hybrid hypothesis

The Hybrid Hypothesis

A new theory of human origins




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 pigs, 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: 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

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 pygmy chimpanzee cranium with that of a human being

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."
Nebraska Man 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."

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Thank the Pig
Not everyone will say it's true,
But pigs are creatures much like you.
We ape an ape in many ways,
Yet pig distinctions win our praise.
— Gene McCarthy

The Times, London: Hesperopithecus Dethroned, February 25th, 1928.


1. (188.7,199). See also (495.3,445a).

2. (101.85,1011)

3. (101.65,175)

4. (136.2,313 & Plate 2; 166.6, Color Fig. 3; 198.7; 220.1d,596; 506.7,309)

5. (533.6,392)

6. (101.65,173; 101.85)

7. See Chapter 6.

8. (79.3; 79.6; 183.1)

9. (198.7)

10. See Chapter 6.

11. (198.7)

12. (360.8,122). See also Chapter 6.

13. (328.68; 621.9)

14. (119.3; 621.9,362)

15. (602.7,701)

16. (507.2)

17. (80.7, 122.3,Table V)

18. (205.9; 326.15; 379.7; 379.8,266; 531.8,35-39; 589.85)

19. (117.5; 541.9; 580.4)

20. (389.4,282)

21. (610.4)

22. (610.4,399)

23. (557.7)

24. (588.89)

25. (610.4,391)

26. (610.4,390)

27. (589.7,394)

28. (495.7,Figs. 1 & 9)

29. (589.42). Translated in (495.3,447).

30. (301.5; 399.55; 588.89)

31. (589.7,Fig. 41)

32. (158.3,239)

33. (259.8,315)

34. (55.1,300)

35. (495.7,74)

36. (220.1d,322)

37. (220.1d,71)

38. (405.7,147)

39. (589.7,395)

40. (285.5,32-33)

41. (470.4,273)

42. (470.4,273)

43. (119.3; 257.8; 328.68,361a; 415.5)

44. Ibid.

45. (470.4)

46. (527.4,6-7)

47. (414.7)

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