only my opinion, but I don’t think so.
Of course, it must be admitted that two human traits do, at first, seem to pose a contradiction. The animal in question lacks a large brain and it is not bipedal. An analysis of the relevant anatomy, however, reveals that these two human features can be understood as synergistic (or heterotic) effects, resulting from the combination (in humans) of certain traits previously found only separately, in the two posited parent forms. (The origins of human bipedality is explained in terms of the the hybrid hypothesis in a subsequent section. Another section offers an explanation of the factors underlying human brain expansion and, therefore, accounts not only for the large size of the human brain itself, but also for certain distinctive features of the human skull that are, themselves, obvious consequences of brain expansion).
Nevertheless, even initially, these two flies in the theoretical ointment fail to obscure the remarkable fact that a single nonprimate has all of the simple, non-synergistic traits distinguishing humans from their primate kin. Such a finding is strongly consistent with the hypothesis that this particular animal once hybridized with the chimpanzee to produce the first humans. In a very simple manner, this assumption immediately accounts for a large number of facts that otherwise appear to be entirely unrelated.
What is this other animal that has all these traits? The answer is Sus scrofa, the ordinary pig. What are we to think of this fact? If we conclude that pigs did in fact cross with apes to produce the human race, then an avalanche of old ideas must crash to the earth. But, of course, the usual response to any new perspective is “That can’t be right, because I don’t already believe it.” This is the very response that many people had when Darwin first proposed that humans might be descended from apes, an idea that was perceived as ridiculous, or even as subversive and dangerous. And yet, today this exact viewpoint is widely entertained. Its wide acceptance can be attributed primarily to the established fact that humans hold many traits in common with primates. That’s what made it convincing. But perhaps Darwin told only half the story. We believe that humans are related to chimpanzees because humans share so many traits with chimpanzees. Is it not rational then also, if pigs have all the traits that distinguish humans from other primates, to suppose that humans are also related to pigs? Let us take it as our hypothesis, then, that humans are the product of ancient hybridization between pig and chimpanzee. Given the facts presented in the discussion of stabilization theory on this website, it seems highly likely that humans are hybrids of some kind. This particular hypothesis concerning the nature of our parentage is, as we shall see, a fruitful one. For the present there’s no need to make a definite decision on the matter, but certain lines of reasoning do suggest the idea should be taken seriously:

A reader’s comment: “Wow! I learned of this site and your pig-chimpanzee-hybrid paper only a few hours ago, and have been stuck here ever since. Fantastic work...Anyway, I look forward to reading more. I know you call this only a hypothesis and not yet a theory, but it sure calls for some ’splainin’. Thanks!”

—Edward Falkowski
Boulder, Colorado, USA

My response to a reader who recently wrote in to say that the only convincing evidence for this theory would be sequence data: I’m not saying pig DNA in the human genome “would not” be detectable. That’s putting words in my mouth. I’m saying “might not.” Or, better, “could easily have been missed without this guiding hypothesis.” You seem to somehow be assuming that it isn’t there. As far as I’m concerned, maybe it is, maybe it isn’t. But if it is, obviously, it’s not obvious. As to sequence data, in my opinion, your view of what constitutes evidence needs to be widened. It seems a bit much to insist that the only thing that can convince anyone of anything is sequence evidence. If that’s true, then law courts will have to throw out all the murder weapons, eyewitness testimony, alibis and everything else, and focus instead on DNA evidence alone, because DNA, if what you’re saying is true, is the only evidence that has any meaning. But you know that’s not right. And I think you therefore have to admit that you’re showing a certain bias here. Besides, I’m not making a strong statement. I’m only saying that, given the likely circumstances (an initial cross between chimpanzee and pig, followed by several generations of backcrossing to chimpanzee), analyzing the genetic data and reaching any strong conclusions is likely to be a pain.

A detailed discussion of the genetics touching on this question appears here.

• First of all, the notion is set forward strictly as a hypothesis. No claim whatever is made that it is actually a fact that humans somehow arose through hybridization of pigs with chimpanzees. In contrast, proponents of the idea that humans are closely related to apes (and not to pigs) often speak as if their case has been proved beyond doubt. But, of course, it has not. The wide acceptance of this idea may actually be due to the lack of any competitive theory. I merely propose an evaluation of two distinct hypotheses by the usual scientific criterion: The hypothesis less consistent with available data should be rejected.
• Even if we could identify some objective unit of measure for “distance” or “similarity” (which is not at all a straightforward problem), we would still expect some crosses to be more distant than others — that is, the various types of possible crosses would constitute a continuum. Many would be “close” and some would be “distant.” But we would expect at least a rare few to be very distant. While these few might be rare, they might be among the most interesting, because they would offer an opportunity to obtain something radically different. Perhaps, it is only a subjective bias, but I believe that a human being, when taken as a whole, is radically different from a chimpanzee.
• On the other hand, if we first compare humans with nonmammals or invertebrates (e.g, crocodile, bullfrog, octopus, dragonfly, starfish), then pigs and chimpanzees suddenly seem quite similar to humans. Relative impressions of “close” and “far” are subjective and depend on context.
• Pigs and chimpanzees differ in chromosome counts. The opinion is often expressed that when two animals differ in this way, they cannot produce fertile hybrids. This rule is, however, only a generalization. While such differences do tend to have an adverse effect on the fertility of hybrid offspring, it is also true that many different types of crosses in which the parents differ in chromosome counts produce hybrids that are themselves capable of producing offspring. As Annie P. Gray noted in the preface to her reference work Mammalian Hybrids (1972, p. viii), which compiled information about all known hybrid mammals, “no close correlation was found between the chromosome count or the duration of gestation and the ability of species to hybridize.”
• There have been no systematic, scientific surveys of the crossability of mammals belonging to different taxonomic orders (a cross between pig and chimpanzee would be inter-ordinal). Any firm opinion on such a point must therefore, necessarily, be prejudiced. In fact, there is substantial evidence on this website supporting the idea that very distantly related mammals can mate and produce a hybrid (see the section on mammalian hybrids and, in particular, look at the videos shown there of ostensible rabbit-cat hybrids). Another relevant case involves ostensible hybridization between dog and cow. In addition, certain fishes belonging to different orders have been successfully crossed, and available information on mammalian hybrids indicates that various other extremely distant crosses have occurred. Evidence published in the journal Nature demonstrates that the platypus genome contains both bird and mammal chromosomes (Grützner et al. 2004). As Frank Grützner, the lead author of the study, stated in a related news story, “The platypus actually links the bird sex chromosome system with the mammalian sex chromosome systems.” How could this be the case if a bird and a mammal did not at some time in the past hybridize to produce a fertile hybrid? Such a cross would be far more distant than one between a chimpanzee and a pig (and platypuses are, of course, fertile — otherwise they would not be able to propagate themselves). And seemingly, a cross between a primate and a pig did occur only a few years ago, in 2008.

A reader’s comment: “Gosh, what a mass of analysis, comparison and so forth. Will take me a while to really read all this rather than skimming through it as I've started doing this evening.”

• Ultimately, the interaction of gametes at the time of fertilization, and the subsequent interplay of genes (derived from two different types of parents) during the course of a hybrid’s development cannot be predicted by any known laws because the interaction is between a multitude of extremely complex chemical entities that each have an effect on others. It is for this reason that the degree of similarity perceived between two organisms is no sure indicator of their crossability.
• Another suggestive fact, probably known to the reader, is the frequent use of pigs in the surgical treatment of human beings. Pig heart valves are used to replace those of human coronary patients. Pig skin is used in the treatment of human burn victims. Serious efforts are now underway to transplant kidneys and other organs from pigs into human beings. Why are pigs suited for such purposes? Why not goats, dogs, or bears — animals that, in terms of taxonomic classification, are no more distantly related to human beings than pigs? (In subsequent sections, these issues are considered in detail.)
• God did not place pigs and humans in different taxonomic orders. Taxonomists did. A great deal of evidence (read a discussion of this topic) exists to suggest that taxonomists are, in no way, infallible. Our ideas concerning the proper categorization of animals are shaped by bias and tradition to such an extent that it would be rash to reject, solely on taxonomic grounds, the feasibility of such a cross.
• The general examination of the process of evolution as a whole (as presented elsewhere on this site) strongly suggests that most forms of life are of hybrid origin. Why should humans be any different?
• It might seem unlikely that a pig and a chimpanzee would choose to mate, but their behavior patterns and reproductive anatomy do, in fact, make them compatible (this topic is considered in detail in a subsequent section). It is, of course, a well-established fact that animals sometimes attempt to mate with individuals that are unlike themselves, even in a natural setting, and that many of these crosses successfully produce hybrid offspring.
• Accepted theory, which assumes that humans have been gradually shaped by natural selection for traits favorable to reproduction, does not begin to account for the relative infertility of human beings in comparison with nonhuman primates and other types of animals (see previous section). How would natural selection ever produce abnormal, dysfunctional spermatozoa? On the other hand, the idea that humans are descended from a hybrid cross — especially a relatively distant cross — provides a clear explanation for Homo’s puzzling and persistent fertility problems.
• If we supposed standard theory to be correct, it would seem most peculiar that pigs and humans share features that distinguish human beings from chimpanzees, but that pigs and chimpanzees should not. Conventional theory (which assumes that pigs are equally as far removed from humans as from chimpanzees) says that pigs and chimpanzees would share about as many such traits as would pigs and humans. And yet, I have never been able to identify any such trait—despite assiduous investigation. The actual finding is that traits distinguishing chimpanzees from humans consistently link pigs with humans alone. It will be difficult to account in terms of natural selection for this fact. For each such feature, we will have to come up with a separate ad hoc argument, explaining how the feature has helped both pigs and humans to survive and reproduce. On the other hand, a single, simple assumption (that modern humans, or earlier hominids that gave rise to modern humans, arose from a cross between pig and chimpanzee) will account for all of these features at a single stroke.

A reader’s comment: “A friend pointed me to your site during a conversation the other day and I spent a while reading the site. It seems more plausible to me than gradualism for what we observe in human, chimp, and pig biological structures and their abnormal distribution across species, as you pointed out. In our conversation we concluded that with pigs and chimpanzees, there would have only had to have been one successful offspring that was still fertile with chimpanzees, with the child having multiple surviving, fertile offspring. We guessed that because of simian group dynamics it would probably have to be a female offspring, a male that was unusual would be unlikely to have mating opportunities with the rest of their group but a strange female would still be acceptable to at least some of the male members of the group. And it would only have to happen once.”

A reader’s comment: “The theory overcomes the creationist’s objection to gradualism and the evidence for pig ape hybridity has no stronger scientific competition. Open your mind and look at the facts. Consider how it might be true. Let go of your prejudices and misinformations. Not all hybrids are sterile. Examples of hybrid crosses are common in nature, including fertile ones. Admittedly transordinal crosses are unusual, but then we are extraordinary.”

For my own part, curiosity has carried me away from my old idea of reality. I no longer know what to believe. Is it possible that so many biologists might be wrong about the nature of human origins? Is it possible for a pig to hybridize with a chimpanzee? I have no way of knowing at present, but I have no logical or evidential basis for rejecting the idea. Before dismissing such a notion, I would want to be sure on some logical, evidentiary basis that I actually should dismiss it. The ramifications of any misconception on this point seem immense. As Huxley put it long ago, “The question of questions for mankind — the problem which underlies all others, and is more deeply interesting than any other — is the ascertainment of the place which Man occupies in nature.”
Are we simply another type of primate, like the chimpanzee or the baboon? Or are we a complex melange, an alloy of two very distinct forms of life? These are questions that can only be resolved by examining the evidence. I invite the reader to consider these two possibilities as simple hypotheses, to consider the data coldly, and then to determine which of the two is more consistent with available evidence.

3: An Initial Analysis

(This is section 3. Go to section 1 >>)

Then felt I like some watcher of the skies When a new planet swims into his ken. — John Keats

(Continued from the previous section)

By the same author: Handbook of Avian Hybrids of the World, Oxford University Press (2006).

Is hairless skin a trait seen only in modern domestic pigs (and not anciently)?

A reader wrote in with the following question about hair: "While some domestic pigs are bred to be relatively hairless, all the wild pigs seem to be fully-furred. In fact, when domestic pigs go feral, they seem to immediately revert to a hairy form. If so, how could we have inherited the hairless trait from pigs?"

I sent back this response: "When a pig escapes from a farm and starts living in the woods it does not suddenly become a hairy animal. Its descendants can, if they interbreed with hairy wild animals, but not otherwise. True, the Eurasian wild boar is hairy (though its hair is nowhere near as dense as that of a cow or sheep, say). But we do not know the history of the domestic pig. It’s usually treated as conspecific with the Eurasian wild boar, but the two differ in chromosome counts (domestic 2n=38, and wild boar 2n=36). So it may be that they are not the same animal and that relatively hairless pigs similar to the domestic pig existed anciently. It may well be that the two have been treated as the same species merely because it has long been known that they can produce fertile offspring together. But these offspring may simply represent hybrids (this is one of many examples, by the way, of animals with differing chromosome counts producing fertile offspring together). The domestic pig has also hybridized with a variety of other types of pigs, but that does not imply that they are the same animal. For example, in addition to the wild boar, the domestic pig has hybridized with the Babirusa, Babyrousa babyrussa (pictures); Bush Pig, Potamochoerus larvatus (pictures); Bearded Pig, Sus barbatus (pictures); Visayan Warty Pig, Sus cebifrons (pictures); Sulawesi Wild Boar, Sus celebensis (pictures); and probably Sus oliveri and Sus philippensis. So why assume that the domestic pig and wild boar are the "same" animal? Relatively naked animals similar to the domestic pig might have existed anciently. We don’t really know what pigs looked like thousands of years ago, but a prehistoric painting in Altamira Cave in Spain shows a pig (pictures) that looks fairly naked to me (except for what looks like a beard and hair at the top of the head, neck, and shoulders).

Video: A bearded pig

A reader’s comment: "I immensely enjoyed the piece on human origins. Clearly well-researched, painstakingly put together, and very convincingly written. I’ve shared a bunch of its more striking points with my girlfriend, who is Chinese (I'm presently in China), and whose diet contains a lot of pork, as is customary in China. She reports that she’s starting to feel pretty bad about eating pigs." —Chase Dumont

Some of the most easily accessible evidence that can be used to evaluate the hybrid hypothesis is visible in the mirror. In this section, we will consider certain external features that link humans with pigs. Much of my research on pigs has centered on the ordinary pig (Sus scrofa). Of course, ordinary pig is really a catchall term for a variety of breeds. "There are currently some 87 breeds of domestic pigs in the world, most of them in Europe and North America," according to Pond and Houpt, and "another 225 or more groups of pigs not recognized as breeds but each having unique characteristics, appearance, or geographical location."¹ However, the focus here will be on traits that are generally characteristic of Sus scrofa.
And now, let’s look a little more closely at some human distinctions that, as it turns out, are characteristics of pigs as well. Traits that distinguish us from chimpanzees and other primates are the only ones that will be discussed, because traits that humans share with primates have no bearing on the question of whether humans are of hybrid origin. Under the hypothesis being considered, it would make no difference if humans are more similar to chimpanzees in most respects than to pigs. The interesting finding is that those features that do distinguish humans from chimpanzees and other primates can be consistently accounted for by reference to the pig.
This physical affinity of humans and pigs is easily observable in certain external features. This fact did not escape Thomas Mann, who once wrote "The pig with its little blue eyes, its eyelashes and its skin has more human qualities than any chimpanzee — think how often naked human beings remind us of swine."² Although I do not concur in Mann’s assertion that pigs share more traits with humans than do chimpanzees, I do think pigs and humans share more than enough traits to suggest a relationship. For example, lightly pigmented eyes, in shades of blue, green, and tan, are never found in chimpanzees or orangutans.³ There is, apparently, only one known case of a gorilla with blue eyes.⁴ Light-colored eyes are also rare in other primates.⁵ Why, then, are they common in certain human populations? Where did this trait come from? One conceivable explanation is that it was inherited from blue-eyed pigs. Blue is a common eye coloration in swine (as are green, yellow, and tan). The dark pigment (melanin), found so consistently in the irises of nonhuman primates, is beneficial. It absorbs ultraviolet light. To protect their eyes from these damaging rays, pigs depend on their narrowly slit, heavily lashed eyelids. Humans shield their eyes in a similar way, unlike the typical wide-eyed, sparsely lashed ape. [A reader, by the name of Chase Dumont, wrote in with the following comment, which is of interest in the present context: "The outer appearance of the eye itself looks quite different from a chimpanzee’s and more like a pig’s — the pupil/iris in a chimpanzee eye covers the entire eye, while the pupil/iris in a pig eye occupy a much smaller footprint, displaying much of the 'white' of the eye — as in humans)."]

In the gorilla, Schultz remarks that he "found a roof cartilage of less than 1 cm² and paper-thin alar cartilages, limited to the nasal center and not extending into the huge wings, which were mere pads of fat. In contrast to this, the prominent nose of man is far more extensively supported by cartilage, which closely determines its shape. While the nearly immobile nasal wings of apes consist of little more than skin and fat, the thin and mobile wings of human noses are extensively stiffened by cartilage to keep them from being sucked shut with every inhalation (495.9,52).

While Schwartz’s statement concerning the uniqueness of the human nose is generally correct, it must be said that certain Asian monkeys (Nasalis, Rhinopithecus) do have protrusive noses (235.4,29).

Walker (588.4,1175) states that "this cartilaginous snout [of pigs], used for turning up surface soil, is strengthened by an unusual bone, the prenasal, situated below the tip of the nasal bones of the skull." Composed primarily of cartilage, this flexible prenasal "bone" finds its equivalent in the cartilaginous tip of the human septum.

Questions or comments about this theory are welcome. Simply send a message to the author through the contact page of this website. He’ll be happy to respond.

Neither is it clear how a protrusive cartilaginous nose might have aided early humans in their "savanna hunter lifestyle." As Morris remarks, "It is interesting to note that the protuberant, fleshy nose of our species is another unique feature that the anatomists cannot explain."⁶ This feature is neither characteristic of apes, nor even of other catarrhines.⁷ Obviously, pigs have a nose even more protuberant than our own. In a pig’s snout, the nasal wings and septum are cartilaginous as ours are.⁸ In contrast, a chimpanzee’s nose "is small, flat, and has no lateral cartilages" (Sonntag⁹). A cartilaginous nose is apparently a rare trait in mammals. Primatologist Jeffrey Schwartz goes so far as to say that "it is the enlarged nasal wing cartilage that makes the human nose what it is, and which distinguishes humans from all other animals."¹⁰ The cartilaginous structure of the pig’s snout is generally considered to be an "adaptation" for digging with the nose (rooting). Rooting is, apparently, a behavior pattern peculiar to pigs. Other animals dig with their feet.
A protruding nose is perhaps the most prominent difference between a human face and that of a chimpanzee, but discussions of human evolution rarely mention the nose, perhaps because its lack of utility precludes explanation in terms of adaptation. Instead, most analyses deal with the fleshless skull, where the protrusiveness of the human nose is a bit less obvious (but visible nonetheless). It is a peculiar omission, because useless (nonadaptive) traits are widely considered to be the best indicators of relationship. What is the evolutionary utility of our unique nasal structure? Is it functional? Or is it the genetic residue of an ancient hybrid cross?

Note: Specifically, Sonntag (533.6,371) notes the absence of a philtrum in chimpanzees.

Another feature to consider is the philtrum, the dent seen on the center of the human upper lip. Apes lack this typical human feature.¹¹ It seems a useless structure from a survival standpoint. Why is it seen, then, the world over in Homo? In both human beings and pigs, during the early stages of development, the upper lip is cleft, though I have not been able to find any evidence of such a cleft in the embryos of any nonhuman primate. As development continues, this cleft usually closes in humans, but persists in pigs.¹² The human philtrum is a visible residue of this primordial split lip. In those human beings where this split never closes, the condition is known as cleft lip, a common birth defect. The frequent occurrence of cleft lip in humans is hard to explain if it is assumed that we are closely related only to primates. If the assumption, however, is that human beings are derived from a pig-chimpanzee cross, this finding becomes far more understandable.
Similar thinking explains the shortness of the human upper lip (distance between mouth opening and nostrils). Why has our upper lip become shorter and thicker in the course of evolution? All apes have upper lips much longer than those of humans,¹³but a pig’s upper lip is so short that it is scarcely more than an appendage of the snout.¹⁴ Morris¹⁵ makes much of the fact that human lips are covered on their exterior surface by glabrous (i.e., absolutely hairless) mucous membrane:

Like the earlobes and the protruding nose, the lips of our species are a unique feature, not found elsewhere in the primates. Of course, all primates have lips, but not turned inside-out like ours. A chimpanzee can protrude and turn back its lips in an exaggerated pout, exposing as it does so the mucous membrane that normally lies concealed inside the mouth. But the lips are only briefly held in this posture before the animal reverts to its normal ‘thin-lipped’ face. We on the other hand, have permanently everted, rolled-back lips.

He goes on to suggest that our peculiar lips are the product of "sexual selection." But other explanations are conceivable: In describing the skin of pigs, Getty¹⁶ states that "there are no true glabrous surfaces other than the labial borders," which are composed of red mucous membrane.

Some disagreement exists in the literature over the question whether earlobes are present in apes. Sonntag says they are not seen in the chimpanzee (533.8,86), but Schultz (495.65,146) claims they are sometimes found in the African apes and even in certain monkeys.

In reference to human earlobes, Morris observes that "anatomists have often referred to them as meaningless appendages, or `useless fatty excrescences.' By some they are explained away as `remnants' of the time when we had big ears. But if we look to other primate species we find that they do not possess fleshy earlobes. It seems that, far from being a remnant, they are something new."¹⁷ Perhaps, however, they are really something old on a new face. Sisson describes the lower portion of a pig’s ear as "strongly convex below, forming a prominence somewhat analogous to the lobule of the human ear."¹⁸

A reader’s comment: "My soon-to-be-eight year old is in fact telling everyone he meets now, matter of factly as if it was today’s weather, 'People are chimp pigs!' Doesn’t phase him in the least. He’s quite proud of it."

An additional feature of the human ear should be mentioned here, the Darwinian tubercle (see Darwin’s illustration below). In his Descent of Man, Darwin comments on this feature sometimes found on the rim of human ears which he describes as "a little blunt point, projecting from the inwardly-folded margin, or helix … These points not only project inward, but often a little outward, so that they are visible when the head is viewed from directly in front or behind. They are variable in size and somewhat in position,

standing either a little higher or lower; and they sometimes occur in one ear and not on the other. Now the meaning of these projections is not, I think, doubtful, but it may be thought that they offer too trifling a character to be worth notice. This thought, however, is as false as it is natural. Every character, however slight, must be the result of some definite cause; and if it occurs in many individuals deserves consideration. The helix obviously consists of the extreme margin of the ear folded inward; and this folding appears to be in some manner connected with the whole external ear, being permanently pressed backward. In many monkeys, which do not stand high in the order, as baboons and some species of macacus, the upper portion of the ear is slightly pointed, and the margin is not at all folded inward, a slight point would necessarily project inward and probably a little outward. This could actually be observed in a specimen of the Ateles beelzebuth in the Zoological Gardens; and we may safely conclude that it is a similar structure — a vestige of formerly-pointed ears — which occasionally reappears in man.¹⁹

Darwinian tubercle (Darwin, 1871)

Primatologist Adolph Schultz (1973), however, flatly contradicts Darwin, saying that "clearly pointed ears, commonly called `satyr ears,' are among monkeys typical for only macaques and baboons and do not occur in any hominoids [great apes], not even in the early stages of development. There is no justification, therefore, to interpret the occasional `Darwinian tubercles' on human ears as an atavistic manifestation of ancestral pointed ears."²⁰ But Schultz has not, perhaps, taken into consideration the pointed ears of swine.

According to Schummer et al. (503.3,497), "The eyebrows [of the domestic pig] are formed by 2 to 3 rows of prominent tactile hairs formed at the base of the upper eyelid; there are more than 40 in all and they are up to 8 cm long. They form into bundles, especially at the medial angle of the eye."

Swine have prominent eyebrow hair. On the brows of the chimpanzee fetus it is possible to discern a region of light-colored bumps following a pattern similar to that of the human eyebrow. Adult apes, however, have no eyebrow hair.²¹ On their eyelids, pigs have luxuriant eyelashes, thicker even than those of human beings. In many pigs these cilia, as anatomists term them, are so thick that the animal seems to be wearing false eyelashes. But apes scarcely have eyelashes at all, despite the apparent survival value of this feature. Also, pongids have prominent brow ridges while pigs and most humans do not. If we choose to explain the development of human eyelashes and eyebrows in terms of natural selection, we must wonder why apes, which have existed at least as long as any hominid, have failed to acquire them. Perhaps their heavy brow ridges sufficiently protected their eyes, but if such is the case, why did not brow ridges also suffice for Homo? What was the pressing need that caused Homo to substitute tufts of hair for ridges of bone?
Dermal Characteristics
That humans lack the hair cover of nonhuman primates is an accepted fact. "It is this single factor that constitutes the chief difference between human skin and the skin of other mammals" (Montagna²²). Some writers say that the hair coat of a chimpanzee is "sparse." But if "sparse" describes chimpanzee pelage, then "naked" accurately describes the skin of human beings. Any human who even approached the hairiness of other primates would be considered abnormal. Pigs, however, are a different case. Many domestic pig breeds have skin just as naked as human skin. As Cena et al. (101.9,521) observe, "Hair densities [of animal coats] range from the sparse residual covering on man and the pig with 10-100 hairs per cm², to [the] dense coats of species such as the fox and rabbit with about 4,000 per cm²." In wild Sus scrofa, according to Haltenorth, the density of hair coverage varies from "sparse to thick," depending on the specimen or variety in question.²³ For example, the hair of the modern day wild variety of Sus scrofa present in Sudan (S. s. senaarensis) is quite sparse.²⁴

Schultz (495.07, Plate 1) pictures a 185-day-old chimpanzee fetus that is virtually hairless except for a thick patch atop its head (in the same region it is seen in human beings). It also has eyebrow hair arranged in the same pattern as do humans.

Why it may not be easy to evaluate this hypothesis with genetic data

---

In connection with the hypothesis that human origins can be traced to a hybrid cross, it’s important to realize that in most mammalian hybrid crosses, the male hybrids are usually more sterile than are the females. This fact means that breeders working with hybrids typically mate fertile females with one of the two parents (that is they "backcross" them). They do not, as a rule, produce new breeds by breeding the first-cross hybrids among themselves.
Often, even after a backcross, the resulting hybrids are still fertile in only one sex. So repeated backcrossing typically occurs. However, after a sufficient number of backcrosses, fertile hybrids of both sexes are often obtained and the new breed can thenceforth be maintained via matings among the hybrids themselves. Repeated backcrossing tends to be more necessary in cases where the parents participating in the original cross are more distantly related and genetically incompatible. So one expects also, in the case of new types of organisms arising via natural hybridization, for backcrossing to be the usual route to fertility and reproductive stability. And the same would hold in the specific case of humans arising via hybridization.
However, consider the effect of such repeated backcrossing on the human genome. The reader may not be familiar with the phenomenon of gene conversion, but its effect on hybrids during backcrossing is to quickly homogenize gene sequences. To understand why this is the case, consider the effects of backcrossing on hybrid DNA.

A Holliday junction. During meiotic recombination, double strands of parental DNA, shown here moving into the junction, separate into two single strands. Each single strand from one parent then joins with a single strand from the other parent. The resulting composite double strands then move out of the junction, undergoing gene conversion in the process.

In the figure above, note that in either of the parental (incoming) double strands each nucleotide in one strand is properly paired with its complementary nucleotide in the other strand, A is always paired with T, and C is always paired with G. So the paired double DNA strand from one parent might look like this:
AGTTCCGACGCG
TCAAGGCTGCGC
while the strand from the other parent might look like this:
AGCTCCGACGCG
TCGAGGCTGCGC
In each of these two double strands all nucleotides are paired with the complementary nucleotide (A always with T, and C always with G). But when one double strand is compared with the other it’s clear that they differ at the third nucleotide base position. In the first double strand the nucleotide base pair is T-A, while in the second it is C-G.
Thus, when these strands associate with their new partner strands after passing through the Holliday junction in meiosis, the two resulting double strands will be:
AGTTCCGACGCG
TCGAGGCTGCGC
and
AGCTCCGACGCG
TCAAGGCTGCGC
So T is paired with G in the first outgoing strand and C is paired with A in the second outgoing strand. Gene conversion converts each such mismatched pair into a matched one by replacing one of the two nucleotides with the complement of the other nucleotide (which remains unchanged). Experimental results suggest that the mechanism chooses at random which of the two nucleotides to replace so that the nucleotides derived from either parent survive at equal rates.
So, in any given case of backcrossing, suppose the genomes of the original parents A and B, which produced the first-cross hybrids, differed at one nucleotide position in five (20%). Then the DNA in gametes of the hybrids would differ from A at one position in ten and from B at one position in ten. That is, the first cross hybrid’s gametes would be right in the middle between A and B with respect to gene sequence.
However, after one backcross to A, and the resulting gene conversion during meiosis in the backcross hybrids, the gametes produced by backcross individuals would differ from A at only one position in twenty (5%). And gametes produced by second backcross hybrids would differ at only one position in forty (97.5% similarity). It’s clear then that it rapidly becomes quite difficult to distinguish, on the basis of nucleotide sequence data, the backcross hybrids from the pure parent A to which backcrossing has occurred. Chimpanzees and humans are about 98% similar in terms of their nucleotide sequences.
The specific genetic underpinnings of the many traits that distinguish humans from chimpanzees are explained, in terms of the present theory, in a separate section entitled Why are Humans Different from Chimpanzees?

Other primates do not have the long mane of hair that tops the head of an unshorn human, nor do they have beards. Haltenorth notes that in some varieties of Sus scrofa, manes are found on the neck and back , beards on the cheeks, and shocks of hair on the forehead and atop the head. He also says that the last of these three traits is found, among pigs, in Sus alone.²⁵ A prehistoric painting of a pig found in Altamira Cave in northern Spain depicts an animal with a beard and thick hair atop its head (pictures). Sus barbatus, an extant pig native to southeast Asia (which forms fertile hybrids of both sexes in crosses with S. scrofa) has little hair on its body, but does have a very thick and bushy beard.²⁶
Panniculus adiposus. In an article on the evolution of human skin, renowned cutaneous comparative anatomist William Montagna notes that, "Together with the loss of a furry cover, human skin acquired a hypodermal fatty layer (panniculus adiposus) which is considerably thicker than that found in other primates, or mammals for that matter. This is not to say that only man has a fat skin, but a thick fatty layer is as characteristic an attribute of human skin as it is of pig skin."²⁷ Similarly, Dyce et al. (160.1,742) note that there is a "well developed fat deposit present almost everywhere in the subcutis." Primatologist F. W. Jones also noted this fat layer:

"The peculiar relation of the skin to the underlying superficial fascia is a very real distinction [of human beings], familiar to everyone who has repeatedly skinned both human subjects and any other members of the primates. The bed of subcutaneous fat adherent to the skin, so conspicuous in man, is possibly related to his apparent hair reduction; though it is difficult to see why, if no other factor is invoked, there should be such a basal difference between man and the chimpanzee."²⁸

Panniculus carnosus. "Another particularity of human skin is its general lack, or loss, of the cutaneous skeletal muscle layer (panniculus carnosus) found throughout the skin of most other mammals. Remnants of a panniculus carnosus in human skin are found at the front of the neck in the apron-like, thin platysma muscle … All other primates, even the great apes, have a panniculus carnosus over much of the body" (Montagna²⁹). As in humans, the cutaneous musculature of pigs is well developed in the neck (platysma muscle) and face, but sparse or nonexistent elsewhere.³⁰
In animals having a panniculus carnosus, the skin receives its blood supply from direct cutaneous arteries (large superficial vessels running parallel to the skin surface in the cutaneous muscle sheath). But when no panniculus carnosus is present, arteries feeding the skin rise up like little trees from deep within the body. Arteries of this latter type are called musculocutaneous. These two forms of dermal circulation are depicted in the illustration below. Both pig skin and human skin are supplied by musculocutaneous arteries.³¹ As Daniels and Williams observed in a 1973 article on skin flap transfer, "Most experimental animals do not have a vascular supply to the skin similar to that of man. The pig’s cutaneous vascular supply has been demonstrated anatomically and surgically to be more comparable than most to that of man … As in man, the pig’s skin is supplied by ubiquitous musculocutaneous arteries and by a few direct cutaneous arteries."³² This observation has been confirmed by other authors: "Except for pigs, whose cutaneous vasculature resembles that of man, loose-skinned mammals are vascularized by direct cutaneous arteries" (Montagna and Parakkal³³). Therefore, in this respect, human skin is more similar to pig skin than to that of nonhuman primates: "Actually, the vascularity of the skin of most nonhuman primates is essentially similar to that of other furred animals" (Montagna³⁴). In particular, Baccaredda-Boy,³⁵as well as Moretti and Farris,³⁶ found that the skin of chimpanzees differs from that of human beings in having numerous large, superficial vessels (i.e., direct cutaneous arteries).

In the paragraph at left, the calculations for the pig capillary separation interval were based on Young and Hopewell’s data (605.4, Fig. 1 and Table 2). In the chimpanzee, the epidermis is richly vascularized only beneath the friction surfaces (palms and soles), not beneath the hairy-skin regions. Thus, regarding the chimpanzee, Montagna (365.5,191) states: "Where the epidermis is flat [i.e., hairy-skin regions], capillary loops are ill-defined … Capillary loops are deepest and most complicated underneath the epidermis of the friction surfaces.

Human skin also stands apart from that of other primates — and from that of most other mammals for that matter — with respect to the quantity of blood that can be circulated through it.³⁷ A certain amount of blood is needed just to feed the skin. This is the amount it receives in most animals. In humans, however, the maximum blood flow can be more than a hundred times greater than this minimum.³⁸ Fed by temperature-sensitive musculocutaneous arteries, the densely spaced cutaneous capillaries of human beings play an essential thermoregulatory role.³⁹ When the body begins to overheat, large quantities of warm blood can be rapidly cooled in these capillaries via sweat evaporation. One measure of cutaneous vascular density is the capillary loop separation interval. In human beings, the typical distance between capillaries ranges from 50 to 100 microns.⁴⁰ In porcine flank skin, this figure is reduced to only about 20 microns, a separation interval so small as to be almost incredible. When white pigs are exposed to high temperatures, the skin flushes pink with blood (even in the absence of sunlight) as it does in light-skinned human beings under similar conditions.⁴¹

Human flea, Pulex irritans

Fleas. Perhaps this difference between our cutaneous vasculature and that of our primate kin accounts for another human distinction: "Ironically," writes Nicole Duplaix, "man is unique among the primates in having fleas."⁴² More than 2,400 distinct types of fleas have been treated as species or subspecies.⁴³ Parasites are usually rather specific in their choice of host. Fewer than twenty of these 2,400 types will readily bite human beings.⁴⁴ Foremost among those that feed on Homo sapiens is the human flea, Pulex irritans, but we are not the only suitable hosts for this species. According to Bennett, "Pulex irritans, the human flea, breeds freely in hog-house litter and may become a serious pest of swine."⁴⁵

Newton’s law of cooling states that the rate at which heat flows out of a warm body into a cooler surrounding medium is proportional to the difference between the temperature at the body’s surface and the temperature of the surrounding medium.

The panniculus adiposus replaces hair as an insulating layer in human beings and pigs. According to Beckett (63.8,2),

The pig increases or decreases the amount of heat lost … by varying the blood flow in the [skin’s] capillary bed … If all blood flow to the outer body parts were stopped, the thermal resistance between the body cavity or muscle tissue and skin surface would approximately equal the resistance of the fat layer plus the resistance of the hair and skin. To the extent that a pig is able to direct a sizable flow of blood through the skin and region just below the skin, the fat layer is by-passed and thermal resistance is at a minimum.

In the figure above, notice that the musculocutaneous arteries pass through the cutaneous fat. This perforated fat layer constitutes an insulating mechanism that can respond quickly to ambient temperature, a characteristic that hair lacks. Dilation of the musculocutaneous arteries in response to heat increases blood flow to the skin. This increase in circulation can raise skin surface temperature to a level almost as high as that within the body, thus increasing the rate at which heat is lost to the environment.^b In cool environments, constriction of these arteries reduces skin temperature and, consequently, the rate at which body heat is lost to the atmosphere because the fat layer can then serve as an insulating blanket.

Obviously, furred animals cannot remove their coats when it’s hot — they shed. But shedding is a process that takes weeks, not minutes. It is a seasonal adjustment, not the moment-to-moment adjustment seen in human beings and pigs.

Possession of a panniculus adiposus allows adjustment to changes in ambient temperature on a moment-to-moment basis — a clear advantage in the temperate zones where much of the human race has made its home, because these regions are much more subject to sudden, extreme shifts in temperature than those close to the equator. Nonhuman primates and other furred animals do not have the option of adjusting their skin temperature. Because their skin is not insulated from the rest of the body by a layer of fat, its temperature must remain near that of the flesh beneath it.
Pig skin is separated from the inner body by a thick fat layer, and it can cool to an extreme degree. Fat, not hair, is the primary insulating barrier.⁴⁷ Alaskan swine can withstand sub-zero temperatures by cooling their skin to as little as 9˚ C (at an ambient temperature of -10˚ C) without suffering tissue damage.⁴⁸ Acclimatized human beings, too, can reduce skin temperature to about 10˚ C without injury.⁴⁹ This mode of insulation is completely different from that of nonhuman primates, more like that seen in certain aquatic mammals (e.g., seal, walrus). With the exception of the pig, it seems that no other land animal has this form of insulation.
More than a naked ape, Homo is a variably insulated naked ape. In hot environments human beings (and pigs) can increase the circulation of warm blood to the skin and raise temperatures almost to the level of body core temperature, thus maximizing heat loss to the surrounding air. If weather turns cold, they can restrict cutaneous circulation, cooling the skin to such a degree that heat loss is significantly reduced. This ability is especially apparent in fat⁵⁰or acclimatized individuals.⁵¹ Although a cultural advance, the invention of clothing, made it possible for Homo to inhabit cool regions formerly off-limits to primates, a biological advance, in the form of a new insulation system, has increased the human ability to withstand the sudden temperature variations found in those regions.

If skin has any hair whatsoever (scalp, forearm, belly) dermatologists refer to it as "hairy skin." Hairy skin in humans, then, is the skin covering most of the body, the general body surface. Other regions, that are absolutely hairless (lips, palms, soles) are called "glabrous."

Besides being a good insulator, human skin is surprisingly thick. "The epidermis over our general body surface ["hairy skin" see note at right] is substantially thicker than that of other primates: the horny layer [stratum corneum] can be peeled off intact as a diaphanous but tough membrane that can be used for experimental purposes … The epidermis in the hairy skin on nonhuman primates, mostly like that of any other furred mammal, is relatively thin, with a relatively thin horny layer" (Montagna⁵²). Pigs, though, have a thick epidermis and stratum corneum, thicker even than that of human beings.⁵³

Another quotation from Montagna (360.3,13): "Elastic fibers are numerous everywhere [in pig skin]. In the papillary layer delicate fibers branch toward the epidermis as they do in the skin of man."

In the case of the chimpanzee’s dermis, Montagna and Yun state that, "Elastic fibers, nowhere numerous, are concentrated in the papillary body and in the deep portion of the reticularis dermis" (365.5,191).

The elasticity of our skin is also unusual. "Whereas the skin of the great apes and that of some of the simian primates have variable amounts of elastic fibers, in no animals, regardless of sex, age, or locality have we found the abundance of elastic tissue characteristic of human skin" (Montagna⁵⁴). This finding comes from the same author who, in an earlier article comparing human skin with that of pigs, observed that "one of the most striking resemblances between these two skins [pig and human] is the large content of elastic tissue in the dermis."⁵⁵
He also remarks that "the surface of both skins [human and porcine] is grooved by intersecting lines which form characteristic geometric patterns."⁵⁶ In a separate paper on the evolution of human skin he provides a little more detail:

The outer surface of human skin is crisscrossed, almost everywhere, by fine intersecting congenital lines … (You can confirm the presence of these lines by looking at the back of your hands). This characteristic is not limited to human skin; creases are also found on the skin of pachyderms, walruses, and, to a lesser extent, pigs. With the exception of occasional, shallow creases, the surface of the hairy skin of nonhuman primates is smooth.⁵⁷

The presence of these lines in both pigs and humans is not easily explained in terms of natural selection since they have no known function.⁵⁸
On the underside of our "hairy skin" (general body surface), where the epidermis meets the dermis, is a different patterning not corresponding in its configuration to the outside patterning described in the preceding paragraph. A similar, though coarser, pattern is also characteristic of the epidermal-dermal junction in pigs. Montagna, however, notes that "in split-skin preparations where the epidermis is neatly removed from the dermis, the epidermis of heavily haired animals is flat.⁵⁹ Even in monkeys and apes, epidermal grooves are found only around the attachment of the ducts of glands and pilary canals." We can account for a finer patterning in humans than in pigs by the fact that a fine mesh is intermediate between the coarse patterning of pig skin and the smooth undersurface of nonhuman primate skin.
So, in the pig, we have a sparsely haired animal with a fatty, stretchy skin supplied by musculocutaneous arteries. The surface of the hairy skin is marked by congenital lines similar to those seen in human beings, and the patterning of the epidermal-dermal junction is also quite similar in the hairy skin regions. Under the hypothesis that we are considering, it makes little difference that pig skin differs from human skin in other ways. The essential point is that, in those cases in which our skin is peculiar for a primate, an explanation for each such anomaly can be found in the skin of pigs.
The Savanna Hunter

A mature pig has about 500,000 large sweat glands distributed over its entire body (503.3,497; 506.5,316). Nevertheless, it is often asserted in the literature that pigs do not sweat. This assumption can be traced to studies by Ingram and by Mount, who studied perspiration rates in immature animals, usually sedentary piglets (247.03; 247.1; 389.7; 390.1; 390.2; 390.3; 390.5). Studies evaluating pig sweating have concentrated on young pigs because they are of greater commercial interest. Immature animals are no more appropriate for determining the evaporative qualities of a boar or a sow than a toddler would be for revealing traits of an adult human—Children sweat much less than do adults (584.4,577). Small animals have a tendency to hypothermia (because their surface area is large in proportion to their size), not hyperthermia, and have little tendency to sweat (390.8,182). Perspiration in pigs is often overlooked because these animals are, apparently, more efficient sweaters than are humans. Their sweat glands seem to be better attuned to thermoregulatory needs (they produce no more sweat than what is necessary to cool cutaneous blood by evaporation). Very little sweat is lost to runoff because sweat rarely builds up on the skin. But observed rates of sweating in mature pigs are approximately comparable to those of humans. Beckett (63.4) found that a 350 lb. sow at rest lost approximately 95 g/m² in sweat per hour at a dry bulb temperature of 98E F and wet-bulb temperature of 81). At a much higher temperature (122EF dry bulb and 79EF wet bulb), Myhre and Robinson found that 70 kg men at rest lost moisture (sweat + respiration) at a rate 250 g/m² per hour (398.7,Table 3). Even in smaller pigs (198 lb. gilts), skin moisture loss is important (387.8,Table 1), ranging from one-third to two-thirds of total moisture loss (lung + skin). The claim that pigs need a wallow when living in hot climates (because they supposedly do not sweat) is also encountered. But Heitman and Hughes exposed hogs without access to a wallow to high temperatures (100E F; relative humidity 35%) for a week without any fatalities—conditions where the only avenue for heat dissipation is evaporative cooling (232.5,176).

Pigs sweat when they are hot. "The apocrine [i.e., sweat] glands of the horse and pig secrete profusely during violent exercise and stress" (Montagna⁶⁰). This sweating serves a thermoregulatory function in pigs just as it does in human beings.⁶¹ The hairy skin sweat glands of nonhuman primates, however, do not respond to thermal stimulation. The failure of nonhuman primates to sweat puzzled Montagna: "One might surmise," he writes,

that, like man, these animals sweat in response to heat stimulation. However, with singular exceptions, if the glands secrete at all, the amount is so small that it cannot be recorded. Sometimes animals show beads of sweat on the facial disc when under deep anesthesia, but our efforts to induce thermal sweating have failed. We have also largely failed to induce sweating with sudorific drugs, either cholinomimetic or adrenomimetic. In the chimpanzee, very few, small sweat drops were recorded even after the administration of shockingly large doses of these drugs.⁶²

In contrast, even a small dose of acetylcholine or adrenaline elicits sweating in pigs.⁶³ Even the immature pigs studied by Ingram (247.1,95) responded to adrenalin.
The notion that nakedness has somehow enhanced sweat evaporation in humans is widely received. Supposedly, our sparse pelage allowed our ancestors to cool their skin more rapidly than hairy animals in hot, dry environments, or somehow improved their ability to dissipate metabolic heat while rushing about the savanna in pursuit of prey. Russell Newman, however, points out that our lack of reflective hair actually increases solar heat load and the need to sweat.¹⁶⁴ To substantiate this claim, he cites a study by Berman showing that cattle exposed to the sun sweat more after their hair is removed.¹⁶⁵ Similarly, panting increases in shorn sheep.¹⁶⁶
Clothing, which replaces hair as a radiation barrier in human beings, has much the same effect on human perspiration. Human beings subjected to solar heat loads sweat more when naked than when wearing light clothing under otherwise identical circumstances. In a study of the effects of clothing on sweat, Adolph¹⁶⁷concluded that "the nude man can save easily as much body water by putting on a shirt and trousers as can the clothed man by finding good shade." Moreover, body hair does not reduce convective heat loss "and has nothing to do with radiation of long-wave infra-red heat to cooler objects," says Newman.¹⁶⁸ He therefore asserts that naked skin,

is a marked disadvantage under high radiant heat loads rather than the other way around, and that man’s specialization for and great dependence on thermal sweating stems from his increased heat load in the sun.¹⁶⁹

My very crude experiment: I took two beakers and lined the bottom of one with a circle of rabbit fur. I then placed water, drop by drop, in equal amounts in both beakers. I continued the experiment for several days, always keeping the fur damp. Day after day, the water level rose in the beaker without fur. But no water buildup at all occurred in the other beaker.

Claims that naked skin confers an evaporative advantage can, for the most part, be traced to a single sentence: Mount (390.8,42) seems only to be expressing an opinion in saying that "In a bare-skinned animal, like pig or man, the evaporation of water from the body surface takes up most of the heat required for the process from the body itself, and so constitutes an efficient cooling system." Nevertheless, many later authors cite this statement in substantiation of the claim that bare skin enhances sweat evaporation. I have not been able to locate any actual data (in the works of Mount or any other author) demonstrating this assertion.

If increased radiant heat loads caused early humans to depend more on sweat for cooling, why has hair loss, which increases those loads, progressed to the degree that it has in Homo? Under the assumption that humans first evolved on the arid, sun-drenched savanna, it is difficult, in terms of survival efficiency, to account for a reduction in hair density that would result in increased rates of water consumption. Newman points out that there is no evidence that hair interferes with sweat evaporation. Actually, I myself performed a crude experiment, the results of which indicate that hair actually accelerates the evaporation of sweat. This finding is surprising in light of evolutionary theorists' frequent claims to the contrary. But with a little consideration, one realizes that a hair coat is not a vapor barrier. Fur’s ability to "breathe" has always distinguished it from less desirable insulators that slow heat loss but don’t "wick away" moisture from the skin. Why should hair not only allow, but even enhance, evaporation rates? There are at least two reasons. First, wet hair presents a more irregular surface to the surrounding atmosphere than does hairless skin, augmenting the surface area available for evaporation. Second, hair allows uniform dispersion of sweat by capillary action, preventing the formation of the individual droplets seen on naked skin. When such droplets form, the skin lying between them does not serve as an evaporative surface and the vaporization rate is reduced.

At 40E C (the approximate temperature of the body surface under hyperthermic conditions) the latent heat of vaporization of water is 2406 J g-1 = 575 cal g-1. The evaporation of just a half-cup of sweat is sufficient to reduce the temperature of a 150 pounds of water by an entire centigrade degree. Evaporation is essential to heat absorption. Runoff merely removes fluid from the body without cooling it (When you pour out a cup from an urn of hot coffee, the temperature of the remaining coffee stays the same.).

This rate (25% of sweat lost to run-off) is for men engaged in intense physical exercise at high temperatures (running without a shirt in the desert) (15.4). Runoff loss can thus be significant even in the desert, let alone on the more humid savanna. Adolph (15.4,93-94), for example, studied sweating in a man exercising strenuously in the desert (relative humidity: 32 percent; estimated wind speed, 10 m.p.h.; the man wore no shirt) and found that "his rate of evaporative loss was 1,300 grams per hour. His measured rate of weight loss, however, was 1690 grams per hour, exclusive of water which accumulated in his trousers." These figures indicate that 23 percent of the sweat went to runoff — even if we ignore the fact some of the water absorbed by the trousers would have run off of a naked human being. On the African savanna, the humidity and solar heat loads would be even higher because the savanna lies closer to the equator than the southwestern American desert where Adolph performed his experiments. On the savanna, then, a larger percentage of sweat would go to runoff (due to lower evaporative rates at the higher humidity) and, at the same time, a larger amount of sweat evaporation would be required to counteract the higher savanna heat loads. This waste of body fluids seems peculiar in a creature that is supposed to be the product of adaptation to a life of strenuous diurnal hunting on the open savanna.

As the amount of sweat on the skin increases, the individual drops do merge to form a continuous sheet of water. But when a large amount of sweat is present on naked skin, another type of inefficiency sets in — runoff. More sweat runs off hairless skin without evaporating. The coat of a hairy animal acts as a sponge, retaining sweat in position until it can evaporate. Perspiration dripping off the body has no cooling effect, because no heat is absorbed by runoff. In contrast, evaporating sweat absorbs a large amount of heat. ^1a But Adolph’s research indicates that about a quarter of human sweat can be lost to runoff, even under near optimal evaporative conditions. ^1b A reflective hair coat, then, has three advantages: (1) lower solar heat loads; (2) increased rate of evaporation; (3) less sweat wasted on runoff. It is therefore difficult to understand how naked skin can be interpreted as an "adaptation" beneficial to a savanna hunter.
Of course, the "savanna hunter" hypothesis is just one of many theories. Hair loss in Homo has been the object of much speculation (for a survey of such theories, see 165.1). Besides those who say we lost our hair on the savanna¹⁷⁰and/or because we were hunters,¹⁷¹there are others who suggest we may have lost it in the forest,¹⁷² or even in the sea.¹⁷³ Some authors suggest that nakedness made us sexually enticing¹⁷⁴or that hairlessness became thermally advantageous when we started wearing clothes.¹⁷⁵
Even if we wished to assume that humans did at one time have a hair coat (there is absolutely no evidence that such was the case), these theories would not explain the advantage of a sparse coat of hair. The hunting hypothesis is untenable because nonhuman terrestrial predators all have thick hair coats. A similar objection can be raised to the sexual enticement scenario. Why haven’t all mammals lost their hair if nakedness is enticing? The aquatic proposal is also dubious, most small (human-sized or smaller) aquatic or semi-aquatic mammals do have hair coats.¹⁷⁶

Some pig-chimp haiku by Chris Millar:

Hogu I

Amo an ape aghast
Hic haec hog hiccup
Pig latin lover

Hogu II

Amo amas amat
An ape aghast agape
Pig latin lover

Hogu III

An ape against a pig
Agape agape agape
Pig greek lover

Hogu IV

An ape agape and a pig
Again and again and again
Then a human

Hogu V

An ape agape
A pig and a poke
Vo-lar-e who-ho-ho-hog

Hogu VI

No pig no gain
Ape agape pig again
Pig in ape inhumane

Hogu VII

Pig in ape chasm
Fakes orgasm
Ham actor

Hogu VIII

Cheeky Chinese chimps
Like a pig in a china shop
Porcelain

The results of my evaporation experiment make it difficult for me to accept Mount’s opinion that naked skin evaporates sweat faster than hairy skin.¹⁷⁷ For the same reason, I doubt Wheeler’s suggestion that the acquisition of erect posture by hominids "was probably the essential pre-adaptation which made it possible for them to shed body hair and develop extensive evaporative surfaces."¹⁷⁸ Also dubious is Kushlan’s "vestiary hypothesis," because it proposes that the invention of clothing left Homo free to lose his body hair and thus obtain "the most efficient cooling system of any mammal."¹⁷⁹ As we have seen, naked skin provides no particular evaporative advantage.
Because nakedness is a handicap on the savanna, Newman concludes, it is unlikely that human ancestors lost their hair after leaving the forest: "If one had to select times when progressive denudation was not a distinct environmental disadvantage, the choices would be between

a very early period when our ancestors were primarily forest dwellers or a very recent period when primitive clothing could provide the same protection against either solar heat or cold. The primary difficulty in arguing for the recent loss of body hair is that there seems to be no single and powerful environmental driving force other than recurrent cold that is obvious after the Pliocene epoch. Furthermore the developing complexity and efficiency of even primitive man’s technology would have decreased the probability of a straightforward biological adaption … The obvious time and place where progressive denudation would have been least disadvantageous is the ancient forest habitat. Radiant energy does penetrate the forest canopy in limited amounts, [but] that portion of the spectrum which is primarily transmitted through the vegetation, the near infrared wavelengths of 0.75 to 0.93 microns, is exactly the energy best reflected by human skin (Gates, 1968¹⁸⁰).¹⁸¹

In desert environments human beings can lose as much as 12 liters in sweat per day (390.3,162). Since the African savannas lie closer to the equator than do most deserts, sweat rates there should be at least as high — if not higher.

Note, however, that Newman does not explain why our ancestors lost their hair in the same environment (forest) where apes did not. If humans came into being via hybridization between pigs and chimpanzees, their genesis would almost surely have occurred in the forest. Chimpanzees live in forests. On the basis of its high rate of water consumption, Yang concluded the pig, too, is functionally a forest animal.¹⁸² Human beings need more water than almost any other animal.¹⁸³
Indeed, it seems incredible that a hominid would spend any more time than necessary away from the forest. Although the savannas of Africa were teeming with game, they were also swarming with ferocious predators. When a human being is chased by a lion, the first impulse is to find a tree. Consider the picture painted by current evolutionary theory: the noble savanna hunter, naked to the brazen sun, boldly erect on an arid and treeless plain, in indefatigable pursuit of a wary and dangerous prey, indifferent to the attack of rapacious carnivores. Certainly, this description has dramatic appeal. It’s like a Tarzan story. But is it plausible?

4: The Bipedal Ape

(This is section 4. Go to section 1 >>)

The Lord will give grace and glory. No good thing will he withhold from them that walk uprightly. —Psalms, 84:11

(Continued from the previous section)

Plato's minimal definition of a human being as a "featherless biped" exploits the fact that it is unusual for a mammal to use only two feet in the course of normal locomotion. Since we're mammals, it's easy enough to understand the lack of feathers. Why, though, do we go about on two feet? Human bipedality has long been a subject of controversy. How long have human beings stood erect? How long did the transition take from quadrupedal locomotion to bipedality? What factors caused the change? Why have other primates not done the same?
Following in Darwin’s footsteps, a wide variety of authors have asserted that human beings gradually developed the ability to walk on two feet in response to selective pressures demanding that two hands be free to manipulate tools. In his book, The Ascent of Man, Darwin stated this view succinctly: "If it be an advantage to man to have his hands and arms free and to stand firmly on his feet, of which there can be little doubt from his pre-eminent success in

the battle for life, then I can see no reason why it should not have been more advantageous to the progenitors of man to have become more and more erect or bipedal. The hands and arms could hardly have become perfect enough to have manufactured weapons, or to have hurled stones and spears with true aim, as long as they were habitually used for supporting the whole weight of the body … or so long as they were especially fitted for climbing trees. ¹

This explanation is not without its flaws. For one thing, should we conclude on the basis of our supposedly “pre-eminent success in the battle for life” that every human trait is superior? Isn’t this line of reasoning a bit vague and self-indulgent? Are our hands really in any way perfect—or do we just see ourselves that way? Isn’t it possible to “manufacture weapons” while sitting down?
And then, there is the presumption that we became “more and more erect or bipedal.” Fossil evidence does not confirm this gradual transition. Apparently, even very early hominids were fully bipedal. Thus, Lovejoy observes, that "for a number of years and throughout much of the literature there has been an a priori assumption that australopithecine locomotion and postcranial morphology were 'intermediate' between quadrupedalism and the bipedalism of modern man. There is no basis

for this assumption...in terms of the lower limb skeleton of Australopithecus. It is often claimed, principally on the basis of this a priori assumption, that morphological features shared by both modern man and Australopithecus do not necessarily indicate similar gait patterns. Although this might be true in terms of a single feature, it is demonstrably not true when the whole mechanical pattern is considered...the only significant difference between the total biomechanical patterns of Australopithecus and H. sapiens is one that indicates that Australopithecus was at a slight advantage compared with modern man (femoral head pressure [i.e., pressure exerted by the weight of the body on the hip joint]).²

Pig tracks were also preserved at Laetoli (357.3,262a).

Fossil footprints preserved in volcanic ash at Laetoli, Tanzania, indicate that hominids were fully bipedal at a very early date (3.7 million years ago).³ Similarly, Straus and Cave concluded that the posture of Neanderthals was not significantly different from that of modern humans.⁴ Homo erectus was also fully upright and bipedal.⁵ This lack of confirmation from the fossil record leaves gradualistic explanations of bipedalism standing on shaky ground.
Even on a hypothetical level, the idea that early humans "gradually" attained erect posture is implausible. One must either go on all fours or stand erect. No feasible intermediate posture exists. Hollywood portrays cave men as slumped over, arms hanging down. Maintaining such a position for any length of time would put an extreme strain on the muscles of the lower back. Millions of years of slouching, then, would surely have produced more than a few backaches. In fact, it seems ridiculous to suggest that hominids went about day in, day out, partially erect. The physical strain would be too great, even for us with our supposedly better-balanced bodies. Gradualistic thought forces the conclusion that early "human beings" spent a portion of their time in the quadrupedal position, but spent a gradually increasing portion of time erect as evolution progressed. Why would there be such a trend? Why have we developed the ability to stand all day on two feet?

These "free" hands seem not to have been taken advantage of for more than a million years: The earliest known stone tools date from 2.6 million years ago (556.6,236), whereas indisputable evidence (Laetoli footprints) indicate that hominids were fully bipedal 3.7 million years ago (104.5; 293.8).

This notion that free hands and intelligence are connected did not originate with Darwin, although he did espouse and popularize it. Perhaps, the earliest thinker to propose it was Anaxagorus who claimed that humans became intelligent by using the hands for manipulation rather than movement. Aristotle thought the opposite (i.e., that humans used their hands because they had become intelligent). Merfield (337.5,51) describes a favorite chimpanzee once on display at the zoological gardens in London. She was an inveterate smoker. "You could hand her a box of matches or a lighter, and she would not only use them properly, but could always be trusted to hand them back when she was finished with them."

The many physical distinctions making a bipedal form of locomotion possible (even necessary, for efficiency) in humans would require many genetic alterations. Anyone who wishes to account for the spread of these mutations in terms of gradual evolution must show how bipedality increased our ability to survive and reproduce. Yet, a comparison of human and simian modes of locomotion, suggests bipedality does not appear to be any great boon. Supposedly, "the freeing of the hands" made tool manipulation possible. The need for such manipulation, in turn, is said to have necessitated enlargement of the brain.
But why must a tool maker or a tool user stand erect for long periods of time? The hand, not the spine, seems to be the essential element in most manipulative processes. Few such activities would require anything more than the facultative bipedality of an ape. A chimp's hands would serve as well as ours in fashioning a spear, bow, or axe — they might even serve better: a chimpanzee has four hands. Human beings commonly sit down to work on such projects, having no need to stand. I can easily picture chimpanzees doing the same — chipping away at an arrowhead or heating spear tips in a fire. Studies of these animals have documented that their hands, too, are capable of performing subtle tasks such as decanting a glass of wine⁶ or even threading a needle.⁷ Surely, their using rocks and sticks to crack nuts⁸ is not so different from the way our forebears would have used hand axes.
Kortlandt has shown that chimps are capable of using weapons when they choose to do so.⁹ In his experiments, he presented various objects to wild chimpanzees and recorded their reactions. In one test, he placed a stuffed leopard on the ground near a troop of chimpanzees. The "leopard" clutched a facsimile baby chimp in its paws, and a concealed tape recorder emitted baby cries. Presented with this phenomenon, the apes attacked, using large broken branches as clubs. Kortlandt says that the blows were of such force that, had the leopard been real, it would surely have been killed. Apparently we are not the only ones with "free" hands.
Jane Goodall has documented "aimed rock throwing" behavior in free-living chimpanzees. ¹⁰ If they can carry clubs and throw rocks, then chimpanzees certainly have the anatomical wherewithal to carry and throw a spear. Physically, chimps may be better equipped for throwing than we are. Their arms are far stronger than those of human beings (about four times as strong, according to van Lawick).¹¹ Our ancestors invented the spear thrower, a hooked stick that, in effect, lengthens the arm, increasing the force of the throw. The arms of chimpanzees are already longer and stronger than those of humans.
If they can carry clubs, apes should also be physically capable of stalking prey with a spear. Human hunters do not stand erect when their quarry is nearby. Rather, they crouch, or even crawl, and approach their prey from downwind, taking advantage of available cover. Only at the last moment when the prey is in range do they spring up and throw the spear. Chimpanzees are quite capable of leaping and throwing an object simultaneously.¹²
For all of these reasons, then, it is at least questionable whether bipedality has enhanced our ability to survive and reproduce. A gradualist would object that, even if we do not understand the selective pressures involved, such pressures must, nevertheless, have existed, and that humans otherwise would never have made the transition to erect posture. But slow selection of minute mutations is not the sole conceivable mechanism that can account for human bipedality.
An Analysis of Human Bipedality
If we listed all the features contributing to our upright mode of locomotion, we would find some of those features in the chimpanzee. Nevertheless, even though chimpanzees do walk on two feet from time to time, such is not their normal mode of progression. They lack certain characteristics that make moving around on the hind limbs not only convenient for human beings, but really, under most circumstances, the only practical way of getting around. But what if pigs possessed all of those features relevant to bipedality that apes lack. Wouldn't it then be easy to understand why a pig-ape hybrid might walk on two feet?
All the human distinctions listed in the remainder of this section were first identified by other writers; I've merely gathered them together. If a scholar somewhere has claimed that a certain characteristic distinguishes human beings from chimpanzees and that that feature contributes to bipedality, then — if I have encountered the claim — I at least mention it. I exclude only those features that relate to the skull; cranial features are discussed in the next section. (It will also be convenient in this section to discuss a few skeletal distinctions of human beings not directly relating to bipedality.)
In the literature, most features said to contribute to human bipedality are located in the spine and lower extremities. For example, our gluteal muscles, large in comparison to those of other primates¹³, enhance our ability to hold our torso erect. Ardrey observes that

Sonntag notes the small size of the chimpanzee gluteal muscles in comparison with those of humans (533.6,356) and that they are small, also, in the gibbons and Old World monkeys (533.8,55,65). Duckworth (158.3,179) observes that the musculature of the upper limb is almost exactly as heavy as that of the lower limb in apes but that in humans the leg muscles are three times as heavy as those of the arms. Although I have not been able to obtain exact data on swine relating to this proportion, a cursory examination of any pig will reveal that the hind legs are far more heavily muscled than the forelegs.

As the brain co-ordinates our nervous activity, so the buttocks co-ordinate our muscular activity. No ape boasts such a muscular monument to compare with ours; and it is a failure more fundamental than his lack of a large brain.¹⁴

Certainly, the gluteus maximus is a significant portion of our anatomy. But, did apes "fail" to alter their bodies in this respect? Or did they simply lack the potential for doing so? Perhaps no pure primate had the potential to evolve into a human being by gradual mutation alone. We could, however, have obtained our big rump by other means. One has only to think of a country ham to realize that pigs, too, have powerful buttocks. Perhaps the very first hominid had a large rump as well as many other distinctively human features.
The Spinal Column
Centralization of the spine, another factor facilitating our erect carriage, is not seen in other primates to the extent that it is in humans.¹⁵ With the spine shifted toward the center of the body, a larger proportion of the trunk lies to its rear. As a result, the anterior portion is better counterbalanced by the posterior and less effort is required to keep the body erect. In pigs, the spine is more centralized even than our own,¹⁶ just as ours is more centralized than an ape's.

The human sacrum is concave on its anterior face while an ape's is rather flat. The anterior face of a pig's sacrum is markedly concave (405.5,I,35,Fig. 50).

Centralization of the vertebral column by itself, however, does not account for the ease with which the human body is held erect. Many other modifications of the spine facilitate our bipedality. At the base of the human spine, where the lumbar vertebrae meet the sacrum, is a sharp backward bend known as the lumbo-sacral promontory (see illustration below). The angle formed by this promontory is more acute on the front side of the spine because of subsequent tapering of the sacrum. This configuration causes the sacrum to form the roof of the pelvic cavity in human beings (instead of the rear wall as it does in other primates).¹⁷

More significantly, it brings the base of the flexible portion of the spinal column into a position directly above the hip joints (when viewed from the side). The force applied to the pelvis by the weight of the upper body is directed straight downward through the hip joints and does not tend to rotate the pelvis around those joints. When an ape is fully erect, a vertical line passing through the base of the spine falls behind the hip joints so that a rearward twisting torque is applied to the pelvis. This torque must be countered by constant muscular exertion.

Barone (55.1,I,439) states that "on the dorsal face of the [pig sacrum] extreme reduction of the dorsal spines is quite characteristic." (translated by E.M. McCarthy)

Krider et al. (280.5,Fig 4-1) provide a photograph of a pig carcass in this position. A lumbo-sacral promontory is clearly visible.

The dorsal, backward-projecting spine of the uppermost vertebra on an ape sacrum is too long to permit backward flexure of the lowermost lumbar. In human and pig,the spines on the dorsal (back) face of the sacrum are quite short and do not interfere with bending at this point (see illustrations above). But, do pigs have a lumbo-sacral promontory? In anatomical depictions of pig skeletons arranged in the typical quadrupedal pose, no promontory is visible. But if a human being gets down on all fours, then the lumbar region is twisted forward relative to the sacrum, and the promontory disappears. Perhaps an erect pig would also develop a sharp bend at the base of the spine. Obviously, pigs do not ordinarily stand upright, and I have never seen a drawing showing the configuration of a pig skeleton in such a position. Nevertheless, anyone willing to examine a hanging side of pork will see that a lumbo-sacral promontory is evident. Hanging a halved carcass by the hind leg causes the leg to swing into a position that closely approximates erect human posture. Here, again, porcine anatomy accounts for a human peculiarity.

Schultz (495.7,77) also points out that "the proportionate length of the lumbar region of man surpasses that of the man-like apes more than [would be] expected from the differences in the lumbar segments." In man the length of the lumbar region is 38 percent of the total length of the trunk, while in the chimpanzee this region is only 22 percent of trunk length.

The human spine contains more lumbar vertebrae and fewer sacral vertebrae than does the spinal column of any great ape.¹⁸ Because sacrals are fused and lumbars are not, the human spine is much more flexible than an ape's. Consequently, we are capable of bending the body backward until it balances over the hip joints (without rotating the pelvis backward). The "small" of the human back is the external evidence of this backward curvature of the lumbars. Pigs have even fewer sacrals¹⁹ than do human beings, and they have more lumbars.²⁰ So here, again, humans are intermediate between apes and pigs.

Cervical vertebra (pig). Tracing.

Cervical vertebra (human)

Cervical vertebra (ape)

Note that the great flexibility of the human neck in comparison with that of apes would make it possible to balance the head, almost regardless of the positioning of the foramen magnum. If the head's ability to swing backward and forward is not limited by long spines on the neck vertebrae, then a balance point will be attainable.

While it is commonly noted that the dorsal spines of the cervical vertebrae slant caudally in Homo, it has also been observed (540.6, 223) that "nonretroverted cervical spinous processes occur frequently in modern Europeans with perfectly normal posture." In the accompanying radiograph tracing (540.6,Fig. 5,223) spines 6 and 7 slope cranially. Pig cervical spines are so short that it is difficult to determine which way they slant except for the long one on the seventh cervical which slants slightly caudally (55.1).

In all the great apes, the cervical (neck) vertebrae have long dorsal spines — significantly longer than those on their thoracic (ribbed) vertebrae.²¹ In consequence, ape necks are stiff in comparison with a human being's. Any nodding motion of the head is severely limited.²² Though all cervical spines are long in apes, the fourth and fifth are usually longer than the sixth and seventh. Humans and pigs, on the other hand, have relatively short cervical spines except on the seventh cervical, where the spine is long (but not so long as the thoracic spines).²³As a result, humans are better able than apes to adjust the balance of the head by tilting it backward to the equilibrium point. Moreover, the figures above clearly show that human cervicals are generally more similar to a pig's than to those of an ape.
The seventh human cervical vertebra differs in another respect from those of other primates: it has transverse foramens or "foramina" (see illustration below). These large openings on either side of the spinal canal "are very rarely missing in even the seventh vertebra of Homo sapiens, but in the other primates it is rare to find corresponding foramina in this segment" (Schultz²⁴). In a work on the comparative anatomy of humans and domestic animals, Barone discusses the seventh vertebra, saying it "is not, in general, pierced by a transverse foramen, with the exception of pigs and human beings. In these two cases it always is."²⁵

Seventh cervical vertebra (human)

Pelvis and Coccyx

Schultz (495.06,429) states that "In the macaque and gorilla, as well as in the other monkeys and apes examined for these conditions, there is no fixed, bony structure opposite the pubic bones, as exists in man in the form of the lower part of the sacrum. In the former, therefore, the sacrum interferes not nearly so much with the passage of the fetus to be born, as in the latter." The obstruction of the birth canal by the sacrum in human beings reflects the shortness of the human pelvis in comparison with the simian. This shortening can be accounted for by the fact that pigs have a very short pelvis. A small pelvic opening does not interfere with parturition in swine because their newborns are relatively small in comparison with those of primates.

At the opposite end of the spine are the coccygeal vertebrae which together form the coccyx, or tail bone. Adolph Schultz observes that these vertebrae are fused in chimpanzees, a lack of flexibility he terms "puzzling."²⁶ Under the assumption that humans stand on a "higher" rung of the evolutionary ladder, chimpanzees should have a longer and more pliable "tail" than do humans. But, in fact, the human coccyx is not fused, but movable — especially in females, where it bends backward when they are giving birth.²⁷
The human pelvis and birth canal are smaller than those of apes.²⁸ Moreover, the sacrum and coccyx curve inward in humans to make a sharp-pointed obstacle that must be negotiated by an emerging infant.²⁹ In apes there is no curvature (see illustration above), which leaves the birth canal unobstructed.³⁰ With their constricted birth canals, human females experience far more difficulty in delivery than do their simian counterparts. "Parturition in the great apes is normally a rapid process," according to primatologist A. F. Dixson, who further states that

Gorillas, orangutans and chimpanzees typically give birth in less than one hour and in most cases there is little sign that parturition is imminent … The rapidity with which the great apes give birth correlates with the fact that the head of the newborn is remarkably small in comparison to the female's pelvic canal. In human females, by contrast, labor may be prolonged and the baby's large head is often turned sideways to facilitate its passage through the canal.³¹

"The antero-posterior diameter extends from the tip of the coccyx to the lower part of the pubic symphysis," says Gray (220.1,267). "It varies with the length of the coccyx, and is capable of increase or diminution, on account of the mobility of that bone."

Turning of the head occurs in Homo sapiens because the pelvis is so short that the birth canal is wider than it is high (unlike other primates).³² In humans, the height (antero-posterior diameter) of the birth canal depends on the length of the coccyx and, specifically, on how closely the tip of the coccyx approaches the front wall of the passage.

Contrary to popular belief, it is not merely the human head, but the entire body that is larger than that of any ape at birth (460.5,73,Table 3). Even the gorilla does not catch up with human babies in size until the second year (460.5,74,Fig. 10). This may be a manifestation of heterosis. In proportion to body size, the head of a new-born ape is as large as that of a human being. In both cases, the brain composes about 12 percent of body weight (188.7).

The human coccyx, then, ought to be relatively short, since the human neonate is larger than any newborn ape. And yet, "man is distinct among higher primates by possessing the largest average number of coccygeal vertebrae, i.e., by having been so far affected least by the evolutionary trend to reduce the tail" (Schultz³³). "In the human coccyx there may be as many as six elements, in the anthropoids there are quite commonly only two. The anthropoids have gone further than man in the reduction of the tail" (Jones³⁴). This longer "tail" is difficult to account for in terms of natural selection. With respect to reproduction, it is clearly a negative factor. Nor does it have any evident utility in other respects. Perhaps we should look elsewhere for an explanation. The sacrum of a pig is curved on its inner side much like that of a human being (see illustration above). Obviously, pigs have tails, albeit short ones. If Homo is a hybrid of ape and pig, we expect the human sacrum to be curved and the coccyx to be longer and more flexible than an ape's. The human pelvis is peculiar in many respects. According to Adolph Schultz,

A similar conclusion was reached by Straus: "The human ilium would seem most easily derived from some primitive member of a pre-anthropoid group, a form which was lacking in many of the specializations, such as reduction of the iliac tuberosity and anteacetabular spine and modification of the articular surface, exhibited by the modern great apes. I wish to emphasize here that the anthropoid-ape type of ilium is in no sense intermediate between the human and lower mammalian forms. Its peculiar specializations are quite as definite as those exhibited by man, so that it appears very unlikely that a true anthropoid-ape form of ilium could have been ancestral to the human type." Quoted in (495.06,431).

distinguishing characters of the human ilium [upper portion of pelvis] are so numerous and in most instances so very pronounced, whereas the ilia of all the anthropoid apes show so many basic similarities, that no theory which derives man from a gorilla-chimpanzee stock can readily account for these conditions.³⁵

The most obvious difference is the shortness of the human ilium. The pelvis of an ape is about half again as long as a human's (as a percentage of body length) and closely approaches the last rib³⁶ (in the great apes, Schultz (495.7,76) notes that the iliac crest approaches the last ribs "far more closely than in any other primates"). A pig has a short pelvis and a wide gap between pelvis and rib cage, just as we do. The upper blades of the pelvis run from side to side in apes but turn towards the front in humans.³⁷ They also turn forward in pigs.³⁸
Human tail
Rarely, human babies are born with tails. The following is from an abstract of a paper (Dao and Netsky 1984) reviewing this phenomenon: "The true, or persistent, vestigial tail of humans arises from the most distal remnant of the embryonic tail. It contains adipose and connective tissue, central bundles of striated muscle, blood vessels, and nerves and is covered by skin. Bone, cartilage, notochord, and spinal cord are lacking. The true tail arises by retention of structures found normally in fetal development. It may be as long as 13 cm [5 in.], can move and contract, and occurs twice as often in males as in females. A true tail is easily removed surgically, without residual effects. It is rarely familial. Pseudotails are varied lesions having in common a lumbosacral protrusion and a superficial resemblance to persistent vestigial tails. The most frequent cause of a pseudotail in a series of ten cases obtained from the literature was an anomalous prolongation of the coccygeal vertebrae."
Lower Extremities
All nonhuman catarrhine primates have longer arms than legs.³⁹ The reverse is the case in humans. But pigs, like humans, have longer hind limbs than forelimbs.⁴⁰ The femur (thighbone) is the largest bone of the body. Paleoanthropologists distinguish the femur of a hominid from an ape's in several ways. On the front of the lower end of the femur in humans and apes, the patellar groove forms a track for the kneecap. In apes, this groove is relatively shallow and its medial lip is more prominent than the lateral.⁴¹ But in humans⁴² (and in pigs⁴³) this groove is deep and the lateral lip is the more elevated of the two.

Physical anthropologists often note that the intercondylar fossa (or notch) is deeper in Homo sapiens than in pongids (325.5,308; 445.5,282; 468.2). Barone (55.1,I,693) describes the porcine intercondylar notch as "très profunde" (very deep).

Also, on the distal (lower) end of the femur are two condyles. In Homo, these condyles are of approximately equal size. In pongids the medial one is markedly larger than the lateral,⁴⁴ but in pigs the femoral condyles are almost exactly equal in size.⁴⁵ Human femoral condyles also differ in shape from those of other primates. "In hominids, both condyles show a distinct elliptical shape, indicating a specialization for maximum cartilage contact in the knee joint only during full extension of the lower limb. In [primate] quadrupeds, on the other hand, the condyles show no such specialization to one position, being essentially circular in cross-sectional outline" (Lovejoy⁴⁶). Nevertheless, many non-primate quadrupeds do, in fact, have elliptical femoral condyles. Among them are most of the domestic animals: cows, sheep, horses, dogs — and pigs (see illustration below).⁴⁷ We have no reason, then, to think that human elliptical condyles represent an adaptation aiding in bipedal locomotion.

Lateral views of femoral condyles in humans, non-human primates and pigs

Lovejoy (325.5,310) suggests that a prominent lateral lip is an adaptation "directly related to a valgus knee position produced by a high bicondylar angle." The horse, however, has a very high bicondylar angle (that is, it is quite knock-kneed) and yet the medial lip is much more prominent than the lateral. Knock-knees, then, do not always result in a prominent lateral lip.

Approximate measurements I have taken from anatomical drawings (405.5,I,89) give a bicondylar angle of about 15 degrees for the pig femur, which suggests that pigs are more knock-kneed than most human beings.

Quadrupedal primates are bowlegged, especially the anthropoid apes.⁴⁸ Human beings, however, are typically knock-kneed.⁴⁹ Preuschoft⁵⁰ follows Kummer⁵¹in suggesting that our knock-kneed stance is an adaptation facilitating bipedal posture, and bowlegs, to quadrupedal posture. But the domestic quadrupeds (dog, horse, cow, pig, etc.) are consistently knock-kneed.>
In pigs (and most other domestic animals), the femoral condyles rest on crescentic menisci that are connected to the tibia (shinbone) in the same way as in humans.⁵² This configuration is significant because, as Tardieu⁵³points out, Homo sapiens is the only primate having a "crescent-shaped [lateral] meniscus with two tibial insertions." In fact, in the vast majority of catarrhine primates (including the chimpanzee and gorilla) the lateral meniscus is ring-shaped. In the tibia itself the most prominent difference is the presence of a long malleolus medialis in nonhuman primates.⁵⁴ In Homo this downwardly directed, spike-like process is reduced to little more than a nub. In pigs it is so short as to be nearly nonexistent.⁵⁵

Romer (470.4,273) remarks that, in artiodactyls, "the astragalus is the most characteristic bone in the skeleton, for it has not only a rolling pulley above, but an equally developed lower pulley surface [articulating with the calcaneus]. This type of articulation gives very great freedom of motion to the ankle for flexion and extension of the limb and a potential springing motion, but limits the movement to a straight fore-and-aft drive even more strictly than is the case in perissodactyls [odd-toed ungulates]."

The upper surface of the talus in human beings is level from side-to-side so that it is parallel to the base of the foot (542.8,52). In chimpanzees (ibid.) this surface lies at an angle so that a perpendicular to it passes through the lateral side of the sole of the foot — this angulation affects the way apes walk when upright. In taking a step, all of the pressure is placed on the outside edge of the foot. Instead of rising on its toes at the end of a step, an ape rolls the pressure point forward along the side of the foot in a rocking motion. According to Sisson (525.3,184,Fig. 197) the porcine tibiotarsal joint is level, as it is in human beings.

We find another human distinction in the foot, in the joint between the heel bone (calcaneus) and the anklebone (astragalus). Szalay and Delson note that one feature distinguishing hominids from apes is the "loss of [the] ancestral helical astragalo-calcaneal articulation, reducing the possible range of movements in this joint."⁵⁶ In apes the articulation is "helical" because the joint allows rotation of the foot in a plane parallel to the ground. In Homo sapiens, this joint is more like a hinge. It allows only flexion and extension.⁵⁷ A pig, too, has a hinge-type articulation between the calcaneus and the astragalus.
The proportions of the human foot are also peculiar for a primate. Duckworth notes that the human heel bone is longer than that of apes.⁵⁸ Baba found that the length of the third metatarsal bone exceeded the length of the calcaneus in all primates in his survey — except in humans, in which the calcaneus is slightly longer(the third metatarsal connects the middle toe with the ankle and composes most of the length of the foot between the ankle and ball of the foot).⁵⁹ Our high ratio of calcaneus to metatarsal makes it easier for us to bear the body's weight on the ball of the foot (as we do each time we take a normal step), because the forepart of the foot and the heel bone can be thought of as two ends of a lever having the ankle as a fulcrum. As in humans, the heel bone is a bit longer than the third metatarsal in domestic pigs.⁶⁰

The fact that our toes are shorter than our fingers can be accounted for under the hybrid hypothesis by the fact that in chimpanzees the toes are markedly shorter than the fingers.

Our fingers and toes, are short compared to those of apes.⁶¹ Our metacarpal bones and phalanges are shorter than a chimpanzee's (not just in relation to the overall length of the hand, but absolutely).⁶² This shortening can be explained by referring to the anatomy of pigs: their digits are even shorter and stubbier, than our own (which, of course, is the case for most quadrupeds).

Shrewsbury and Sonek (510.6,237) feel that the difference between human nail phalanges and those of other primates is so marked that a distinction in terminology is called for, saying, "For humans we reserve the diagnostic term ungual tuft; for non-human primates the term ungual tuberosity is to be employed … [because] the roughened development of the volar aspect of a broadened ungular tuft, characteristic of humans, is not evident in the prevailingly conical ungual tuberosities of the other primates." While it does seem that a distinction in terminology is called for, it makes more sense to use a new term in connection with nonhuman primates instead of with human beings, because the term ungual tuberosity was originally used in describing humans. Moreover, no tubercle being present in these animals, the choice of the term tuberosity seems inappropriate.

Lastly, consider the ungual tuberosities. These small, hoof-shaped processes tip the bones (nail phalanges) that underlie the nails of our fingers and toes (see illustration below). Nonhuman primates do not have such processes. "When comparing the nail phalanges of apes to those of man, a pronounced slenderness of the former can be observed. If the impressive strength of pongid hands is taken into consideration, this is surprising" (Preuschoft⁶³). Shrewsbury and Johnson state that "the distinguishing features of the human distal phalanx are the broad spade-like tuberosity with proximal projecting spine and the wide diaphysis, which is concave palmarly to create an ungual fossa. These features are not seen in primates such as the monkey and gorilla."⁶⁴ This distinction, which was also present the various extinct hominids (395.5,539,541), has been explained as an adaptation facilitating the manipulation of objects with the fingertips. If such is the case, why should these processes also be found on the tips of our toes? Do these hoof-like ungual tuberosities actually reflect a relationship between humans and ungulates like the pig? That is, are they vestiges of ancestral hooves?

Human distal phalanx (ungual tuberosities circled).

Convergence or Relationship?

Elsewhere on this website, some of the problems with thinking in terms of homology and analogy are considered at length. Access this discussion >>

Our hypotheses have accounted for a number of traits in Homo. From the standard neo-Darwinian perspective, it is hard to understand why the parallels between human being and pig should be so extensive. Biologists call the existence of similar traits in animals that they consider to be distantly related analogy. They say analogy is found when animals live under similar conditions or have similar habits. The same needs in each case are supposed to cause structures of similar function to develop during the course of evolution. But when the organisms under consideration are considered to be closely related, such features are termed homologous. Homologous features are usually judged to be so when the similarities are numerous and extend to detail. As Dobzhansky et al. put it, "Examination of the structure of convergent features usually makes it possible to detect analogy because resemblance rarely extends into the fine details of complex traits."⁶⁵
In this section we have considered one complex trait (bipedality) in a fair amount of detail. Any attempt to account for these details in terms of natural selection seems inadequate. It is difficult to see what “selective pressures” could have caused human beings and pigs to “converge” in so many different respects. Under neo-Darwinian theory, to explain most of the human features that we have just discussed, we have to posit pressures selecting for bipedality (some human features — long tail bone and ungual tuberosities — cannot be explained in this way). But pigs are quadrupeds. How will we account for the fact that they, too, have these features? Perhaps it is all just a coincidence, but after a certain point coincidence begins to assume the color of relationship.

5: The Cranium and the Brain

(This is section 5. Go to section 1 >>)

The work of teaching and organizing the others fell naturally upon the pigs, who were generally recognized as the cleverest of animals. —George Orwell, Animal Farm

From ancient times, the large size of the human brain has been regarded as the crowning distinction of Homo sapiens. And upon first consideration of the idea that humans might have originated via hybridization between pig and chimpanzee, this particular trait seems to pose a major objection to the hypothesis. Why would a cross between a chimpanzee and a pig yield a hybrid with a big brain? The average cranial capacity of a chimpanzee is about 375 cubic centimeters; that of a pig, just 150. If humans are derived from crossing between chimpanzees and apes, the initial expectation is that the human brain should be of an intermediate size. But, of course, it is not: the average cranial capacity of a human being is about 1,350 cc. Clearly, an explanation in terms of hybrid intermediacy, then, would be unsatisfactory. But, as has already been shown, many other facts are consistent with the idea that a hybridization of this sort actually did occur. Is there then some other explanation? As it turns out, yes, there is.
The human brain is large not only in absolute terms, but also in proportion to body size. This human distinction becomes apparent only during the course of postnatal development. "A variety of sources place the neonatal brain/body ratio at about 12% in both pongids and humans, as in other primates" (Frost¹). However, by adulthood the human brain is about twice as large in proportion to body size as is an ape's (1.8% vs. 0.9 %). These facts suggest that we should look for human characteristics conducive to brain growth outside the womb.
Comparing absolute brain weight with body weight, we find that the former increases rapidly with the latter for small primates, but that for the larger primates a large increase in body weight corresponds to only a small increase in the size of the brain. It is as if the large nonhuman primates have hit a ceiling that limits brain size below a certain level. Only the human brain continues the trend set by the smaller primates. There seems to be some hidden factor allowing brain size in humans to pass through an otherwise impenetrable barrier.

The adverse effect of increasing brain size on cooling is a simple consequence of a geometric relationship taught in introductory calculus classes: The ratio of surface area to volume declines as the size of an object increases. A large object has a small surface area in proportion to its volume. If heat is flowing out of an object, the rate at which it passes out will be directly proportional to the object's surface area. The object in question in the present case is a brain. The total amount of metabolic heat produced by a brain is in proportion not with its surface area, but with its volume. Therefore, since the rate of heat production rises in proportion with volume while the rate of heat dissipation increases with surface area, and since volume goes up more rapidly than surface area, cooling becomes a serious problem as brain size increases.

Intrauterine development of the human head:

A new type of cooling system. An evaluation of the evidence suggests the human brain was able to break through this barrier because it had a new type of thermoregulatory system. Although brain cells are easily damaged by elevated brain temperatures, they carry out metabolic processes that actually generate heat. If this heat is not dissipated, brain damage will result. If an object, such as a brain or an engine, produces heat internally, it is more difficult to cool when large than when small. Cooling large objects is harder, not simply because they produce more heat, but also because they have a smaller surface-to-volume ratio. It is for this reason that a cooling mechanism may not be feasible for a large engine when the same mechanism is sufficient for a smaller, but otherwise identical, engine. The engine of a lawn mower or a small car can be air-cooled; in a large car or a ship, water-cooling is needed. If air-cooling were the only option, there would be an upper limit on engine size. The same rules must, almost certainly, place limits on brain growth.

Under sedentary conditions, humans and other primates cool their brains with arterial blood. But cooling cannot be accomplished under all circumstances by ordinary circulation of blood through the brain. Arterial blood is an inefficient coolant. Even before entering the brain, blood in the carotid artery is at, or very near, body core temperature and so is not much cooler than the brain itself. During periods of physical activity, incoming carotid blood becomes so warm that it cannot cool the brain, and even so warm that it would damage the brain—if other cooling mechanisms were not available. In addition, cerebral arteries are in extensive contact with, and absorb heat from, the cerebral veins. This contact causes heat to be fed back to the very areas in need of cooling.² As Cabanac and Brinnel point out

the highest temperature tolerated by the human brain seems to be only about 40.5E C [104.9E F] which is below that of other core tissues … Because of its relatively large mass, the human brain needs to be cooled more than that of most other species. At rest, this is accomplished by the carotid blood: when the temperature of the arterial blood is raised, the brain is in jeopardy and there is need for a mechanism to cool the brain directly.³

Exercise-induced hyperthermia: During exercise carotid (arterial) blood becomes so warm that its heat would cause brain damage if alternative cooling mechanisms did not ameliorate its effects.

And human beings do have such a mechanism. During exercise-induced hyperthermia, it actively pumps cool blood to the brain from the surface of the cranium. My first inkling that such a mechanism might exist came when an initial investigation of skin structure revealed that humans and pigs share an unusual thermoregulatory system not seen in chimpanzees and other nonhuman primates. This system was described at length in Chapter Six. Its basic units are: 1) a highly vascularized skin, permeated with fine capillaries at a density far in excess of what is required simply to feed the skin; 2) an insulative subcutaneous fat layer pierced by musculocutaneous arteries that regulate blood flow to the skin's surface on the basis of temperature; and 3) a dense population of heat-responsive sweat glands. Comparative anatomists have documented these distinctive features linking human and pig, features that make for a skin that is markedly superior to that of nonhuman primates with respect to thermoregulation, especially with regard to evaporative cooling.

Sonntag (533.6,330) notes that in the chimpanzee the veins of the pterygoid region "do not form a large diffuse plexus [as in humans], but consist of tributaries accompanying the large arteries and opening into an external maxillary vein." But in the course of my research into the comparative anatomy of humans, pigs, and apes, I have noticed what appears to be an additional, more general distinction. The vessels of the human and porcine circulatory systems seem to have a much greater tendency toward anastomosis than those of chimpanzees and other nonhuman primates. When vessels anastomose, they not only divide like the branches of a tree, but also reconnect with one another like the strands of a web. Sometimes the division and interconnection of these circulatory structures is extremely complex and minute, in which case they are termed plexuses. Although it may only be my subjective impression that a greater tendency toward anastomosis distinguishes humans and pigs from nonhuman primates, it is a definite fact that chimpanzees lack the venous plexuses that line the upper respiratory tract of humans and pigs in regions adjacent to the base of the skull.⁴ "There is no close pterygoid plexus," in the chimpanzee, and "the pharyngeal veins do not form a rich plexus" (Sonntag⁵). These plexuses represent ideal surfaces for exchanging heat between the body and inhaled air via water evaporation (from saliva and mucus).
Anyone familiar with heat-exchange mechanisms knows that a single large pipe containing hot water will lose heat far less rapidly than will a large number of small pipes carrying the same amount of water. The difference in rates of heat loss is a consequence of the difference in surface areas (heat flow varies in proportion to surface area when all other factors are held constant). A large pipe has a much smaller surface area than does a network of small pipes of equal capacity. Engineers make use of this fact to achieve rapid heat exchange.
Thus, in humans and pigs, but not in chimpanzees, the brain is in close proximity to evaporative surfaces that are highly efficient sites for the dissipation of heat (externally, to a skin with enhanced thermal characteristics, and internally, to the plexus-lined epithelia of the respiratory tract). But no particular advantage would inhere to a mere proximity to such surfaces if they were unable to communicate freely with the brain through the skull. In humans and pigs, however, a highly developed, intricate system of veins serves this need. These vessels are called emissary veins.

The adjective emissary is somewhat misleading; it is now known that these vessels do more than just drain blood from the cranium (as was once supposed). Blood flows inward through these vessels more often than it flows outward and their most important function seems to be regulation of brain temperature rather than simple drainage (101.65; 101.85).

Emissary veins are sensitive to thermal conditions. When the arterial blood feeding the brain gets too hot and the brain begins to overheat (i.e., under hyperthermic conditions), the pattern of cranial circulation changes so that cool venous blood runs rapidly inward through the emissary veins from the superficial evaporative surfaces of the cranium to the brain.⁶ Under normal or cool conditions, the system shuts down; blood flow in the emissary veins slows to an ebb and ceases, or even reverses (flow reversal does not occur in other veins of the body).⁷ In short, the human brain has a water-circulated evaporative cooling system that is actively responsive to thermal conditions. The cranial thermoregulatory mechanism in question here is similar to the cutaneous one discussed in Section 1. In the earlier case, the insulating material was a fatty sheath (panniculus adiposus). In the present case, the skull forms a thermal barrier between brain and environment. Here, the vessels perforating the insulating medium are the emissary veins, there, the musculocutaneous arteries. In both cases the rate of heat flow is actively regulated by the rate of blood flow.

For example, Boyd (79.3,109-112; 79.6,113-114) surveyed the frequency of three types of emissary foramina (mastoid, parietal, and condyloid) in primates. The expected number of mastoid foramina in humans is 1.128/skull, while in chimpanzees it is only 0.17/skull (The human rate is therefore 6.6 times as high). Similar differences apply to other foramina. For parietal foramina the human rate is 8.54 times as high, for the condyloid, 6.2 times as high.

The emissary veins pass into the skull through small holes termed emissary foramina, which are numerous in human skulls, but not in those of apes and other nonhuman primates.⁸ Obviously, when these openings are absent, the corresponding emissary vein must also be absent. The ability of these animals to pump in blood from superficial regions to cool the brain would be limited, then, even if they did possess the efficient evaporative surfaces present in humans and pigs.
Pigs, on the other hand, boast a system of emissary veins that is, if anything, better developed than that in human beings.⁹ Their skin is everywhere densely perfused with sweat glands and capillaries.¹⁰ In these animals, venous plexuses do exist in the upper respiratory tract and they are maintained in direct connection with the brain via numerous emissary vessels.¹¹
The nonhuman primate approach to evaporative cooling is different; sweat plays no significant role.¹² Since the upper respiratory tract of a chimpanzee has no venous plexuses, evaporative cooling takes place in the lungs. But this mechanism is not brain-specific. After cooling in the lungs, blood flows next to the heart. The cooling effect of any evaporation that does occur is therefore distributed equally to all parts of the body, not concentrated on the brain. In human beings and pigs, blood cooled on the evaporative surfaces of the cranium, and carried inward by the emissary veins, is held separate from the general blood supply for direct delivery to the brain.

It may also be that the presence of this efficient cooling system in pigs accounts for the perception that they are among the most intelligent of animals even though they have small brains.

When the human brain obtained an evaporative water-cooling system like the one in pigs, it escaped thermal constraint. The simian brain continued to deal with hyperthermic conditions in the same old way. Hence, it is easy to understand why brain size reached an upper bound in nonhuman primates as body size increased. But this new, heterotic mechanism would do more than simply allow humans to fully realize an inherent primate tendency to have a large brain. It would permit a more active brain by allowing dissipation of the additional heat generated by an elevated cerebral metabolism. Because it has a super-efficient cooling system, the human brain can utilize oxygen and other nutrients at higher rates than would be possible in an ape. Thus, an assumption of porcine ancestry helps to explain not only the volumetric preeminence of the human brain, but also the more subjective observation that our mental function is somehow qualitatively superior.
Common Chimpanzee or Pygmy Chimpanzee?
Having accounted for the large size of the human brain, we can turn to a consideration of other, less obvious cranial traits. Because there are two types of chimpanzee, treated as separate species, which differ significantly with respect to many such traits, we must first decide which to compare with humans, the common chimpanzee (Pan troglodytes) or the pygmy chimpanzee (P. paniscus). The pygmy chimpanzee has been increasingly known in recent years as the “bonobo." However, for reasons explained below, the name pygmy chimpanzee will be used within within the context of subsequent discussion. Pygmy chimpanzees are quite similar to ordinary chimpanzees, but they are, on average, smaller with respect to most physical measurements.¹³ Average body weight in wild-shot pygmy chimpanzees is just 85 percent of that in P. troglodytes.¹⁴

The common chimpanzee produces abnormal spermatozoa at a low frequency (0.2%). The pygmy chimpanzee produces none. These facts are documented separately.

Zihlman and Cramer compared 20 skeletal measurements in P. troglodytes and P. paniscus (621.9,89,Table 1). The standard deviation of every measurement was higher in the common chimpanzee.

Any modern chimpanzee that has been genetically influenced by past hybridization would not be an appropriate choice. An ideal analysis would compare the human skull with a skull typical of chimpanzees before the time of hybridization. In this way, all human traits indicating porcine ancestry would stand out. For several reasons, the pygmy chimpanzee appears to be the best modern representative of this ancient type. In particular, there is the empirical finding that the pygmy chimpanzee is far less variable than the common chimpanzee with respect to a broad range of traits. The pygmy chimpanzee is monotypic, that is, it has no recognized subgroups or races.¹⁵ This lack of variability in comparison with the common chimpanzee, which is commonly asserted to have three, or even four, different races, is not merely with respect to external features. It extends also to a wide variety of less obvious characteristics, ranging from sperm morphology,¹⁶ to skeletal measurements, to cranial features,¹⁷ to blood groups and various other genetic traits.¹⁸
This variability is consistent with the idea that the common chimpanzee is itself a kind of hybrid. When two types of organisms come into contact and hybridize, hybrids are at first limited to the region of initial contact. But, with time, the genetic effects may spread outward into surrounding populations because of backcrossing and migration. The introduction of new genetic material into a previously uniform parental population, generates variability. Eventually the entire population can be affected unless some geographic barrier, such as a river or mountain range, happens to protect some isolated group. When the backcrossing is rare or the parental population is relatively large, the amount of variability induced in the parental population will be relatively modest.

Fenart and Deblock (183.6,11) note there is one point where it is possible to cross by leaping from rock to rock at low water upstream of Boyoma Falls (formerly Stanley Falls) above Bubundu, but that a survey (See 583.63) of that area revealed no evidence of natural hybridization. While common chimpanzees and pygmy chimpanzees are not known to hybridize in nature (because they are isolated from each other by a water barrier), they have been reported to hybridize in captivity (583.67).

Thus, a scenario can be pictured in which an ancient, homogeneous chimpanzee population, similar to the modern pygmy chimpanzee, 1) engaged in hybridization with pigs in North Africa or, possibly, even the Near East; 2) the genetic influence of this hybridization spread south into the previously uniform chimpanzee population generating new variability; 3) this variability continued to spread until it reached an impassable geographic barrier. The most obvious barrier in the region in question is the wide and crocodile-infested Congo River, which in conjunction with its eastern tributaries, today completely separates the range of the pygmy chimpanzee from that of the ordinary chimpanzee (chimpanzees cannot swim).¹⁹

An email from a reader

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Thank you for your precise answer and your time.
I stumbled on your page by complete accident (although, I don't believe accidents exist at all).
While reading, I was wondering what will you say when you get to human origins. I had no idea what was awaiting. :D
Wow, I'm absolutely stunned. Pig-ape? But the evidence is there. I can't think of any other explanation.
Also interesting for me is the fact that it is the Bonobo that is the primate parent. It's very synchronistic for me, as I saw a documentary about them just a week or so ago, and really liked their disposition. They immediately became my favourite apes. I thought it was a great name for a band. :) It also just strengthens your theory that they are probably one of the most sexual of all animals. It is easy to imagine a Bonobo female giving herself to a boar, or even going after him herself :D. I believe you've seen them in action yourself.
Anyway, I want to wish you many more years of brilliant research and luck in getting this into the mainstream.
Just stand your ground and keep up the good work.
The world deserves to finally break off from the evolutionistic influence on so many of our basic concepts of civilization, society and nature.
Stabilization theory is the perfect name as it implies stability of life and the need for humans to form stable relationships to their surroundings.
Thank you so much for your work and please keep it up.
Now, if you'll excuse me, I have to read on. :D

The pygmy chimpanzee (P. paniscus) can thus be interpreted as a genetically stable, geographically isolated enclave of homogeneous chimpanzees that probably resemble ancient chimpanzees (i.e., those that existed prior to the time of the posited cross). Conversely, the common chimpanzee (P. troglodytes) can be thought of as a variable population influenced to some extent by hybridization. In addition, a great deal of evidence suggests that the common chimpanzee hybridizes on an ongoing basis with the gorilla at the present time (read about probable hybridization between the gorilla and chimpanzee). Such hybridization would also tend to alter the northern chimpanzee populations from the ancient type. These various considerations, then, suggest that the pygmy chimpanzee is the best extant representative of what chimpanzees probably were like prior to the hybridization events leading to the production of the human race. Such, at least, is the supposition under the hybrid hypothesis.
Two Types of Cranial Traits
Features distinguishing a human skull from a pygmy chimpanzee's can be conveniently analyzed by breaking them down into two separate sets: those belonging to the cerebral capsule, and those belonging to the noncerebral set. "The cerebral capsule is formed by the tissues which surround, and are intimately responsive to the functional demands of the neural mass [brain]" (Moss and Young²⁰). "Cerebral capsule" is nearly a synonym of braincase. The noncerebral set can be defined as those portions of the skull not influenced by brain growth. Generally speaking, features in the latter category are not in intimate contact with the brain. Most features in the noncerebral set that distinguish a human skull from a chimpanzee's can be ascribed to porcine influence, whereas most differences in the cerebral capsule are best interpreted as consequences of brain expansion. We turn first to characteristics of the second type.

Comparison of a pygmy chimpanzee cranium (left) with that of a human being (right)

The Cerebral Set: Consequences of Brain Expansion

Sexual differences with respect to cranial traits are minimal in the pygmy chimpanzee. For twenty cranial measurements compared by Cramer, average sexual dimorphism (degree of difference between the sexes) in Pan paniscus was only 1.4 percent (122.3,31-32). The pygmy chimpanzee skull in the figure is thus essentially representative of both sexes. In both skulls the various dimensions reflect mean expectations wherever possible. With respect to the human skull, the figure follows Gray (220.1,160,Fig. 180) rather closely, but after consulting Howells (242.8) It was necessary to lengthen the occipital posteriorly to bring the drawing into agreement with worldwide norms. Howells' figures also suggested a slightly widening of the palate and bizygomatic width. Sources used for pygmy chimpanzee skull: (117.5; 122.3; 183.6; 257.8; 271.4; 282.7; 589.7)

As can be seen from the figure (comparing a human skull with that of a pygmy chimpanzee), it appears to be inaccurate to speak, as do some writers, of the foramen magnum "migrating" forward in the course of human evolution. This "migration" is an illusion created by the backward expansion of the occipital.

Numerous researchers have demonstrated that both tension and internal pressure can stimulate bone growth. See (301.5; 399.55; 557.7; 588.89; 610.4,405).

The figure above compares a pygmy chimpanzee skull with a human being's.^a The skulls are viewed from below. Note how all of the features between the incisors and foramen magnum match up from one side of the drawing to the other. The major differences all seem to be related to the obvious expansion of the human braincase. The region where little difference exists could perhaps best be defined as those regions that are not in close contact with the brain.
The correspondence in the drawing is not a result of scaling, which is the same in both skulls. For example, the center of the foramen magnum is about equidistant (on average) from the incisors in both humans and pygmy chimpanzees. Although there is a remarkable degree of correspondence between the two skulls, certain differences are obvious. In humans, the forehead and cheek-bones bulge forward. The whole braincase, especially the rear portion of the skull, has expanded. Any given feature on the portion of the skull adjacent to the brain lies farther from the center line than it does in the chimp skull. The skull is sharply constricted behind the eyes (postorbital region) in Pan paniscus, but not in Homo sapiens. Can these differences reasonably be interpreted as consequences of brain expansion in humans?
Research by Richard Young²¹ of Columbia University shows that the growing skull is, in fact, responsive to intra-cranial pressure. Young studied the effect of brain growth on the shape of rat skulls by manipulating the volume of the cranial contents. In one group of rats he removed a portion of the brain at birth. Survivors developed various degrees of microcephaly as adults. Rats in another group received intra-cranial injections of kaolin that interfered with the drainage of cranial fluids. The result was increased pressure within the cerebral capsule. These rats became macrocephalic. As Young puts it, "Expansion of cranial contents yields intra-cranial pressures which are translated into tensile forces within the [cerebral] capsule. These forces stimulate proliferation of capsular tissues, producing compensatory capsular expansion."²² It is well-known that the soft tissues contained in or attached to the skull govern the growth of the bone adjacent to them. Thus, Taylor²³ showed that, in individuals who lose an eye at an early age, the affected socket never reaches full size. Bony crests normally develop at the point where muscles are attached to the skull and other bones of the body. When Washburn²⁴ surgically removed muscles from young animals, he found that the crests associated with those muscles never developed.

Altered patterns of development in the underlying brain may also contribute to the expansion of the frontal and postorbital regions of the human skull. Numerous researchers have observed that the temporal and frontal lobes compose a larger proportion of the brain in humans than in apes. See, for example, (240.1,22). The same is true of pigs (525.3).

Prominent cheek bones and a high, bulging forehead make human beings appear less prognathic than apes. In addition, the large size of the human cerebral capsule relative to the face downplays any prognathism that would otherwise be present.

Aficionados of physical anthropology will also be interested to learn that according to Young (610.4,401) "Angulation of the foramen magnum, often ascribed to factors of body posture alone, was altered by changing cranial contents. With cerebral dilation the posterior border of the foramen (opisthion) was displaced caudally. The position of the anterior border (basion), however, was not altered, with the result that the foramen was tilted downwards. Inversely, by the same principle, it was deflected upwards in microcephaly." A similar difference in angle, usually attributed to human bipedality, distinguishes Homo from Pan.

The diameter of the foramen magnum is larger in humans than in chimpanzees, but a variety of authors (37.3; 122.3; 450.3) have shown that a positive correlation exists between the size of this foramen and the size of the brain. vNeanderthals are the only prehistoric hominids that had brains as large as ours, and they are also the only ones that had chins. Although it is frequently claimed that Neanderthals tend to have less of a chin than modern humans, it seems there are no statistical studies comparing this trait in the two. But judging from photos of available materials there seems to be no reason to suppose that Neanderthals had weaker chins than modern humans. See (540.8,figs.30,31,37,44,50,79).

Schultz (495.7,65) notes that a very commonly cited, alleged distinction of the chimpanzee jaw, the simian shelf, "is not at all a constant feature of chimpanzees, as has been claimed."

Young's study did more than simply show that intra-cranial pressure stimulates growth of the cerebral capsule. It also demonstrated that certain alterations of the cranium are characteristic consequences of brain expansion. Many features that distinguish macrocephalic and microcephalic rats also distinguish humans from chimpanzees. In macrocephalic rats, the braincase was smooth, globular, enlarged, and thin (all as in human beings). "In the orbital region, rapidly expanding contents displaced the jugum [i.e., cheek bones] anteriorly." The skull base widens²⁵ in rats with extreme cases of macrocephaly and the frontal bone bulges outward as well. The rear of the skull in these rats expanded so that the foramen magnum was no longer at the extreme rear margin of the skull base—in human beings the foramen magnum lies further from the rear of the skull than in chimpanzees.
Of those features distinguishing the human cerebral capsule that we have mentioned thus far, only one is not accounted for by Young's findings: the expansion of the postorbital region in humans. The eyes of rats are not enclosed by orbits as are human eyes. With these animals, then, it is not possible to speak of postorbital constriction. But it is possible to establish a link between postorbital constriction and stunted brain growth by other means: Weidenreich²⁷ notes that the skulls of microcephalic human beings exhibit "pronounced postorbital constriction." He also points out that, in these small-brained individuals, several other features of the cerebral capsule (foramen magnum angle, configuration of forehead and brows, development of air sinuses, placement of temporal lines relative to the sagittal suture, etc.) bear a greater affinity to apes than to normal human beings.
Chin and jaw. The presence of a chin in human beings has been cited by many authors as a distinctive human trait. But so long as the developing chimpanzee is able to keep pace with respect to brain/body ratio, it, too, has a prominent chin, as is evident from examination of the skulls of both fetal and neonate chimpanzees.²⁸ It is only later in development, when chimpanzee brain growth flags and falls behind that of humans, that a chin ceases to be evident. Why the presence of a chin should depend on the size of the brain can easily be explained. If the condyles of a flexible model of a primate jaw are stretched apart, the chin juts out, as in human beings. If instead they are pushed toward each other, the chin recedes and the incisors turn outward, as in the chimpanzee. The lateral expansion of the human skull base forces the jaw joints much further apart than is the case in a chimpanzee. So an expected consequence of brain expansion would a jutting chin. A human jaw is rather similar in other respects to a chimpanzee's (if the teeth are excluded from consideration), the human type falling within the chimpanzee range of variation.

If a human gets down on all fours like an ape or a pig, or even leans forward, the sterno-cleido-mastoid muscle is relaxed. It becomes tense when the head leans to the rear. It is slightly tense when the head is held upright and becomes fully tense whenever it is necessary to counter a movement of the head to the rear. The circumstances under which this muscle is tensed can be easily checked by placing a finger on the root of the muscle (just below the mastoid process) and leaning back.

Prominent mastoid processes may not even be a human distinction. Fenart and Deblock (183.6,49,Fig. 25) provide photographs of an adult female Pan paniscus with large mastoid processes similar to those of human beings. Illustrations in Duckworth (158.3,173,Fig. 116) and Schultz (495.2,150,Fig. 113) also convey the impression that this is not a true human distinction. See also (495.2, 241-242).

Mastoid process. The mastoid process is a bony bulge on the base of the skull. It lies behind the base of the ear, so close to the skin's surface that it can be felt with the fingertip. It is often asserted in the literature that the size of the mastoid process distinguishes humans from the apes. Thus, Weber²⁹ claimed that "in man alone does the mastoid attain the size of his processus mastoideus, most likely in consequence of the importance of the sterno-cleido-mastoid muscle, which inserts on it, for the maintenance and rotation of the head in the erect position." Standing erect, humans put a strain on this muscle that does not occur in apes and pigs. As has already been mentioned, experiments show that bone tissue responds to such strains by developing bulging processes and crests at the point of muscular attachment.³⁰ It is not surprising, then, if the mastoid process is large in humans.
Neoteny. Noting that baby apes have flat faces without brow ridges, a rounded braincase, and a prominent chin, certain authors have suggested that humans are essentially apes that have somehow retained juvenile characteristics. The retention of such traits is termed neoteny. But baby apes lack multipyramidal kidneys, protrusive cartilaginous noses, light-colored eyes, a panniculus adiposis, epidermal patterning, and a wide variety of other human features. They do, however, have a very high brain/body ratio. The characteristics they share with human beings (but not with adult apes) generally seem to be traits that are consequences of having a large brain (features of the cerebral capsule).

The Noncerebral Set: Features Attributable to Porcine Ancestry
The dimensions of many cranial features, distinguishing humans from apes, show no correlation with brain size. These can be directly attributed to porcine influence.
Nasal bones. Human microcephalics have the large nasal bones³¹ that set humans apart from other primates.³² "In the large size and permanent separation of the nasal bones, man is in marked contrast with all of the anthropoids" (Jones³³). Both the large size and the peculiar protrusion of these bones can be accounted for by a supposition of porcine ancestry.
Occipital condyles. The same line of reasoning explains the large size of these bony prominences in humans relative to other primates (see Figure 8.2). Pigs have large occipital condyles, at least as big as those of human beings.³⁴
Divergent eyes. When an ape skull is viewed in profile, the interior of the orbit (eye socket) is invisible. The same cannot be said of the human skull, where the orbits diverge, not lying perpendicular to the sagittal plane. A pig's eyes are even more divergent than a human being's.

Zuckerman et al. (632.7) seem to imply that the presence of a styloid process may not really be a distinction of human beings. Braga (80.7), however, examined a larger number of ape specimens. He found the styloid process tends to be more ossified and better developed in older apes (particularly the orangutan), but found a well-developed styloid process in only a very small percentage of the 351 Pan troglodytes skulls he examined, and in 166 Pan paniscus skulls he found none at all.

Styloid process. On the base of the human skull, inside the angle of the jaw, is the styloid process, a curved, projecting stalk of bone. In size and shape it is similar to the rib of a rat. "Bony styloid processes, fused with the petrosum, have become a specialization of adult man" (Schultz³⁵). The stylohyoid ligament "continues the styloid process down to the hyoid bone, being attached to the tip of the former and the small cornu of the latter" (Gray³⁶). The styloid process is not initially bony, but ossifies out of the stylohyoid during postnatal development.³⁷ In pigs, the term styloid process is not generally used, but the cranial end of the stylohyoid ligament changes to bone with age and takes on a form similar to that of the human styloid process.³⁸

Gantt (191.8,285) asserts that "Humans have much thicker enamel than any extant primate." Human tooth enamel exceeds that of Pan troglodytes in thickness by about 50 percent. Pigs are artiodactyls—a group as a whole noted for thick tooth enamel.

At one time human premolar roots were considered more primitive than those of apes because they were believed to have fewer roots. Thus, in 1929 F. W. Jones (259.8,321) wrote that "the fact that the upper premolars of man are single-rooted, groove-rooted, or two-rooted, whilst the corresponding teeth in all the Old World Monkeys and Apes are three-rooted, is remarkable." More recent studies have shown that upper premolar roots are sometimes grooved, or double in chimpanzees as well (Pan troglodytes and Pan paniscus). Fenart and Deblock (183.6,Figs. 17 and 21) provide figures establishing this fact beyond doubt. This commonly noted distinction, then, is illusory (or at best statistical) and need not concern us.

The anterior teeth of pygmy chimps of either sex are the same size as those of humans because sexual dimorphism of pygmy chimpanzee teeth is negligible, with the possible exception of canine teeth, which may be slightly larger in females (257.8,46-47).

The enlargement of the molars without comparable enlargement of the jaw forces the teeth together in a tight row that tends to bulge outward. Any outward pressure exerted on the jaw condyles by expansion of the brain case would tend to stretch the left molars away from the right. The result of these two factors would be the "arcuate" or "parabolic" tooth row, which is one of the features distinguishing humans from apes. Oversized molars and cranial expansion would have a similar effect on the dentition of the upper jaw. Cramping of the tooth row would also squeeze shut any gaps or "diastemata" (often seen adjacent to the canines in apes).

The bunodont crowns of human molars, because they are rounded and can slide against each other, contribute to our distinctive manner of chewing. According to Szalay and Delson (545.6,495), "The helical chewing pattern seen on Homo sapiens is not seen in other primates." But Herring and Scapino (233.4,454) state that in pigs "lateral movements are produced by rotation of the jaw as in man, selenodont artiodactyls, and rabbits … Thus it is possible in pigs for the mandible on one side to remain virtually completely adducted while moving laterally with the teeth in contact."

An artist's reconstruction of Nebraska Man, drawn on the basis of a single molar. Source: Forestier, A. 1922. "The earliest man tracked by a tooth: An 'astounding discovery' of human remains in Pliocene strata (A reconstruction drawing by A. Forestier)," The Illustrated London News, June 24, pp. 942-943.

Teeth. Porcine ancestry may also have had an effect upon our teeth. The dimensions and form of our teeth do not seem to depend on brain size. The teeth of microcephalic human beings "do not show essential deviation from the norm" (Weidenreich³⁹). But human premolars do differ from those of apes. The peculiar form of the human premolar has long puzzled both paleontologists and physical anthropologists: "We know of no case in which a comparable specialization [as seen in apes] has been lost once attained [as it supposedly once was in human ancestors], by a reversal to the primitive condition," says a puzzled Bjorn Kurtén, "The specialized premolar of the ape does not change back into the primitive premolar of man."⁴⁰ In artiodactyls, the order to which pigs belong, "the premolars, in contrast with those of perissodactyls, do not usually assume the full molar pattern but remain comparatively simple" (Romer⁴¹). Moreover, among artiodactyls, pigs are considered to have the most "primitive" teeth of any group.⁴²
Many authors have asserted that human incisors and canines (which are termed anterior teeth) have decreased in size during the course of evolution. It is surprising, then, that the absolute measurements of these teeth are almost identical in human beings and pygmy chimpanzees.⁴³ Perhaps the claims that human anterior teeth have decreased in size can be traced to the fact that they look small beside human molars, which are larger than those of chimpanzees.⁴⁴ Consequently, there is a sharp change in tooth size at the molar-premolar boundary in humans that is not evident in simians. Pig molars, too, are quite large relative to the other teeth. In human beings, "the molar cusps are blunt ["bunodont"] rather than sharp ["crenate"] as in apes" (Romer⁴⁵). Pig molars are also bunodont, and so similar to those of human beings that fossil pig teeth have actually been mistaken for those of prehistoric human beings.
One such molar, found in Nebraska in 1917, prompted scholars to name a new genus, Hesperopithecus or "Ape-Man of the West" (see Figures 8.7 and 8.8). This tooth, which was thought to date to the Pliocene (which ended about 1.6 million years ago), became a center of controversy, both in the popular press and in academe. By 1924, prominent English anatomist Sir Grafton Elliot Smith had joined the bandwagon of scientists trumpeting what had by then been dubbed Nebraska Man: "The discovery of this tooth may seem rather a frail and hazardous basis upon which to build such tremendous and unexpected conclusions; and many,

if not most, scientists have grave doubts as to the justification of such an interpretation. But the specimen was discovered by a geologist of wide experience, and its horizon has been satisfactorily established. Moreover, the determination of its affinities and its identification as one of the higher primates closely akin to the Ape-Man of Java, Pithecanthropus, has been made by the most competent authorities on the specific characters of fossilized mammalian teeth, Professor Osborn and Drs. Matthew and Gregory, who not only have had a wider experience of such material than any other paleontologists, but also are men of exact knowledge and sound judgement. ⁴⁶

After the furor had continued unresolved for three years, Osborn, then director of the American Museum of Natural History, was prompted to exclaim, "In the whole history of anthropology no tooth has ever been subjected to such severe cross-examination as this now world famous tooth of Hesperopithecus. Every suggestion made by scientific skeptics was weighed and found wanting."⁴⁷ In the same year (1925), this tooth was actually introduced in court as evidence for the defense at the Scopes "monkey trial." Imagine the blush that came to erudite cheeks when further excavation revealed that all the great ballyhoo had been inspired by the humble molar of a pig! "An ancient and honourable pig no doubt," quipped The Times when the news came out, "a pig with a distinguished Greek name, but indubitably porcine."^†

6: Additional Evidence

(This is section 6. Go to section 1 >>)

The first distinguishing characteristic of thinking then is facing the facts — inquiry, minute and extensive scrutinizing, observation. —John Dewey Reconstruction in Philosophy

Questions or comments about this theory are welcome. Simply send a message to the author through the contact page of this website. He'll be happy to respond.

Gathered from diverse sources, the traits included in this section have not previously appeared on a single list. Any one of them, encountered in isolation, could be shrugged off as insignificant or coincidental. Taken together, though, and in the wake of information considered in previous sections, their implications become more intriguing. Why should the pig, usually considered no more closely related to us than a dog, have a close affinity to humans with regard to a broad spectrum of traits that distinguish us from nonhuman primates? Why should this animal be a stellar performer in the fields of human biology and medicine? As Britt points out, "Pigs' physiological similarities to man—surpassed only by the primates'—make the animals invaluable to medical science." ¹ Many other authors echo this sentiment.² Why the pig? The situation is even more puzzling if we consider the many cases that we have already examined where pigs actually exceed nonhuman primates in similarity to human beings.

Nicolaides and his co-authors (410.5,88) assert that "the lipid sample of the pig represents primarily its hexane extractable epidermal lipids rather than those of its sebaceous glands whereas the other samples [in the survey] represent primarily sebaceous excretions." They specifically state that they make this assumption because they believe that a pig has "a sparse pilosebaceous population on its back and sides, the sites from which our samples were taken." In fact, however, pig bristles are more densely placed on the sides and back than on other regions of the body and in pigs the sebaceous glands are largest and most numerous in the vicinity of the bristles (503.3,497; 531.8). Moreover, Cox and Squier (120.1,743,Table 1) demonstrate that triglyceride and free fatty acid concentrations are highest at the surface of the porcine epidermis and decline steadily with depth, which is consistent with a sebaceous origin of these lipids.

SURFACE LIPIDS
The principal structural components of living cells are proteins, carbohydrates, and lipids. Lipids are fats and fat-like substances, only sparingly soluble in water. Nicolaides et al.³ compared the skin lipids of human beings with those of eighteen different types of mammals (including a pig, a baboon, a chimpanzee, as well as others: horse, cow, sheep, goat, dog, cat, rat, mouse, hamster, rabbit, guinea pig, chicken, duck, goose, and turkey). They note⁴ that human surface lipids are rich in triglycerides and free fatty acids, but that "none of the other animals [in the survey] had significant amounts of triglycerides or free fatty acids in their skin surface lipid except the pig … Additionally, pig surface lipid showed the same lipid classes as those obtained from the nonpolar epidermal lipid samples of different [i.e., various] human sources, namely, sterol esters, triglycerides, free fatty acids, and free sterols for sole epidermis."
MELANOCYTES

Chimpanzee dermal melanocytes are most common in the face, scalp, and genital regions (365.5,190).

Melanocytes are found in the dermis of chimpanzees (see green box, right), and most other nonhuman primates,⁵ but not in the dermis of human⁶ or pig,⁷ where these pigment cells are limited to the epidermis. Skin expert William Montagna (360.1,77) states that "In man, melanocytes are found consistently in the dermis only during fetal life, and in some nevi. Many other primates, however, have rich populations of them." In addition, according to Montagna,⁸ humans and pigs (but no other known mammal) have melanocytes in the hair follicle matrix.

In a study of porcine melanoma, Millikan, et al. (355.8,28) found that pigs exhibit a "broad variety of pigmentary tumors paralleling much of human pigmentary pathology including flat, junctional nevus-like lesions, elevated, compound nevus-like lesions, raised, blue, nevus-like tumors and a vitiligo that had an uncanny similarity to that developing in humans. Associated with vitiligo were some lesions very similar to the Sutton's halo nevus, and occasionally wide-spread, rapid growing tumors resembling various types of melanoma. Ultrastructural [i.e., microscopic] studies further confirmed the similarity of these tumors to human pigment pathology. This animal model is of particular interest because of the very high degree of spontaneous regression of the tumors which in some instances has been associated with the vitiligo-like picture and in some animals, a wide-spread, almost total depigmentation that can result in even ocular retinal involvement, all very reminiscent of the extreme situation in human vitiligo … Of the larger mammalian quadrupeds, it is our bias that the swine offers the greatest potential at the present time for fruitful further investigation in melanoma. It possesses one of the closest parallels to the full spectrum of human pigment pathology."

Vocal cords and intelligence It has been recognized for several decades that exposure to language plays an essential role in the development of intelligence. Deaf children who are not exposed either to spoken or sign language at a sufficiently early age develop significant, permanent mental handicaps. So it may well be that the presence of vocal cords in humans was essential to the initial emergence of human intelligence. Thus it's easy to see why humans (if they really are pig-ape hybrids) would have taken an upward leap in intelligence after inheriting a chimpanzee-like brain made heterotic by a pig-like cooling system (as discussed in the previous section), plus a pig-like vocal apparatus (in particular, vocal cords). After all, even human beings with brains that are structurally normal, never achieve normal cognition without language. So an imaginary chimpanzee with a fully human brain, but who lacked vocal cords, would not be expected to exhibit a fully human intelligence.

MELANOMA
Malignant melanoma is a deadly form of cancer that initially affects cutaneous melanocytes and rapidly spreads to other parts of the body. Melanoma in swine closely parallels melanoma in human beings. I have, however, been unable to locate any observation of melanoma in a primate; apparently none has been recorded. Schultz⁹ notes that all types of cancer are rare in nonhuman primates.
VOCAL CORDS
In The Descent of Man, Darwin asserted that "the fact of the higher apes not using their vocal organs no doubt depends on their intelligence not having been sufficiently advanced."¹⁰ But later anatomical studies have shown that the chimpanzee vocal apparatus is not suitably constructed to produce human speech. Thus, after dissecting the larynx of a chimpanzee, Kelemen¹¹ reached the conclusion that "even with the help of the high grade of intelligence of this animal every attempt to make him utter human voice and speech must fail on the basis of anatomy. An imaginary being equipped with a human brain and the larynx of a chimpanzee could not produce any other phonetic effect than this animal actually does." Although chimpanzees can be taught to communicate visually using symbols and gestures, efforts to teach them to speak have failed miserably.
A key difference between humans and chimpanzees is found in the structure of the vocal cords. A horizontal slit (ventricle) marks each of the two interior sides of the human larynx. The upper borders of these ventricles are defined by the superior vocal cords (also known as the false vocal cords because they are not concerned with the production of sound¹²), which contain the ventricular ligaments.¹³ On their lower edges, the ventricles are bounded by the inferior vocal cords, which, in humans, contain strong bands of yellow elastic tissue (vocal ligaments) that can be placed under tension to produce vibrations (voice) as air escapes from the lungs.¹⁴
In contrast to the human situation, Sonntag¹⁵ found that the inferior vocal cords of the chimpanzee "are soft and flaccid, consisting almost entirely of mucous membrane, and a little soft tissue. The false cords are likewise soft, and between them are well-marked ventricles." Because they are soft and contain no vocal ligaments, chimpanzee "vocal" cords cannot be tensed, and so, cannot produce human-like vocal tones. Pigs, however, do have a ligament on both sides of each ventricle¹⁶ (in positions corresponding to the human vocal and vestibular ligaments¹⁷). So although Darwin is no doubt correct in suggesting that human intelligence plays a critical role in the production of speech, it may well be that a second essential ingredient of human phonation is found in pigs, but not in chimpanzees.
LARYNGEAL SACS
The great apes have laryngeal air sacs (large, irregularly shaped, air-filled organs opening into the larynx through the ventricles and spreading outward from the larynx through much of the neck and chest).¹⁸ The function of these organs is unknown. They are not present in pigs and human beings.

Pig stomach (Sus scrofa)

STOMACH

Keith and Jones (268.5) were the first to note this human peculiarity. Their observations were later confirmed by Schwalbe (503.5) and Pernkopf (430.1,65-78; 430.3). Also see Langer (290.9). According to Schwalbe (503.5), the diverticulum regresses in the human fetus as it passes from 150 to 200 mm in length.

I would have preferred a more extensive embryological comparison between human, pig, and chimpanzee, but there is surprisingly little information available on the embryonic chimpanzee. Writing in 1969, for example, Hill (235.47,17) noted how little is known of chimpanzee embryology, saying that a single brief article (four pages) had been published on the topic. My research has turned up nothing published since that date. Even for the fetal chimpanzee there is very little information available, but see (74.7; 74.8; 495.07; 495.43).

It has never been disputed that circular valves like those of human beings are absent from the bowels of nonhuman primates other than apes. Some dissension, however, has existed over whether they are present in apes themselves. Burkl (98.3,158) resolves this question, however, by noting that when the walls of the intestine are tightly stretched, the valvulae conniventes remain visible in human beings, but that in apes the "analogous" structures disappear. Therefore, apes lack true circular valves because persistence under distention of the intestine is an essential, defining characteristic of these structures (220.1d,916). With respect to their absence in chimpanzees, see Symington (1891, p. 304).

The stomach of the human fetus passes through a stage during which it has a diverticulum ventriculi, i.e., a small side cavity in addition to the main stomach chamber (Figure 9.1). This blind pouch is commonly found as a transient differentiation at the upper end of the stomach in human fetuses. This feature does not recapitulate anything seen in the stomachs of nonhuman primates. The stomach is simple (single chambered) in all primates except the langurs and the guerezas, which have stomachs that are highly aberrant by primate standards, having no resemblance to the human organ.¹⁹ A simple stomach has only a single chamber. The stomach of the domestic pig is shaped like that of a human being, but it does have a small blind pouch identical to that seen in human fetuses.²⁰ Why should the human fetus exhibit the same pouch in the same position if human beings are related to apes alone? Hirsch²¹ has even described a diverticulum ventriculi in a 46-year-old human adult. It is much easier to account for these facts under the hypothesis that humans are pig-ape hybrids. At any given stage of development, a hybrid combines traits derived from each of its parents.
VALVULAE CONNIVENTES
In nonhuman primates the small intestine contains no circular valves (valvulae conniventes). Also called the “valves of Kerkring”, these circular membranous folds are found on the interior surface of the small intestine of humans and certain animals. They are so large that they may be felt through the wall of the intestine. Nelsen notes the presence of "the valves of Kerkring in the human and pig small intestine."²³ These structures are thought to enhance the absorption of nutrients by increasing the internal surface area of the intestine.
MESENTARY
In human beings the superior mesenteric artery supplies blood to the small intestine and colon. Connecting it with the small intestine are the intestinal arteries, which form an arcade (a series of interconnected arches); this arterial arcade is connected to the intestine by numerous, small straight vessels (220.1,678-679 & Fig. 577). No similar configuration is found in chimpanzees (533.8,241). The pig, however, does have human-like arterial arcades connecting the superior mesenteric with the small intestine (196.2,1332; 405.9,Fig. 145).
HEART
"In human hearts placed on their apices, both the auricles lie on nearly the same level. In chimpanzees, as in the rest of the Pongidae, the left auricular appendage lies distinctly lower" (Frick²⁴). Sisson shows that the auricles of a pig heart are approximately level with each other.²⁵
Heart attack is, of course, a very common cause of death in human beings (as it is in pigs when they are allowed to get old instead of being slaughtered young²⁶). Chimpanzees and other primates do not have heart attacks.²⁷ When the tricuspid valve of a human heart is diseased, currently accepted medical practice specifies its replacement with a pig tricuspid valve—not that of a dog or a baboon.²⁸ In fact, according to Osther et al., "Porcine products have been used extensively in many human therapeutic replacement regimens. Their use is primarily due to genetic similarity of humans and pigs at the amino acid level, which results in a high acceptance of grafted prosthetics and excellent tolerance of repeatedly administered biologicals."²⁹ For example, pig skin is used in treating human burn patients.³⁰ Human diabetics are given pig insulin.
ATHEROSCLEROSIS
In biomedical research the pig has proven to be the model of choice for investigating human atherosclerosis, another commonly fatal disease in humans. Pigs are much preferable in this respect to nonhuman primates.³¹ "In apes, atherosclerotic lesions are absent or minimal despite reported high cholesterol levels" (Munson and Montali³²).
KIDNEYS
In terms of gross morphology, the human kidney bears little resemblance to the kidneys of other primates — for example, Elftman and Atkinson (168.8,201) speak of "the fundamental contrast between the human kidney and that of primates in general." — and is certainly much more similar to that of a pig than to that of a chimpanzee. As can be seen in the figure below, pig kidneys have the same bean-shaped cortex seen in humans, while a chimpanzee's is almost triangular. The interior cavity of this organ is large in humans and pigs, but small in chimpanzees. Note especially the numerous inward projections (pyramids) in the human and pig kidneys. In the ape kidney, the projection is single, centrally placed, and quite flat. This latter type of configuration is termed unipyramidal. The configuration in human beings and pigs is termed multipyramidal. In this respect, human kidney structure contrasts sharply not only with that of chimpanzees, but also with that of all other primates.³³

Chimpanzee kidney	Human kidney	Pig kidney

Terris (560.4,1676) states that the distribution of "pacemaker cells depends on the general form of the upper [urinary] tract. In animals possessing relatively simple collecting systems, e.g., dog, cat, rabbit, rat, and guinea pig [and nonhuman primates as well (540.6)], cells are arranged as a continuous layer extending across the renal pelvis and ending in the region of the pelviureteric junction. The whole region acts as a single pacemaker unit. In the multicalyceal kidneys of pigs and humans, however, pacemaker cells surround the renal attachment of each minor calyx. Multiple pacemaker sites exist, therefore, and their number depends upon the number of minor calyces present in each kidney."

Straus (540.6) did not examine any gorilla kidneys in his survey, but Elftman and Atkinson (168.8,200-201) confirm that gorillas also have unipyramidal kidneys. Steiner (537.3,Plate 4) provides photographic proof of this fact.

Pig kidneys are similar to those of human beings not just in being multipyramidal. The similarity extends to fine details such as regional distribution of pyramid type (compound vs. simple),³⁴ renal lymphatics, and distribution of pacemaker cells.³⁵ The pig is the only creature other than human beings known to have this type of kidney (multipyramidal with a smooth, bean-shaped, undivided cortex).³⁶ After personally dissecting 117 kidneys, representing 67 animals of various primate genera (including Pan), Straus emphasized the sharp distinction between the human kidney and that of nonhuman primates:

It is apparent from the writer's findings, and from a survey of the literature as well, that the kidney in Primates regularly possesses but one true or primary pyramid [an appendix below deals with this topic]. The sole exceptions to this rule, at least so far as is known at the present time, are the spider monkey and man. In the former animal the presence of more than one pyramid in a single kidney is an inconstant feature, and many specimens exhibit a primarily undivided medulla [i.e., they have unipyramidal kidneys], as in the other monkeys, the lemurs, tarsiers, and the anthropoid apes. Thus the kidney of even the spider monkey contrasts markedly with that of man, in whom multipyramids always occur. And it is unlikely that the number of pyramids in Ateles [the spider monkey] ever equals that normally found in man. It is obvious, in view of its general ordinal constancy, that the character of primary pyramid formation is possessed of no real taxonomic value among Primates. The only features of interest here are the peculiarly isolated position of man and the occasional approach to the human condition made by the spider monkey, an animal that by every ordinary taxonomic criterion is far removed from the human line of descent.³⁷

Elsewhere in the same paper Straus notes that he never observed more than two pyramids in any spider monkey kidney.³⁸ Certainly, then, humans are peculiarly isolated from other primates; Homo averages ten pyramids per kidney (as does the domestic pig³⁹). But, renal pyramid count is not devoid of taxonomic value: It is suggestive of a link between human and pig. Strauss’s attitude toward this trait exemplifies a more general problem: Biologists tend to ignore data that tends to contradict accepted notions concerning the nature of phylogenetic relationships.
BACULUM
"In all primates except Tarsius [tarsiers], Lagothrix [South American wooly monkey] and man, the distal part of the penis is strengthened by a rod-like bone (baculum)" (Hill⁴⁰). Thus, human males lack a structure common to virtually all their primate kin, including all apes.⁴¹ This finding is consistent with the idea that humans are pig-ape hybrids: the pig lacks a baculum.⁴²
PENIS
It is sometimes claimed in the literature that Homo sapiens has the largest penis of any primate (252.5; 385.5,10,58), but photographic evidence clearly demonstrates that the erect chimpanzee penis is comparable in size to a human’s (585.3,127). The obvious feature differentiating the penis of Homo from that of Pan is the presence of a glans penis. The chimpanzee glans clitoridis is almost identical in form to the glans penis in Homo, but the male chimpanzee has no expanded glans (36.2; 140.5,429,Fig. 3h; 235.7,115; 252.5; 490.3,Plate 4; 533.8,270; 585.3,127). Gorilla males also have a glans similar in shape to that of chimp females (140.5). This glans structure may have been transferred between the sexes in humans and gorillas during the process of recombinational stabilization. Banding studies lend some support to this idea because both gorillas and humans have a larger Y than do chimpanzees (269.5; 317.8; 613.5) and, in both cases, the Y shows brilliant quinacrine fluorescence that is not seen in the chimpanzee Y, or in the Y of any other primate (348.6; 429.93; 541.6). The chimpanzee X, however, does fluoresce brilliantly (as do, also, those of humans and gorillas). It could well be, then, that a piece of the brilliant chimpanzee X has been transferred, during the process of chromosomal rearrangement characteristic of recombinational stabilization, to the Y in Homo and Gorilla and that the traits common to the human penis and chimpanzee clitoris are specified by genes present in this transferred chunk of DNA (subsequent discussion will eventually explain why it seems likely that gorillas are also of hybrid origin).
HYMEN

In particular, a hymen is absent in the chimpanzee (533.6,420).

"The presence of the hymen is always cited as a human peculiarity, and several authorities have definitely asserted its absence in the anthropoids. Sonntag has declared that it is absent in all apes" (Jones⁴³). Duckworth says the hymen is present in "the hominidae [i.e., humans] alone among the primates."⁴⁴ Nickel et al., note of the pig that "at the junction [of the vagina] with the vestibule, in the virginal animal, is an annular fold 1-3 mm. high, homologous to the hymen."⁴⁵
SCROTUM AND TESTICLES

A correlation exists between small testicle size and sterility in human beings (549.6), as it does in hybrids.

Hill asserts that the scrotum is sessile in chimpanzees and gorillas (i.e., it does not hang away from the body).⁴⁶ This is accurate with regard to gorillas, but a photograph by van Lawick of a mature chimpanzee male demonstrates that the chimpanzee scrotum is pendulous.⁴⁷ This trait, then, is not a human distinction, as some authors have claimed. Both the chimpanzee and the pig greatly exceed human males in the size of testicles.⁴⁸ The lack of human intermediacy can here be ascribed to hybridity. Hybrids often have underdeveloped testicles.
PROSTATE

According to Taber's Medical Dictionary (547.3,108), "Enlargement of the prostate is common, especially after middle age. This results in urethral obstruction, impeding urination and sometimes leading to retention. Forty to 50% of men over 60 have prostate trouble." Retention of urine can cause inflammation of the bladder and kidney damage. Surgery is often needed to remove the enlarged section of the prostate. Perhaps this obviously disadvantageous trait is an example of hybrid inviability and related to the fact that the prostate gland is extremely large in pigs.

The prostate gland is a firm, partly muscular, partly glandular organ situated at the base of the male urethra just below the bladder. It secretes a sticky, alkaline fluid which is a major constituent of semen. In primates, "the prostate is generally bi-lobed, a traverse groove dividing it into a cranial and caudal lobe, but only in Homo and Ptilocercus [tree shrew] does it completely encircle the urethra. In other primates it leaves the ventral wall of the passage uncovered" (Hill⁴⁹). In boars, however, it does form a complete circle, just as it does in men.⁵⁰
BULBO-URETHRAL GLANDS
Also known as Cowper's glands, the bulbo-urethral glands are found only in males. In humans they are about the size of a pea and are located on either side of the prostate gland. Each has a duct about an inch long, terminating in the urethra. During copulation, the pre-ejaculatory fluid secreted by these glands serves to clear the urethra of urine and to lubricate the penis and vagina. Bulbo-urethral glands "have never been reported in anthropomorphs [i.e., apes] despite diligent search" (Hill⁵¹). Nevertheless, large bulbo-urethral glands are found in boars.⁵²
COITUS AND SEXUAL CLIMAX

The huge quantity ejaculated by a boar (up to 1 liter) compares with about 2-3 ml in a human male. The semen of the boar is of three types and comes in three stages. First comes the injection of copious amounts of lubricating non-sperm-bearing fluid. Next comes five to six minutes of sperm injection. In the last stage, the penis withdraws somewhat and secretes a jelly-like plug that seals the vagina (485.2).

Morris notes that in nonhuman primates "pre-copulatory [behavior] patterns are brief … Copulation itself is very brief."⁵³ He cites the baboon, whose time from intromission to ejaculation is only "7-8 seconds," with no more than 15 strokes. The duration of coitus in anthropoid apes is scarcely longer. The pygmy chimpanzee averages 24.7 strokes per intromission⁵⁴ and the common chimpanzee only 11.3 strokes.⁵⁵ This figure is about 36 in gorillas.⁵⁶ Dixson says chimpanzees ejaculate five to ten seconds after intromission.⁵⁷ The human male, then, who takes several minutes or more to reach climax,⁵⁸ stands in marked contrast to the apes. In swine, however, coitus lasts 20 to 30 minutes. In itself, the ejaculation of a boar can continue nearly ten minutes,⁵⁹ and the volume of the ejaculate can be as much as a liter (although the average is closer to 200 ml).⁶⁰ Domestic boars have been observed to ejaculate as many as 25 times in a single day.⁶¹

See (30.8,100; 92.4; 246.9). Dewsbury (138.4) confirms that there exists little or no evidence of orgasm in nonhuman primate females. Chevalier-Skolnikoff (102.4) and Zumpe and Michael (650.4) have suggested that the "clutching response" sometimes seen in stumptail and rhesus macaques shows certain similarities to the spasmodic hand reflex displayed by the human female at orgasm described by Masters and Johnson (334.6). Fox and Fox (187.8,333), however, point out that they have observed the "clutching response" to occur independently of orgasm in human females and that orgasm in macaques should be confirmed by "alternative orgasmic criteria." Even the few researchers who do assert that "orgasm" occurs in nonhuman primates admit that it is a rather different phenomenon than that seen in human females. Consider, for example, a comment made by orgasm proponents Allen and Lemmon (27.1,23): "Of considerable interest is the fact that relatively few blatant emotional responses (e.g., vocalizations) associated with orgasm in women were manifested by the chimpanzee, even when intense vaginal contractions were being palpated. In fact, a female colleague who observed several experimental sessions found it incredible that this female chimpanzee was experiencing anything even approaching the emotion characteristic of orgasm in women."

Given the lengthy attentions lavished upon the sow by the boar, it is, perhaps, not surprising that "the female [pig] exhibits an 'orgasm-like' response" (Hafez⁶²). Sexual climax has not been observed in nonhuman primate females. Morris says that "female orgasm in our species is unique amongst primates⁶³… If there is anything that could be called an orgasm [in nonhuman primates], it is a trivial response when compared with that of the female of our own species."⁶⁴ Others affirm this fact. If we are related only to primates, then it is odd that female orgasm is found in humans but not in other primates.
AFFECTION AND GRIEF
Pigs are also very affectionate animals, capable of forming strong attachments. Following the loss of a close associate, many pigs exhibit a grief-like syndrome "causing the animal to reject food, be depressed and frustrated, angry and to search almost frantically for the missing animal or person. In some instances, the animal may die" (Bustad and Horstman⁶⁵). Pigs enjoy close physical contact and like to snuggle while sleeping. In contrast, excepting mother and infant, chimpanzees sleep alone.⁶⁶ Famous chimpanzee observer Jane Goodall writes, "Chimpanzees usually show a lack of consideration for each other's feelings which in some ways may represent the deepest gulf between them and us. For the male and female chimpanzee there can be no exquisite awareness of each other's body—let alone each other's mind. The most the female can expect of her suitor is a brief display, a sexual contact lasting at most half a minute, and, sometimes, a session of social grooming afterward. Not for them the romance, the mystery, the boundless joys of human love."⁶⁷
UTERUS FORM
In a bicornuate uterus, the main body of the organ is divided into two long chambers ("horns"). A bicornuate uterus is the normal configuration found in swine and most other domestic animals,⁶⁸ as it is, also, in lemurs, lorises, tree shrews.⁶⁹ All monkeys and apes have a simple, undivided uterus, which is also the condition found in most humans.⁷⁰ But according to Luckett "a bicornuate uterus occasionally occurs in the human, and pregnancy may ensue and proceed to term under these conditions."⁷¹
MENSTRUAL CYCLE
The chimpanzee menstrual cycle lasts 36-37 days;⁷² the human, an average of about 28; and the porcine, 21.⁷³ The human cycle is intermediate.
SEXUAL SWELLINGS
Chimpanzee females (and females of many other kinds of primates) develop large sexual swellings. These structures shrink away as the female goes "out of season." In chimpanzees, they are pink, larger than in most other primates, and have a stimulating effect upon the male.⁷⁴ No similar swelling of the female genitals occurs in human beings or pigs. In fact, a breeder has to be something of an expert to be certain whether a sow is ovulating or not.
LABIA
Flanking the entrance to the urino-sexual opening of human females are two sets of lips, differing in structure and size.⁷⁵ The external lips, the labia majora, are thicker and coated with ordinary skin. The labia minora lie inside and are more delicate in structure and are coated with epithelial tissue. The typical nonhuman primate has labia minora, but not labia majora. In particular, "the chimpanzee's vulva never, at any age, attains the typically human form of a simple linear pudendal cleft, guarded by two parallel cutaneous folds (labia majora) which hides from view, in the undisturbed state, the labia minora, the clitoris and its praeputial appendages and all deeper structures … true labia majora, lying more or less parallel to the median cleft, are found only in foetal life, as in monkeys" (Hill⁷⁶). A sow's vulva exhibits the opposite configuration, having labia majora only,⁷⁷ and so is quite similar in external appearance⁷⁸ to the corresponding human structure. The human configuration, then, can be accounted for by assuming labia minora are derived from chimpanzees and labia majora, from pigs.
ISCHIAL CALLOSITIES
Ischial callosities are large, thick, hairless patches of skin seen on the rumps of nonhuman primates. When these animals sit, their callosities provide a tough cushion between pelvis and ground. "Humans are unique among catarrhines [i.e., humans, apes, and Old World monkeys] in their total lack of ischial callosity development" (Schwartz⁷⁹). Ischial callosities are not found on a pig's rump, either.
BREASTS

Jones (259.8,324) says "One other well-known peculiarity of man … is the fact that the human nipples are situated considerably lower on the chest than they are in any other known primate." See also (495.06,281 & 445,Table 6). Schultz (495.65,140) notes that, among primates, "in man alone do they [i.e., the nipples] migrate caudally [downward] during growth." Pigs have nipples on their bellies (405.9).

"Except in Homo, mammary glands, during the quiescent phase, are poorly represented [in nonhuman primates], and even in the gravid phase and during lactation they do not form external swellings, enlarging instead diffusely subcutaneously. The only external manifestation is an enlargement of the nipples, which elongate and become pendulous" (Hill⁸⁰). On the other hand, enlarged breasts are characteristic of mature human females. During lactation, sows' breasts also enlarge and are similar in form to those of human females.⁸¹ In this respect, female pigs are more similar to human females than are nonhuman primates. Nevertheless, nonlactating breast enlargement is a characteristic that distinguishes Homo even from Sus.
This discrepancy, perhaps, can be accounted by considering how certain traits are combined in human beings. A typical mammary gland is divided into chambers (milk sinuses). Each sinus is connected to the teat by a separate milk duct. The number of milk ducts, and, hence, the number of milk sinuses, in a single human mammary gland ranges from 15 to 22.⁸² In the chimpanzee, the range is narrower, from 20 to 22.⁸³ In contrast, pig mammae have a relatively simple structure, being divided into only two or three milk sinuses.⁸⁴ As noted in the section on human skin, a thick layer of subcutaneous fat (panniculus adiposus) is characteristic of pigs and human beings, but not of chimpanzees. In the human breast this fat is interspersed between the milk sinuses, as it is in pigs. The volume of the interspersed fat is much greater in human beings because numerous milk sinuses imply numerous intervening spaces for fat storage. Fat is the major constituent of the nonlactating human breast. If each of the numerous sinuses of the chimpanzee mammary were ensheathed in fat like human sinuses, then the mammary region might be as prominent in chimpanzees as in humans. The novel combination of a panniculus adiposus with a complex sinus system may thus be responsible for nonlactating breast enlargement in human females.
ALCOHOLISM
Although Cheeta, Tarzan's chimpanzee sidekick, often drank beer when he wasn’t making movies,⁸⁵ nonhuman primates are not heavy drinkers—even when they get the opportunity. On the other hand, according to Tumbleson, "the pig is the ideal model for human alcoholism because it is the only mammal other than man that will drink enough ethanol to be classified as an alcoholic."⁸⁶ One of Tumbleson's pigs drank the equivalent of four quarts of 86-proof vodka every day for a week. The average pig in Tumbleson's herd was a more modest consumer, taking in only a quart a day. Moreover, pigs drink alcohol, even distilled alcohol, voluntarily, if it is available. With continued use they become dependent on alcohol, exhibiting withdrawal symptoms similar to those seen in human alcoholics.⁸⁷
TEARS
When upset, chimpanzees whine and moan, but they never shed tears.⁸⁸ Since tears lack commercial significance, little attention has been directed towards this particular physiologic response in pigs, but according to Dexter et al., one of the withdrawal symptoms seen in distressed alcoholic pigs is lacrimation (i.e., the secretion of tears).⁸⁹
NOCTURNAL ACTIVITY
With the exception of the South American night-monkey (Aotus), monkeys and apes are not active at night.⁹⁰ They retire with the setting of the sun. This inflexible behavior pattern is by no means characteristic of Homo sapiens. By choice, pigs are active during the day, but when they are hunted they follow a nocturnal schedule.⁹¹
TERRESTRIALISM
The typical primate, with the two notable exceptions of baboons and gorillas, spends much, or even most, of its time in an arboreal setting. Such, of course, is not the case with humans and pigs.
SWIMMING AND BATHING
In general, primates have a greater fear of water than the members of perhaps any other mammalian order. Until very recently, ape experts generally believed that chimpanzees could not be trained to swim. Thus, Vernon Reynolds in his book, The Apes observes that

swimming ability is almost nil in the apes. A young gibbon that fell into the pond at a zoo drowned while its mother watched, too afraid to rescue it. No report of swimming by a gibbon exists, but several others confirm its inability to swim. Orangutans have never been observed swimming, and probably cannot. Gorillas, faced with the alternative of capture by man, dare not go into water more than two or three feet deep. They have never been observed to swim, and at least one immature male has drowned in a zoo water enclosure, sinking as soon as it fell in … In captivity, attempts to teach chimpanzees to swim have met with failure, and deep water barriers have often proved efficient in keeping apes on islands … The general response of chimpanzees to water is universally agreed to be one of avoidance and even fear … I am quite convinced they cannot swim.⁹²

However, in the July 2013 issue of American Journal of Physical Anthropology ("Swimming and diving behavior in apes (Pan troglodytes and Pongo pygmaeus): First documented report") Bender and Bender document that chimpanzees and orangutans actually can be trained to swim (My thanks to John Law for notifying me of this paper!). So the ability to swim is not, as has long been thought, a characteristic that clearly distinguishes humans from chimpanzees (to see videos of a chimpanzee and an orangutan swimming, click here). It still appears safe to say, though, that non-human primates generally have a strong aversion to water and do not learn to swim unless they are raised by humans and taught to do so.
In contrast, the domestic pig, "like many of its wild relatives, is very fond of water. Pigs can obtain food underwater, are fond of fish and are very capable swimmers" (Bustad and Book⁹³). The 1984 recipient of the American Humane Association Stillman Award for animal heroism was a pet pig named Priscilla who earned the prize by saving a drowning 11-year-old boy. Pigs are even capable of catching fish underwater.⁹⁴ Diving pigs, for example, regularly perform for the public at Aquarena Springs in Texas. A pig is at least as good a swimmer as most human beings.
Given access to water, pigs not only swim, but also use it to keep themselves clean. The actual behavior of pigs defies the stereotype that casts them in the role of filthy mud-wallowers. According to swine experts L. K. Bustad and V. G. Horstman, "Under normal conditions, pigs are instinctively neat and fastidiously clean. They much prefer a clean dry bed and are exceedingly fond of comfort and warmth."⁹⁵ Pigs confined to a muddy pen are forced to be dirty, just as humans would be under similar circumstances. When given the option, pigs are actually rather tidy. For their pigs, Bustad and Book "provided outside runs with a cold water misty spray. Our swine would select a remote area in an outside run for their defecation and urination, and appeared very neat and clean under these conditions."⁹⁶ Swine are easily housebroken.⁹⁷
In contrast, apes do not bathe even when given the opportunity. Their bodies constantly emit a powerful, reeking odor. This disregard of personal hygiene is also reflected in a rather nonchalant attitude toward bodily wastes—despite their much vaunted intelligence, they are difficult to house train. "In their habits of excretion, the apes have not developed specific patterns of behavior. They have no defecating areas, and no set times for defecation. This is typical of primates. Man learns the place to defecate; but wild apes do not, and apes raised in captivity mostly do not either" (Reynolds⁹⁸).
OMNIVOROUS DIET
At one time it was believed that Homo sapiens was the only meat-eating primate. In recent years, however, researchers have shown that meat and insects are important in the diets of many primates.⁹⁹ Chimpanzees not only eat meat, but hunt together in cooperative groups.¹⁰⁰ There are even well-documented cases of chimpanzees preying on human infants,¹⁰¹ and elsewhere on this website there is even a video showing a chimpanzee troop hunting down a weakened chimpanzee and eating him (watch the video >>). But chimps more typically attack small animals such as monkeys and the young of ungulates.¹⁰² It may be, however, that chimpanzees do not digest meat as well as human beings. Goodall studied the diet of wild chimpanzees, using a method called dung-swirling (swirling of dung in water to separate its components for analysis). She states that "it is amazing how much of a chimpanzee's food seems to pass through the digestive tract only partially digested," and notes in particular that "dung-swirling was an excellent method of determining how often members of our group ate insects and meat."¹⁰³ I assume that the method was a good one for insects and meat because foods of this type were not well-digested. If this assumption is correct, then chimpanzees are not as omnivorous as humans. Certainly, meat cannot pass through the human digestive tract and remain in recognizable form. Nor would it pass through a pig unscathed.
The pig is the archetypical omnivore. Walker¹⁰⁴ notes that foods eaten by wild pigs include fungi, roots, tubers, bulbs, green vegetation, grains, nuts, cultivated crops, and animals, both vertebrate and invertebrate—a list that corresponds rather closely with the items normally found on the human menu. "In its nutrient requirements," say Pond and Houpt, "the pig resembles humans in more ways than any other mammalian species."¹⁰⁵ One of the main attractions of pigs as domestic animals has been their willingness to eat all of the leftovers from the farmer's table. In fact, the porcine digestive tract is so efficient that it can even extract nutrients from human feces. Thus, if human beings really are more omnivorous than chimpanzees, a close relationship to pigs would explain why.
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Works Cited >>
Mammalian hybrids >>
Online Biology Dictionary >>

The Hybrid Hypothesis - © Macroevolution.net

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APPENDIX A: NOMENCLATURE OF THE MULTIPYRAMIDAL KIDNEY
Straus¹⁰⁶ found only one renal pyramid in nonhuman primates. This finding is supported by Bolau,¹⁰⁷ Ehlers,¹⁰⁸ Bischoff,¹⁰⁹ Huxley,¹¹⁰ Chapman,¹¹¹ Weber,¹¹² Symington,¹¹³ Hill,¹¹⁴ Sperber,¹¹⁵ Tisher,¹¹⁶ Gagnon,¹¹⁷ and Elftman and Atkinson.¹¹⁸ Elftman and Atkinson note that Gerhardt¹¹⁹ described a gorilla kidney as "multipyramidal" but showed unipyramidal kidneys in his illustration (Plate III, 13a and 13b), and therefore reach the conclusion that "it seems highly probable that the kidneys described by Gerhardt bore a closer resemblance to those of man in verbal description than in actual morphology, due to the confusion of nomenclature reviewed by Straus."¹²⁰
This confusion was over the terms "pyramid" and "sub-pyramid". Straus points out that certain writers (Deniker,¹²¹ Sonntag,¹²² Jones¹²³) used the term pyramid (or papilla) to describe the structures called "sub-pyramids" by most other writers. He notes that this is made clear by Deniker's illustrations (as well as by Jones')¹²⁴ and by Sonntag's use of the term "fused pyramid".¹²⁵ Because sub-pyramids are subdivisions of pyramids, they will obviously always be more numerous than pyramids themselves. When Sonntag¹²⁶ says that "Symington¹²⁷ found the pyramids fused to form one papilla," or that in the chimpanzee "The blunt apices of the pyramids do not project much, and they are not embraced by large calyces," it becomes obvious to anyone familiar with the actual renal anatomy that Sonntag is referring not to pyramids, but to subpyramids. I include Straus' discussion of the nomenclature problem:
The usual text-book description of the human kidney states that the number of pyramids far exceeds that of the papillae. This concept of human renal structure is derived from the investigations of Maresch.¹²⁸ Following his work, the pyramids are structural units of the medulla that are separated from each other by inward cortical projections (the columns of Bertin) of various depths. Some of these renal columns extend to the renal pelvis and thus completely separate the pyramids. In the latter instance, two or more pyramids end in a single papilla [by definition].
Hou-Jensen,¹²⁹ however, has convincingly demonstrated that the above concept of renal pyramid is neither correct (historically) nor satisfactory. According to him, the pyramid is a division of the medulla, completely isolated by inward cortical projections (true renal columns or septa interpyramidalia) and ending in its individual and isolated papilla. Hence the number of pyramids and papillae exactly coincide. The pyramid is usually divided into two or more secondary pyramids by less extensive inward projections of the cortex (false renal columns or septa pyramidis).
The difference between the above two descriptions of the human kidney is really but one of definition. Yet that of Hou-Jensen is more satisfactory in that it recognizes and classifies minor subdivisions of the medulla. Thus his definitions of pyramid, etc., are more applicable to comparative renal study, and have therefore been adopted by the present writer.
It is this standard definition of "pyramid" that is employed in the present work to compare humans, apes and pigs.

CITATIONS:
1. (87.6,406)
2. (577.6)
3. (410.5)
4. (410.5,88)
5. (360.8,115b)
6. (103.45; 360.1,77)
7. (360.3,13)
8. (360.5,334). See also (360.1,243; 360.3,13; 360.7,587; 365.5,194).
9. (495.44,969). See also (413.2).
10. (135.6)
11. (269.3,254)
12. (220.1d,960)
13. (220.1,1182)
14. (220.1d,960,1182)
15. (533.6,398)
16. (196.2,122)
17. (531.3, Figs. 317, 318)
18. (411.5; 452.8,23 & Figs. 2,24,25; 495.65,141; 533.6,333,398)
19. (158.3,149,Fig. 94; 533.8,9)
20. (508.7,495)
21. (237.6)
22. (533.6,374; 533.8,220)
23. (399.3,699); See also (399.3,629,Fig. 298b).
24. (188.65,299)
25. (525.3,754,Fig. 605)
26. (534.6)
27. (475.6)
28. (87.6,406)
29. (414.9)
30. (34.4; 87.6,406)
31. (443.6,32)
32. (393.3,103). See also (67.4).
33. (235.4,44)
34. (452.4; 770.4)
35. (560.4)
36. (560.4)
37. (540.6,97-98)
38. (540.6,96)
39. (525.3,579)
40. (235.4,44; 235.7,113,115)
41. (334.58,143); Dixson (140.1) says the spider monkey (Ateles) also has a baculum.
42. (525.3,602)
43. (259.8,323-324)
44. (158.3,209)
45. (405.9,376-377)
46. (235.4,44)
47. (292.6)
48. (495.063)
49. (235.7,115). See also (348.2; 452.8,88a).
50. (196.2,1300; 525.3,602)
51. (235.4)
52. (196.2,1300)
53. (385.5,63)
54. (490.3,336)
55. (583.3)
56. (399.1)
57. (140.1,149)
58. (334.6)
59. (245.1,200; 485.2)
60. (485.2)
61. (100.1,12; 231.88)
62. (225.4,162)
63. (385.5,79)
64. (385.5,63)
65. (101.1,13)
66. (205.1)
67. (205.1,194)
68. (405.9,376,Fig. 543, 485.2)
69. (235.7,116)
70. (53.9,250; 235.7,116)
71. (325.8,209)
72. (460.5,72,Table 3)
73. (245.1,207; 405.5,II,380; 588.4,1177)
74. (360.7,108; 490.3,Plates 1-5; 495.65,142)
75. (220.1d,1025-1026)
76. (235.1,134-135). See also (533.6,420).
77. (405.6,377)
78. (530.9,Fig. 250)
79. (505.1). See also: (495.06,266; 495.65,139-140).
80. (235.4,32)
81. (405.9,377,Fig. 545)
82. (503.3,471)
83. (74.7,60)
84. (503.3,471)
85. (198.3)
86. (187.6,412)
87. (139.6)
88. (192.2,115; 230.6,39; 231.82; 273.2; 603.1,30; 603.2,299b)
89. (139.6,192)
90. (36.4,386-387)
91. (200.1,210)
92. (460.5,87)
93. (100.1)
94. (316.7)
95. (100.1,11)
96. (100.1,11)
97. (100.1,11)
98. (460.5,72)
99. (588.4)
100. (23.4; 205.1; 205.4; 558.8)
101. (205.1,198)
102. (205.1,App. D)
103. (205.1,132)
104. (588.4,1175)
105. (443.6,276)
106. (540.3,97-98)
107. (74.3)
108. (167.5)
109. (73.3)
110. (246.5)
111. (102.1)
112. (589.4)
113. (544.3)
114. (235.4,44)
115. (534.1)
116. (573.3)
117. (191.6)
118. (168.8,200-201)
119. (194.3)
120. (168.8,201). These authors refer to Straus’s comments in (540.3).
121. (136.9)
122. (533.6; 533.8)
123. (259.8)
124. (540.3,97)
125. Ibid.
126. (533.6,399)
127. (544.3)
128. (334.4)
129. (242.7)

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