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The Hybrid Hypothesis

A new theory of human origins




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.


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."


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.


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.


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.


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
Pig stomach (Sus scrofa)


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.


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.


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).


"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.


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).


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 kidneyhuman kidneypig kidney
Chimpanzee kidneyHuman kidneyPig 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.


"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


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).


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


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.


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


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


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.


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


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


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.


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.


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 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.


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).

Supernumerary nipples
Supernumerary (extra) nipples are fairly common in human beings (Brown and Schwartz 2002). In women, these extra nipples produce milk during lactation. The prevalence of supernumerary nipples varies, depending on the population, but in certain ethnic groups, such as Arabs and Japanese, they are present in one person in twenty (see Brown and Schwartz for prevalences in various human populations). They usually arise within the embryonic milk lines, that is, they occur along lines corresponding the the lines of nipples present in various animals such dogs, cats, and pigs. I have been unable to find any reported case of a supernumerary nipple in a non-human primate (in these animals the normal number of nipples is two). If anyone knows of such a case, please forward the relevant citation to this website. In humans with supernumerary nipples, most commonly, only one is present. But cases are known in which as many as ten are present, for example, the woman pictured in the illustration below.
extra nipples
A woman with four pairs of supernumerary nipples, breasts lifted to reveal bottommost pair (Bateson 1894, p. 183, fig. 29)

extra nipples
A man with a pair of supernumerary nipples, as well as a single, smaller supernumerary nipple (below, and in line with, the upper two on the left), five nipples in all.

"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.


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


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


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


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.


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).


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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Mammalian hybrids >>

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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.


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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