The Hybrid Hypothesis

A new theory of human origins


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The Gorilla and the Koolokamba
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The gorilla is something of a paradox on the African scene. One thinks one knows him very well. For a hundred years or more he has been killed, captured and imprisoned in zoos. His bones have been mounted in natural history museums everywhere, and he has always exerted a strong fascination upon scientists and romantics alike. He is the stereotyped monster of horror films and adventure books, and an obvious (though not perhaps strictly scientific) link with our ancestral past. Yet the fact is we know very little about gorillas.
—Alan Moorehead
No Room in the Ark

BY EUGENE M. MCCARTHY, PHD GENETICS (Continued from previous section) — This section considers the gorilla, its likely origin, and the nature of its relationship to humans and chimpanzees. It represents an effort to expand the theory of human origins presented in previous sections. After all, most biologists think the chimpanzee and gorilla are our closest living relatives.1 So the nature of the connection between humans and gorillas is directly pertinent to the subject under discussion. For many years, there was controversy over which of the two, the gorilla or the chimpanzee, is more closely related to humans. The question was which pair — chimpanzees and humans, or gorillas and humans — shared the most recent common ancestor. The difficulty was that gorillas shared certain traits with humans that chimpanzees did not and vice versa: chimpanzees had certain human characteristics that gorillas didn't. Recently scientific consensus has come down on the side of the chimpanzee, and the general belief now is that the chimpanzee is a little more closely related to the humans than is the gorilla — but only a little (and there are some primatologists who still disagree).

gorilla face
Gorilla (Gorilla gorilla)

But from the perspective of stabilization theory, which, of course, holds that new forms of life typically come into being via stabilization processes associated with hybridization, the entire notion of divergence from a common ancestor makes no sense. That point of view suggests that the connection between Homo, Gorilla, and Pan would likely be of an entirely different nature, and that their origins would in some way involve hybridization. The previous sections have presented a strong case for the idea that humans are derived from hybridization between chimpanzees and pigs. The human case, then, lends credence to the idea that the gorilla, too, might be of hybrid origin, the question examined in the first half of this section.

The first gorillas reported were western lowland gorillas. The eastern race of the gorilla was unknown until about 1900 (71.6; 158.2, 314).
In the wild, chimpanzees outnumber gorillas by at least ten to one. The total number of gorillas in all Africa is probably less than 15, 000 (602.7, 686). Pygmy chimpanzees, alone, number between 100,000 and 200,000 (602.7, 702). The even more numerous ordinary chimpanzee (P. troglodytes) is abundant over a wide geographic range, although it is in decline, or even extinct, in some localities (602.7, 706, 712-719). In captivity, chimpanzees are easily bred and maintained, whereas the breeding of gorillas is notoriously difficult and captive populations are not self-sustaining (186.6; 456.7).

With the exception of the pygmy chimpanzee (which in recent literature is now usually referred to as the "bonobo"), the gorilla was the last of the apes to be recognized, remaining unknown to European science until 1847, when its existence was first reported by Thomas Staughton Savage (1804-1880), who obtained several gorilla skulls in what is now coastal Gabon.2 During the intervening years, much has been learned about this now famous animal, but in all ways, our knowledge of Gorilla gorilla is sparse in comparison with what we know of Homo sapiens, or even with what we know about the chimpanzee. The meager supply of human fossils seems abundant when one considers that paleontological remains of the gorilla are practically nil.3 Because of its rarity, both in the wild and in captivity, its intimidating size, and its inaccessible habitat, the gorilla has not been so readily available to researchers as have the chimpanzee and most other primates.

Notwithstanding the absence of a fossil record, one can still consider the implications of reports made on the living animal in the years since its discovery. The gorilla's infertility and abnormal spermatozoa (see appendix) have not been satisfactorily explained by those who suggest that the presence of the same trait in a close relative (Homo sapiens) makes it "normal." Nor is any real elucidation provided by the claim that dysfunctional semen has "somehow" been favored by natural selection — this trait is an obvious impediment to reproduction. For my own part, I can only take the observations of infertility in humans and gorillas as evidence that their reproductive systems have somehow been disturbed. In the case of human beings, the evidence is consistent with the idea that the observed symptoms of infertility are a consequence of hybridity. Why seek a different explanation for the gorilla?

So let's consider the possibility that the gorilla came into being via hybridization and attempt to identify the two types of animals that crossed to produce it. The procedure will be the same as it was in the case of human beings. The absence of symptoms of infertility in the chimpanzee and its close affinity to the gorilla concur in recommending it for a return appearance as "parent number one."

In prehistoric times, there was in fact a primate much bigger than a gorilla. Known to paleontologists as Gigantopithecus, it stood about ten feet tall. Fossils of Gigantopithecus are, however, only from eastern Asia, far from the modern-day range of the gorilla.

The infertility of the gorilla is, if anything, more severe than that of human beings, which is consistent with the idea that the cross producing the gorilla was at least as distant as the one we have posited for human beings. This inference is seconded by another line of reasoning. The assumption that the chimpanzee crossed with some other type of animal to produce the gorilla entails the further assumption that the cross involved was a broad one: If this clause is excluded from the contract, the second parent has to be some type of primate. But then, what primate would it be? Because the chimpanzee is so much smaller than the gorilla, the second parent would almost certainly have to be very large, as large or larger than the gorilla itself. Otherwise the gorilla would not be intermediate in size between its parents, as is usually the case with a hybrid. The gorilla is far larger than any other primate. No primate can therefore be cast in the role of second parent for this cross unless we wish to assume that two little animals crossed to produce a giant.

Thus, if we still cling to hybridity as an explanation of gorilla infertility, we are compelled to search outside the primate order. Actually, we have already seen a great deal of evidence consistent with the idea that just such a cross produced the human race. And a similar approach in the case of the gorilla turns out to be informative as well.

If one grants, then, that it's reasonable to seek a nonprimate to play the role of the second parent, the next step is to compile a list of traits distinguishing the gorilla from the chimpanzee. When one compiles such a list, and considers the geographic range of the gorilla, the most likely candidate turns out to be an animal that has done a better job of escaping the eye of science than any other large land mammal on the planet: Hylochoerus meinertzhageni, the giant forest hog (see photos below).

Hylochoerus meinertzhageni
Hylochoerus meinertzhageni, the giant forest hog Image: Michelle Zappa, Wikipedia

Hylochoerus meinertzhageni
A sounder of giant forest hogs
Image: S. Fimpa

The reclusive habits and inaccessible habitat of H. meinertzhageni concealed it from science until the early 20th century (505.8; 567.8). Fossils indicate that this pig was already in existence in the late Pliocene or early Pleistocene (563.1; 563.3). Even today, it is far less studied than the gorilla itself.
gorilla face
Gorilla's distribution

The western range or the gorilla includes virtually all of Gabon, much of Cameroon, Congo, Equatorial Guinea, and Cabinda, and small portions of Zaire and Central African Republic).
The eastern range or the gorilla also extends over the border into Rwanda, Burundi, and Uganda (602.7). In the early part of this century, the eastern gorilla's range extended further to the north than it does today, at least as far as the Uele River in northern Zaire (223.07). Four gorillas were collected in Bondo, Zaire just north of the Uele in 1927 (140.1, 28; 223.07).
As will be shown, characteristics that distinguish the gorilla from the chimpanzee can be found consistently in Hylochoerus — at least to the extent that they can be verified in such a poorly researched animal. In short, then, the hypothesis under consideration becomes that not only humans are derived from hybridization between chimpanzees and a pig (S. scrofa), but also that gorillas are derived from crossing of the chimpanzee with a different type of pig (H. meinertzhageni).

Certainly, the forest pig is big enough to account for the huge size of the gorilla. Hylochoerus really is a giant. An average weight of 230 kg (507 lb) makes it the largest type of pig now living, which compares with 153 kg (337 lb) for an average male gorilla.4

Even today, the gorilla is found only in geographic regions where the range of the chimpanzee and H. meinertzhageni overlap. Two gorilla populations exist in Africa, one on the west coast (bounded by the Cross, Congo, and Oubangui Rivers5), and a smaller population, located about 600 miles away in eastern Zaire.6 (See map at right) The common chimpanzee (Pan troglodytes) has a far larger range that stretches from Senegal to Tanzania, while the pygmy chimpanzee (P. paniscus) is confined to a southern enclave defined by the Congo, Zaire, and Lualaba rivers (see map below). The two gorilla ranges lie entirely within the range of P. troglodytes, and wherever the gorilla is found, the forest hog is found also. The gorilla and giant forest hog frequent the same habitats and have similar diets.7

map showing chimpanzee's distribution
Chimpanzee's distribution in Africa (click to enlarge)


In fact, in the gorilla's habitat, there really is no other large animal that might by any stretch of the imagination mate with a chimpanzee. In Africa, most large mammals are confined to the open grasslands. Few such mammals, other than the chimpanzee, are present in the dense thickets and rain forests frequented by the gorilla. Elephants, hippopotami, leopards, okapis, an occasional buffalo, and the giant forest hog are about the only animals on the list.8

I have only seen photographs and drawings of the forest hog, but when viewed from certain angles, the living animal must bear some resemblance to a gorilla: Dian Fossey, author of Gorillas in the Mist, actually mistook one for a gorilla at close range in full daylight (perhaps its head was turned or obscured by brush). She sat about a hundred feet away, using binoculars to observe a large, stationary animal she took to be a gorilla. Not having seen the animal move in quite some time, she decided to investigate. "I awoke my guide and asked him to remain where he was so that I could creep closer to the sunbathing animal," writes Fossey, "I'll never forget the chagrin felt upon realizing that the 'gorilla' I had been observing for over an hour was actually a giant forest hog (Hylochoerus meinertzhageni)."9 Fossey had "near daily encounters" with the giant forest hog while she was out observing mountain gorillas.10 Other gorilla observers frequently mention accidental encounters with Hylochoerus in the same habitat.11 Traps set for the forest hog often capture and kill gorillas instead.12

So, in this animal, we have a creature that lives in the same places, and bears some resemblance to, the gorilla, even eating similar foods. Moreover, morphological features distinguishing the gorilla from the chimpanzee can be consistently accounted for in the anatomy of the forest pig (or at least that of pigs in general in those cases where specific information for Hylochoerus is unavailable).

As in the case of human beings, I have compiled a list of traits distinguishing the gorilla and the chimpanzee. I have included as many such features as I have been able to identify. This list is necessarily shorter than the one for humans. The amount of information available for the gorilla is minuscule in comparison with what is known about humans.

In considering this list of traits, it is important to keep one fact in mind: if gorillas and humans are both derived from hybridization involving pigs, they would have some pig traits in common, but in other respects we would expect them they to differ. Homo and Gorilla, then, would be more similar to each other, with respect to certain porcine traits, than to chimpanzees. But not every porcine trait present in the one would be present in the other. So even if the gorilla is a hybrid of a type akin to that posited for humans (pig x chimpanzee), and thus had certain traits in common with pigs, there is still no reason to expect it to have the same set of porcine traits as humans.

In particular, it is not necessarily expected that gorillas should have large brains if they are hybrids of a type of pig and chimpanzee. Gorillas lack key traits that seem to have made brain expansion possible in human beings: They have poorly vascularized skin (except on the palms, soles, and knuckle pads).13 Even if the skin were suited to heat dissipation, it would be to little avail — gorillas have relatively few emissary veins (the number of emissary foramina in a gorilla skull is comparable to the number in a chimpanzee's), which precludes the possibility of cooling the brain in the way that humans do. It seems that this crucial pig trait is one that they happened not to inherit.

Now let's consider a brief list of traits distinguishing gorillas (and Hylochoerus) from chimpanzees.

Traits Distinguishing Gorillas from Chimpanzees

Sub-orbital warts. In Hylochoerus, says Ewer, "the sub-orbital wart is enormously hypertrophied [i.e., extremely large]. The bone of the zygoma [i.e., cheek bone] is roughened and thickened and provides a firm anchorage for it."14 These gigantic swellings are visible beneath the eyes of the pigs in the photos above. Unlike other primates, gorillas sometimes also develop huge, suborbital growths.15 They arise on one or both cheeks. In gorillas, as in giant forest hogs, the bone affected is the zygomatic arch. No one knows why these bizarre structures are seen in the occasional gorilla. A relationship to the forest hog, however, would explain this mystery.

Sagittal crest. Mature male gorillas have a bony sagittal crest running down the center of the upper rear portion of skull, a feature that is uncharacteristic of chimpanzees. Mature Hylochoerus boars, however, are distinguished from other types of pigs by the presence of a ridge of bone down the center of the supraoccipital.16

Brow ridges. Brow ridges are more pronounced in gorillas than in chimpanzees, particularly in mature males (see gorilla photo above). According to Ewer, the bone over the eyes "thickens" in old Hylochoerus males.17

Jaw. The gorilla's jaw is massive in comparison with a chimpanzee's, a fact that would be fully accounted for by a connection with the giant forest pig.18

Eyes. As can be seen in the photos above, giant forest pigs have darkly pigmented eyes like those of gorillas and chimpanzees, not the light colored eyes of Sus scrofa and many humans.

Rhinarium. The gorilla's nostrils are bordered by a thick pad of tissue not present in the chimpanzee.19 The nose of the giant forest hog, too, bears a thick tissue pad (rhinarium) around the nostrils (see photos above).20 The nose of the forest hog is unsuited for rooting because the nasal septum is composed of fused bone, instead of being cartilaginous (like that of Sus scrofa and humans).21 The gorilla also lacks a cartilaginous nose.22

In human beings the lobes of the liver are "five in number -- the right lobe, the lobus quadratus, the lobus Spigelii, and the lobus caudatus, the last three being merely parts of the right lobe. The right lobe is much larger than the left; the proportion between them being six to one" Gray (220.1d, 936). "In the chimpanzee the liver resembles that in man" Sonntag (533.8, 225). Elftman and Atkinson (168.8, 199) note that "the most striking point of contrast between the liver of the gorilla and that of man [and that of the chimpanzee] is the presence of more extensive lobation. On the fundamental division into right and left lobes ["of nearly equal size" (533.8, 225)] is superimposed a further division on each side. This results in the recognition of a right central [i.e. medial] and right lateral lobe, with similar lobes on the left." The left side of the gorilla's liver, then, although still slightly smaller than the right, is proportionately much larger than in humans and the other apes. A pig's liver has "four principal lobes -- right lateral, right medial, left medial and left lateral; the last of these is usually considerably the largest" (196.2, 1280). The smaller accessory lobes of the right human lobe (Spigelian, caudate and quadrate) are present in both chimpanzees and gorillas, but the pig lacks the Spigelian.
Sus and Homo chew with a grinding motion, but gorillas and forest hogs chew with a chopping action of the jaw (180.5, 151-152; 545.6, 495).

Liver. The gorilla's liver is unusual for a primate. Elftman and Atkinson (1950) observe that

Although the liver is one of the few viscera of the gorilla which have been described in a sufficient number of individuals so that we have some assurance that we are not unduly influenced by chance variation, we are still left in the quandary so well first expressed by Flower (1872) after the examination of one specimen: "The abnormal form of the liver of the gorilla is one of the most interesting facts that has been brought to light by recent researches into this subject. It shows either that the modifications of the liver are not very characteristic in natural and related groups of animals, or that the gorilla ought not to occupy that position in the system which has hitherto been assigned to it."23

Specifically, "the liver of the gorilla differs from those of the other large anthropoids and of man in the more marked tendency to subdivision of right and left lobes, a character which assigns a comparatively lowly place to the gorilla in a comparison of the higher primates based on the anatomy of the liver" (Duckworth24). Descriptions of the gorilla liver sound suspiciously like those given of this organ for the domestic pig.

Canines. Adult male gorillas have enormously developed canine teeth.25 The giant forest hog has huge canines, far bigger even than a gorilla's.26

Molars. Gorilla molars are larger than those of chimpanzees, are longer from front to back, bear more prominent cusps, and have deep grooves on their sides.27 These characteristics are also seen in Hylochoerus.28

Femur. The section on human bipedality discussed several attributes linking the human thigh bone with that of pigs. These traits are all absent in the gorilla. One way in which a gorilla's femur is similar to a pig's, however, is in its overall robustness. That is, it is extremely thick in proportion to its length.32

Sexual dimorphism. The average chimpanzee female (P. troglodytes) is nine-tenths as large as a male.33 Gorilla females are only about half the size of males.34 This distinction of the gorilla is readily accounted for by reference to the forest hog; Hylochoerus shows more sexual dimorphism in body size than any other type of pig.35

Diet. The major component of the diet in both the gorilla and the forest hog is herbaceous vegetation.36 For example, both eat bamboo and vines.37 In the case of Hylochoerus, even grass can be consumed in quantity.38 The chimpanzee diet is more generalized than that of the gorilla. Dixson points out that "whereas gorillas favour secondary forests and feed mostly at, or near, ground level on leaves, pith, shoots, roots, etc., chimpanzees prefer primary forests where they climb a great deal and feed principally upon fruits."39 Chimpanzees supplement their diet with a variety of foods including leaves, insects, small animals, and eggs.

Social structure. Unlike chimpanzees, gorillas travel in large groups led by a big silverback male.40 Walker notes that the giant forest hog also goes about in sounders composed "of up to 20 individuals," and that they "are led by an old male."41 (See photo of a forest pig sounder above.)

Pigmentation. The gorilla is darkly pigmented throughout life. Even the youngest specimens are as dark as adults.42 Chimpanzee skin is usually much lighter than that of the gorilla, particularly in juvenile specimens. The skin of Hylochoerus is also dark.43

Hybrid Traits

Under the hypothesis that the gorilla is a hybrid, it is possible to attribute some traits to the mere fact of its hybridity:

Abnormal spermatozoa

Testicular atrophy

Low sperm density (41 × 106/ml),44 about one-sixtieth that seen in chimpanzees (i.e., 2561 × 106/ml).45

Low mtDNA variability in conjunction with high nuclear DNA variability.

High levels of morphological and biochemical variability.

Porcine Characters Shared by Humans and Gorillas

Nasal bones. The gorilla has longer nasal bones than the other apes, as do human beings.46 As a result, the upper portion of its nose tends to form a somewhat elevated bridge.47 Long nasal bones are also characteristic of Hylochoerus.48

Short upper lip. A gorilla's upper lip is short in comparison with that of a chimpanzee.49 As in other pigs, the upper lip is short in Hylochoerus.50

Hands and feet. In shape of hands and feet, the gorilla, and not the chimpanzee, is most similar to human beings.

As in humans, the gorilla palm is wide in proportion to its length.51 "Man and gorilla are distinguished by the shortest hands of all higher primates" (Schultz52). They also differ from the chimpanzee in having shorter hands than feet.53

Although human toes and fingers are proportionately the shortest, the gorilla comes in a close second.54

Although still opposable, the big toe is, as in humans, more extensively connected with the main body of the foot.55 The other toes also tend to be webbed together to a much greater degree (and are much shorter) than in the chimpanzee. The thumb and great toe are about the same size in proportion to body size (trunk length) in chimpanzee, gorilla, and human being.55b It is only the other digits that are markedly shorter in gorillas and human beings.55c

According to Keith, "The projection [of the heel bone behind the astragalus] in the chimpanzee is always less than 1.5 cm.; it is never less than 3.5 cm. in the gorilla" (267.8, 304). See also (495.65, 165, Fig. 7-28). Schultz (495.65, 163) notes that the tarsus in human and gorilla is practically equal in size in relation to their trunk lengths." The ratio of tarsus to metatarsus (or of tarsus to the overall length of the foot) in these two species is higher in these two species than in any other primates (Ibid).
The crural index (tibia length in percentage of femur length) does not vary significantly among humans and apes (495.65, 130).

While human beings have the longest heel bone in proportion to foot length of any primate, the gorilla has the second longest.

Brachial index. Forearm length as percentage of upper arm length is about 80 percent in both humans and gorillas, the smallest value among catarrhine primates. The humerus is also longer than the radius in pigs.

Terrestrialism. Gorillas, particularly adult gorillas, are almost exclusively terrestrial, a trait shared with human beings and pigs, but with few other primates.

Malleolus medialis. This downward directed spike present on the lower end of the tibia in other nonhuman primates is absent in the gorilla,58 as it is in humans and pigs.

Melanocytes. Ellis et al. note that "In spite of the heavy pigmentation of the [gorilla] epidermis, there are no dermal melanocytes."59 This trait links gorillas with humans and pigs to the exclusion of chimpanzees and monkeys (see the previous page). As Montagna notes, "Dermal pigment occurs normally in most primates."60 So its absence from the dermis in the heavily pigmented skin of the gorilla is unusual.

Hymen. Raven notes the presence of a well-defined hymen in the gorilla.61 This structure is not found in chimpanzee nor, apparently, in other nonhuman primates.62 I have not found descriptions of the genitals of the forest hog, but in the ordinary pig (and various other ungulates) a hymen is present in the virginal animal.63

Labia. "In the gorilla the labia majora are distinct but small, the labia minora absent" (Raven64). This configuration is the opposite of that found in other nonhuman primates, but the same as that found in domestic pigs (see the previous page).

Sexual swellings. Neither human beings nor gorillas have the conspicuous sexual swellings seen in the females of most other nonhuman primates, including the chimpanzee.65 These swellings are not characteristic of pigs.

Menstrual cycle. Chimpanzees have a cycle length of 36-37 days; the human cycle is about 28 days long; that of pigs, 21 days. A gorilla's is also shorter than a chimpanzee's (31-32 days66).

The gorilla, then, shares quite a few traits with humans and pigs, traits not seen in chimpanzees, and this commonality is explained by the hypothesis that both humans and gorillas are derived from hybridization between chimpanzees and pigs, albeit to quite different types of pigs. Other traits, however, link gorillas, not with humans, but with chimpanzees and other nonhuman primates. Characteristics in this second category have all been mentioned in previous sections, but they can be summarized briefly here:


Nonhuman Traits Seen in Both the Gorilla and the Chimpanzee:

Gorillas and chimpanzees are linked by: a thin epidermis, a high intermembral index, a long pelvis with a large birth canal, a short coccyx, poorly vascularized skin containing few elastic fibers, small brains, dark eye pigmentation, non-circular prostate, fear of water, brief copulation, and identical chromosome counts.

Both gorillas and chimpanzees lack numerous human features which have already been covered in previous sections: a panniculus adiposis, a gap between pelvis and rib cage, a lumbo-sacral promontory, a centralized spine, a projecting cartilaginous nose, a philtrum, a chin, a diverticulum in the stomach, a styloid process, knock knees, epidermal patterning (external and internal), heavy gluteal muscles, seventh cervical vertebra with prominent spine and transverse foramens, ear lobes, thermoregulatory sweating, bulbo-urethral glands, ungual tuberosities, glabrous lips, valvulae conniventes, multipyramidal kidneys, bunodont molars, nocturnal activity, and female orgasm, as well as susceptibility to flea infestation, sunburn, alcoholism, melanoma, atherosclerosis, and heart attack.

Both have traits that humans and Sus lack: a panniculus carnosus, a hairy pelt, knuckle pads, brow ridges, non-divergent eyes, long dorsal spines on cervical and sacral vertebrae, various features of the femur distinguishing chimpanzees from humans (circular condyles of unequal size, shallow intercondylar notch, shallow patellar groove with high medial lip), circular lateral menisci in the knees, laryngeal sacs, and a baculum.




Hybridization between the Gorilla and Chimpanzee

Continuing hybridization of the gorilla and chimpanzee would have an influence on the chimpanzee population itself even though it is far larger than the gorilla population (a large population tends to be less affected by hybridization than a small one). The effect would be limited to chimpanzees north of the Congo-Zaire-Lualaba river barrier. The common chimpanzee (Pan troglodytes), the gorilla, and the giant forest hog occur only north of this barrier. Therefore, the pygmy chimpanzee (P. paniscus) which occurs only to the south of it would be protected from any genetic interaction with the gorilla.

The common chimpanzee produces abnormal spermatozoa at a low frequency (0.2%). The pygmy chimpanzee produces none.
Zihlman and Cramer compared 20 skeletal measurements in Pan troglodytes and P. paniscus (621.9, 89, Table 1). The standard deviation of every measurement was higher in the common chimpanzee.

Pan paniscus (bonobo or pygmy chimpanzee) is, in fact, much less variable than P. troglodytes, which also fits with the idea that the former has been protected from hybridization (populations affected by hybridization are generally more variable than those that are not). The pygmy chimpanzee is treated as a monotypic species (there are no recognized subgroups or races).67 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 variety of less obvious characteristics, ranging from sperm morphology,68 to skeletal measurements, to cranial features,69 to blood groups and other genetic markers.70

Even if we eventually find that humans and gorillas are not hybrids after all, evidence that the gorilla and chimpanzee hybridize in nature would still be interesting under traditional hypotheses: any such interbreeding would do much to explain why it has been so difficult to determine which of these two apes is more closely related to human beings.

But, beyond the fact that P. troglodytes is more variable, there is the interesting fact that members of the northern (common) chimpanzee population tend to be more like gorillas. Beyond the fact that pygmy chimpanzees are smaller than common chimpanzees (85 percent as big71), various other traits characteristic only of the common chimpanzee, such as large canines and robust jaws, seem to reflect a gorilline influence. Traits that are never seen in pygmy chimpanzees72 (e.g., gorilla-like noses, sagittal crests) are reported for Pan troglodytes in rare instances73 (from areas where the gorilla is also present). Most telling, however, is the fact that numerous researchers have reported wild-caught apes of indeterminate status, morphologically intermediate between gorilla and chimpanzee. These reports strongly speak for the existence of ongoing hybridization between gorillas and chimpanzees at the present day. In contrast, no intermediates are known between humans and chimpanzees. As Dobzhansky et al.74 point out, "no individual has ever been seen about which one would have the slightest hesitation as to whether it belongs to the species humankind or the species chimpanzee." Reports relevant to these intermediate apes are so numerous and have persisted for so many years that I would be surprised if a genetic survey of the pertinent geographic regions failed to confirm what available reports already strongly suggest. In the remaining pages of this section these reports have been condensed into a history — that of an intermediate ape, the koolokamba.

The Koolokamba — A Probable Natural Hybrid of Gorilla and Chimpanzee

The accepted view of science is that gorillas and chimpzanzees cannot produce hybrids together. And yet, for more than a century, naturalists have been aware that the indigenous peoples of equatorial Africa claim that they do. The assertion that they cannot cross is somewhat strange, since there is no known anatomical or physiological impediment. They have the same chromosome counts (2n = 48), and indeed, in the author’s experience, many other organisms that differ far more at the genetic level are in fact able to produce offspring together. One of the foremost modern authorities on the gorilla, A. F. Dixson (1981, p. 19), states that "a cross between the two might be feasible in captivity, if artificial insemination were used, but I have not heard of such an experiment."

But beyond these questions concerning the possibility of hybridization in captivity, the fact of most interest in connection with this cross is that there is a great deal of hard evidence that natural hybridization does occur, much more evidence, in fact, than is available for many other mammalian crosses where the occurrence of hybridization is well accepted. It does seem as if the nearness of relation of these two apes to human beings has resulted in a bias on this point that has led the scientific community to overlook facts that would be absolutely convincing in the context of other crosses of a more mundane nature.

In particular, there have been many reports of individual "chimpanzees" with gorilla-like traits. These are individuals with traits that are in between those of chimpanzees and gorillas. In some cases the intermediacy of these specimens has been such that expert primatologists have had trouble classifying them definitely as either chimpanzees or gorillas. Some naturalists have interpreted these chimpanzee-gorilla intermediates as hybrids, while others have claimed they are merely chimpanzee "variants" (that is, ordinary chimpanzees that are gorilla-like, but that nevertheless fall within the ordinary range of variation for chimpanzees).

Those who study natural hybridization know that it is common for disputes of this sort to arise when specimens of an intermediate nature ("intermediates" as they are often called) are found in a natural setting. It is in fact typical in such cases, for some experts to say the specimens are hybrids, and for others to say they are variants. However, the standard method of resolving such debates is to determine where the intermediate specimens are coming from. Locality is key: If the question is examined geographically and the specimens have all been found in areas where the two putative parents come into potential breeding contact, then it makes sense to conclude that the specimens are hybrids. For if the unusual individuals were variants of a given type of organism A, and not hybrids arising from matings between organism A and organism B, then they would be expected to occur at random throughout the range of A, and not just in that part of A’s range where A comes into contact with B. This reasoning is straightforward.

But to make a discrimination of this kind, it is necessary to look at the origins of the intermediate specimens, preferably all the intermediate specimens — which can be a lot of work. But if they all come from regions where A comes into contact with B, it becomes obvious that they are hybrids. On the other hand, if it is found that they occur at random within the range of A, the conclusion is that they are variants.

We will apply this same method to the case currently under consideration, that of the chimpanzee and the gorilla. Doing so requires an examination of all available reports of such animals ("gorilla-like chimpanzees," as they are usually described), in order to determine the geographic locales where each was found. A detailed examination of this question follows below, but suffice it to say here that the finding of a detailed investigation of this matter indicates that all such intermediate animals have, in fact, come from areas where both gorillas and chimpanzees occur. This fact alone, in the case of any other cross, would constitute conclusive evidence that hybridization does occur. Therefore, it does also in the case of gorillas and chimpanzees. But here are the relevant facts, documented and laid out in the form of a history of what has been reported about chimpanzee-gorilla intermediates:

Reports of Chimpanzee-Gorilla Intermediates: Only five years after the discovery of the gorilla itself, Franquet (1852) reported that the indigenous peoples of the Gabon (the region of western equatorial Africa adjacent to the Gabon River) believed that gorillas cross with chimpanzees to produce a hybrid ape of intermediate form. A few years later, explorer Paul Du Chaillu (1861a) shot an ape and sent its skull to the British Museum from the Gabon — a region where both chimpanzees and gorillas were present. This male specimen was intermediate in character between the skulls of ordinary gorillas and chimpanzees. It was "smaller than the adult male gorilla and stouter than the female gorilla" (Du Chaillu 1861b, p. 408). Du Chaillu suggested that it should be treated as a separate species of chimpanzee, to which he gave the scientific name Troglodytes kooloo-kamba, an epithet derived from one of the local dialects. Koolokamba is the name used since then in most of the papers dealing with this creature (a variety of spellings appear in the literature (kooloo-kamba, kulu kampa, kouloukamba, etc., but the most common is koolokamba.

The names given this animal in the languages of equatorial Africa, confirm that the locals think it is a hybrid. As Cousins (1980, p. 91) notes, "The local names given by the indigenes vary, but all appear to agree on the basic meaning of the description: chimpanzee-gorilla or gorilla-chimpanzee." These aliases occasionally sneak into the scientific literature. Koula-nguia is the term used by Raingeard (1938), but he obviously refers to the same animal: "In addition to the gorilla and chimpanzee, the indigenous peoples of the Gabon distinguish a third ape with habits very different from those of the other two. Chimpanzee-gorilla is the name given it in the various dialects" (Raingeard 1938, p. 81) — Koula-nguia, means "chimpanzee-gorilla" in the Akele language (ibid). The Fang people of equatorial Guinea use the term N’gui-Moun which means gorilla-chimpanzee (Cousins 1980, p. 88). Strictly speaking, however, this ape’s name is not derived in every dialect from the words for chimpanzee and gorilla. Good (1947, p. 45) writes that

The Bulu people of southern Cameroon strongly claim that there is a distinct animal, a cross between the gorilla and the chimpanzee. They have names for all the principle animals of the country, and this has a distinct name. The gorilla is called nji or ngi, the chimpanzee, woo, but the hybrid they call ebot. They describe it as usually very large, combining the characteristics of the gorilla and the chimpanzee, but claim to be able to distinguish it from either.
mafuka
Mafuka

According to Keith (1899, p. 296), the earliest such intermediate animal to be dissected was a specimen described under the name Troglodytes aubryi by Gratiolet and Alix (1866). During the ensuing half century, several of these peculiar apes made their appearance in Europe. The best known was a female ape named "Mafuka," residing in the Dresden Zoo (see figure right). In 1877, primatologist A. B. Meyer (1877) asserted that Mafuka was a hybrid of chimpanzee and gorilla. This paper "initiated a storm of controversy that" ape expert W. C. O. Hill (1969, pp. 23-24) described as "not yet been finally settled."

Six years after Meyer’s assertion, another expert on the anthropoid apes, Robert Hartmann (1886, pp. 215-219), discussed the Mafuka dispute: "When I first saw this savage creature, early in September, 1875, it was full of vigour, and I was almost convinced that I saw a female gorilla, not quite an adult, an opinion shared by such zoologists as K. Th. von Siebold and others, while it is vehemently opposed by Bolau [1876] and A. B. Meyer [1877]." Hartmann goes on (p. 220) to express his own doubts about Mafuka’s classification. In the end, he wrote: "For me and many other naturalists Mafuka remains up to this time an enigma, which is slurred over by others with a few phrases." At one point (pp. 218-219), he admits having leaned toward the idea that she was a hybrid:

To what species, then, did Mafuka belong? A cross between the gorilla and the chimpanzee was often suggested at the time. I was myself inclined to take this view, and it was advocated by C. Vogt in his contemporary treatise on the subject, as well as in [another of Vogt’s works (see Vogt 1882, p. 11)]…Von Koppenfels heard much of such crossings while he was on the Ogowe [i.e., the Gabon River], nor is their occurrence by any means impossible, and indeed they have been directly observed among other species of apes while in confinement. Koppenfels also affirmed that he had shot two such cross-bred animals, which were associating with a troop of gorillas…The skins and skeletons of the supposed cross-breeds were brought to the Natural History Museum in Dresden. A. B. Meyer [the same who had pronounced Mafuka a hybrid] observed that the traveller [von Koppenfels] was mistaken in these instances, and that the remains sent by him to Europe were undoubtedly those of chimpanzees [see Meyer 1881]. It must be remembered that Koppenfels was a clever hunter, and on the whole a good observer of nature, but that he was no zoologist, and may have been mistaken as to the nature of the animals he had shot. At the same time, the possibility of the existence of such cross-bred animals cannot by any means be disputed…If the trophies of von Koppenfels’ hunting are merely chimpanzees, it is, at any rate, very interesting to learn that these animals were found in the company of gorillas.

Before going on with this story, it’s worth mentioning that J. C. G. Allen, in his Gorilla hunting in southern Nigeria, also reported mixed groups of wild chimpanzees and gorillas feeding and playing together. Gorilla expert A. F. Dixson (1981, p. 19) also says, "that kooloo-kambas have been observed in normal groups of chimpanzees, which suggests a cross between a male gorilla, perhaps a lone silverback, and a female chimpanzee." According to Meyer (1881), this was the direction pictured for the cross by von Koppenfels, who thought the careful eye that male gorillas keep over their harems is too vigilant for a male chimpanzee too slip in and carry out the required insemination. Meyer himself, however, felt that a cross between an old chimpanzee male and young gorilla female would be more plausible because the size difference would be minimized. And it can be pointed out, too, that the time from intromission to ejaculation is very short for a male chimpanzee — on the order of ten seconds.

Be that as it may, nearly a century later Groves (1970a, 19) continued the story of von Koppenfels’ "hybrids" (or of Meyer’s "chimpanzees"):

In 1877 the French anatomists Alix and Bouvier obtained a female gorilla from the Loango Coast ([obtained at] Brazzaville, Congo [but shot on the Kuilu River at 4°35’S, 12°35’E]) and described it as a new species which they called Gorilla mayêma after a local chieftain. In 1913 Daniel Giraud Elliot (1913) of the American Museum of Natural History discovered, in the museum at Frankfurt, three more specimens: a male, a female and young from a single troop, shot at Fernan Vaz on the Gabon coast a long way north of Loango. After studying them, especially the male, he concluded that the species mayêma differed so much from both gorilla and chimpanzee that it should be put in a genus of its own, Pseudogorilla.

Elliot (1913, p. 224) compared his proposed "Pseudogorilla" with the other African apes as follows:

The remarkable species of this genus exhibits affinities to both the gorilla and chimpanzee, possessing characters belonging to each. In size it is nearest the chimpanzee but with a distribution of the hairy covering more like the gorilla. The braincase with the prominent forehead and absence of crests is very like that of the chimpanzee, but the rest of the cranium is gorilla. The side lateral and occipital flat expansion, shown in a most limited degree if at all in the chimpanzee, the lengthened narrow rostrum, like the gorilla and contrasting strongly with the broad short rostrum of the species of Pan; the root of the zygomatic arch reaching only to the first molar as in the chimpanzee; the narrow palatal floor, and the rounded lower horizontal line of the mandible like gorilla and not straight as in chimpanzee, all exhibit an animal which, while possessing characters found in one or other of its near relatives in Pongiidae, has combined them in so extraordinary a manner as to entitle it to be separated from both and regarded as representing a distinct generic form connecting the other two genera.

But Groves (1970a, p. 19) dismissed the idea that these animals were hybrids or that they perhaps deserved treatment as a distinct species:

The specimen described by Alix and Bouvier is in the Paris museum: it is a female gorilla, rather a small specimen it is true, but within the expected size variation. Of the specimens in Frankfurt on which Elliot placed so much reliance, the male is a young animal [i.e., young gorilla] and the female is a perfectly ordinary female gorilla, not even of particularly small size.

Grove’s use of the word young here is hard to understand, since Elliot (1913, p. 225) says the middle of this male’s back, its flanks and the hinder parts of its thighs were iron gray. Grey hair on the back is the mark of maturity in male gorillas. Gorilla troops are usually led by a mature "silverback," as such individuals are called. Dixson (1981, p. 31), a leading expert on gorillas, says that as silverbacks age "the grey areas spread to include the flanks and buttocks," the same regions where Elliot’s specimen was gray.

Moreover, because the area was being colonized by the French during the latter half of the 19th century, von Koppenfels seems to have been the only German collector in that entire region at the time in question.

The specimens "discovered" by Elliot in 1913 are apparently the very same "cross-breeds" sent to A. B. Meyer some thirty years earlier by von Koppenfels. Elliot’s specimens had been shot on the Fernan Vaz, an estuary on the Gabon coast (where both chimpanzees and gorillas exist) while in association with a troop of gorillas, and they were stored in a German museum. Von Koppenfels had 1) operated out of "Eliva Comi on the Fernan Vaz" (Meyer 1881, 231); 2) had shot his specimens in a troop of gorillas; and 3) sent them to Germany. If Elliot’s specimens were in fact the ones sent to Meyer by von Koppenfels, four different judges assigned them four different identities: 1) hybrids (von Koppenfels); 2) ordinary chimpanzees (Meyer); 3) "pseudogorillas" (Elliot); 4) ordinary gorillas (Groves).

Other intermediate apes continued to meander into scientific journals over the years. In 1898 we find W. L. H. Duckworth (1898, p. 989) at Cambridge University puzzling over a preserved carcass he had just received from Africa:

The specimen under consideration, which is an aged female, was shipped to this country from the Gaboon River [a region where both chimpanzees and gorillas occur], West Coast of Africa. In placing on record the results of the dissection of this anthropomorphous ape, I am met with the difficulty of being unable to refer to it with confidence as either a true chimpanzee (Anthropopithecus troglodytes) or a genuine gorilla (A. gorilla) … On its arrival the specimen was thought to be a female gorilla, the principal reasons, so far as I can ascertain, for the opinion being the facts of its great bulk and the dark color of the face and extremities. But from the first time that I saw it, I have had misgivings about the correctness of this view, and these up to a certain point have been strengthened by further observations.

In his article he compares his specimen to several other apes (some of these also of indeterminate status), and points out that his ape has many chimpanzee features. But, ultimately, although he rejects an identification of the specimen as an ordinary gorilla, he also says (Duckworth 1898, p. 994),

it can hardly be described as an ordinary specimen of Anthropopithecus troglodytes; and I am inclined to think, in the absence of contradictory evidence, that we have here a specimen of Du Chaillu’s Kooloo-Kamba. Its great size gives it some claim to an intermediate position between A. troglodytes [i.e., chimpanzee] and A. gorilla [gorilla].

One of the other intermediate apes mentioned by Duckworth (1898, p. 994), "Johanna" of the Barnum and Bailey Circus, is discussed at length by primate anatomist Arthur Keith. While Johanna’s owners claimed she was a gorilla, Keith (1899, p. 296) claimed that she was a chimpanzee — or rather, a koolokamba:

"Johanna" shares all of the features of "Mafuka"; she answers to the description given by Du Chaillu of the species he names "Troglodytes kooloo-kamba." The animal dissected and described by Gratiolet and Alix [1866] under the name of T. aubryi [mentioned above] was also of the same variety of chimpanzee which approaches the gorilla in so many points that it is evident that the characters which separate the two African anthropoids are not so well marked as many suppose.
Raingeard’s (1938) specimens were both males. Merfield (337.5, 52), who collected apes in Lower Guinea for export to zoos, encountered "a number" of male animals with traits that were transitional between chimpanzees and gorillas. He referred to these animals as "chogas." He says that "Like gorillas they have prominent eye-brow ridges, and they are coal black all over; male chogas have small, gorilla-like [sagittal] crests and the same smell as gorillas, though it is much less powerful...Unlike gorillas, female chimpanzees and chogas have conspicuous menstrual swellings. Another male koolokamba is mentioned by Garner (1896). Also the koolokamba specimen donated to the British Museum by Du Chaillu was a male (157.5, 270, 360; 509.2, 5). Frechkop and Marit reported 11 intermediate males (188.55). Censuses of wild gorilla populations (140.1, 103) indicate that their social groups contain twice as many adult females as adult males (36.7% vs. 18.9%).

In the 1920’s Charles Sonntag was one of the world’s foremost authorities on chimpanzee morphology (Sonntag 1923). In 1924, he brought together in a single volume what was then known about primate anatomy (The Morphology and Evolution of the Apes and Man). In it he lists many features by which the African apes can be distinguished from each other, but he includes a caveat (p. 86): "It occasionally happens, however, that it is difficult or even impossible to tell whether a particular animal is a gorilla or a chimpanzee…[and] when a difficulty arises, it is in connection with the females." This assertion was echoed 56 years later by Cousins (1980, p. 89): "It is a strange coincidence that so far all captive chimpanzees described as Koolookambas have been females." It does seem that females predominate in the literature, though a few males have been reported. However, this predominance is not at all surprising if these animals are interpreted as hybrids, in fact it constitutes a strong corroboration of the hypothesis: According to Haldane’s Rule, when in the offspring of a cross between two mammals one sex is absent or rare, it is expected to be the male. It is only if we interpret them as ordinary chimpanzees that it becomes difficult to see why nearly all of the many reported intermediate specimens should be female. Under that hypothesis we would expect the male-female ratio to be near 50-50.

Nevertheless, Ernst Schwarz (1934a) not only denied that koolokambas are hybrids, but even refused to admit that they exist as entities distinct from the chimpanzee. He argued against the idea that specimens described as koolokambas should prompt the erection of a separate species (or subspecies), and insisted they were mere variants of the common chimpanzee. When two more possible chimpanzee-gorilla hybrids surfaced in the literature four years later (Raingeard 1938), Schwarz (1939) didn’t change his opinion. He promptly pounced on them, too, saying he had "no doubt that the ‘koula-nguia’ was only a black-faced chimpanzee."

In saying that Raingeard’s specimens were "only" black-faced chimpanzees, Schwarz implies they were ordinary chimpanzees, but this conclusion does not follow: In Cameroon, many apes passed through the hands of veteran collector Fred Merfield (1956, pp. 52-53) on their way to zoos overseas. One of them was named Bo-Bo.

Bo-bo was a black-faced chimpanzee, or choga [choga may be an anglicization of some indigenous name resembling n’tchego used by Franquet (1879) to refer to the koolokamba], a rare variety about which I was given from time to time a great deal of intriguing but inaccurate information. Chogas were alleged to be very dangerous, since they had the strength of gorillas and the cunning of chimpanzees. In the Batouri district [i.e., a region of eastern Cameroon adjacent to Central African Republic ] they are called N’Killingi which means ‘gorilla’s brother’. After collecting and studying a large number of gorillas, chimpanzees, and chogas, I came to quite different conclusions. I once discovered a female choga that made repeated and unprovoked attacks on village children, but generally they are no more dangerous than common chimpanzees; on the other hand, they differ from all other chimpanzees and resemble gorillas in a number of interesting ways… In some parts of the Batouri district chogas and common chimpanzees share the same habitat [as do, also, gorillas (Wolfheim 1983, p. 693)], so it is quite likely that they interbreed and that this might account for the considerable individual variation in colour.

Although Schwarz was a brilliant zoologist, he might be better described as a mammalogist than a primatologist. Primatology, however, was certainly the calling of W. C. O. Hill. Schwarz wrote a blizzard of articles on a wide variety of mammals; Hill’s major work was the monumental, multi-volume Primates: Comparative Anatomy and Taxonomy. So if anyone could judge what constituted the unusual in a chimpanzee, it was Hill. He did not consider the matter of the koolokamba so cut-and-dried as Schwarz, and he clearly expressed his opinion (Hill 1969, p. 35) that "review of the literature and personal experiences" had convinced him "of the general validity of Schwarz’s results [in revamping the taxonomy of the chimpanzee], but a lingering doubt remained, amounting almost to a tacit belief, in the existence of two races inhabiting the Lower Guinea region" (see map). In a separate paper (Hill 1967), he expressed his view in more detail:

map of lower guinea
Lower Guinea, as per Hill (1967)

Schwarz was a critical observer and, though inclined to simplify matters unduly, his work resulted in many supposedly distinct species and/or races being justifiably reduced to synonymy. In one respect, however, Schwarz appears to have given little credence to the abundant and constantly repeated testimony for the existence of two kinds of chimpanzees in the general area of Lower Guinea [i.e., the region of equatorial Africa bounded by the Sanaga, Congo and Oubangui rivers, and the Atlantic. Countries included: Gabon, Cameroon, Congo and Equatorial Guinea]. Further evidence has accrued of this since Schwarz’s original publication and though I cannot categorically claim that the problem is finally settled, I think a review of the situation is called for, and pending this, I am prepared tentatively to recognize the koolo-kamba (P. t. koolokamba Du Chaillu, 1861a) as what Rode refers to as a "biological race"… A number of experienced collectors have further affirmed either verbally or in litt. that two forms of chimpanzee occur in the region under consideration, and I have also received photographs from Professor W. E. Kershaw of examples he saw in the Leopoldville Zoo, which he submitted to me for identification (i.e., old chimpanzees or young gorillas) as no one had been able to state categorically what they were. In all these reports the examples are stated to exhibit features linking the more typical chimpanzee with the gorilla. Perret and Aellen (1956 [p. 445]), too, repeated the native tradition that two kinds of apes besides the gorilla had their habitat in the Cameroons; different names were employed for them, one being the product of hybridization, but these authors maintained that such animals are merely very old chimpanzees…It may be asked of those that maintain koolokambas are merely aged examples of P. t. troglodytes, why is it that only the Lower Guinea race produces such anomalous results in old age?

Rode’s suspicions that the koolokamba might be a distinct race of chimpanzee were based in part on a study of two unusual skulls, one from Congo, the other from Gabon. Quoted in Malbrant and MacLatchy (1949, p. 58) concurred with this view and quoted Rode as follows:

The two skulls differ from those of other chimpanzees with respect to the following essential characters: the lofty development of the supraorbital ridges, which are more clearly demarcated from the cranial portion of the skull; the orbits are deeper and their margins are elliptical rather than circular. The zygomatic arch stands out less from the brain case…Finally, the palate is markedly larger in the koolokamba than in ordinary chimpanzees … Koolokambas may constitute a separate subspecies of chimpanzee, but to reach any decision we will need a larger series of well-preserved specimens to compare with the other three subspecies Pan t. troglodytes, P. t. verus, and P. t. schweinfurthi.

Similar taxonomic woes were voiced by famous ape experts Ada and Robert Yerkes (Yerkes and Yerkes 1945, p. 393), who also noted that it can sometimes be difficult to tell gorillas from chimpanzees:

A somewhat embarrassing complication for the student of classification results from the existence of what seem to be forms intermediate between the chimpanzee and gorilla. The appearances naturally suggest hybridization…To various authorities the widely differing morphological characters and the nature of social relations in the genera constitute negative arguments. Nevertheless, the existence of forms which are with difficulty classifiable as either chimpanzee or gorilla is established and the necessity for explanation is evident.

Louis de Lassaletta (1958) describes yet another intermediate ape, shot in the Nsork Rain Forest in Rio Muni (modern Equatorial Guinea) in 1958. More recently, primatologist R. V. Short (1980, 5) discussed the question of whether the koolokamba should be treated as a subspecies of the chimpanzee, and concluded that

the status of the Kooloo-Kamba is still uncertain…The matter cannot be resolved until more animals have been studied in the wild in the type locality … And though it may seem improbable that a new species or sub-species of chimpanzee could remain undiscovered for so long, it must be remembered that the existence of the pygmy chimpanzee, Pan paniscus, was not established until 1929.

Short seemed particularly impressed with the koolokamba skull sent to the British Museum by Du Chaillu. Merfield had also pondered the question of whether koolokambas (or "chogas" as he called them) constitute a separate race. Although he was at one time convinced that they do, he seems ultimately to have recanted. After finding one of these animals in a troop of ordinary chimpanzees, he became an apostate, stating in his field notes (quoted in Shea 1984, p. 8) that "it shakes my faith as to really black-faced chimps or chogas being a separate race."

But is there really any reason to speak of a separate "race" of chimpanzees? The very rarity of these specimens is consistent with the idea that koolokambas are either hybrids or unusual variants. Of 141 chimpanzees in a collection examined by Reynolds and Luscombe, only two were koolokambas, and this appears to be a larger representation than is typical of most chimpanzee colonies (Reynolds and Luscombe 1971, Table 1). These authors (ibid, p.131) cautioned against Hill’s tentative treatment of P. t. koolokamba as a subspecies:

Nevertheless, this chimpanzee type keeps cropping up in the literature. Ferris (1981) examined the mitochondrial DNA of koolokambas (184.2). In a 1986 study of chimpanzee serum esterases Davidson (135.9) included two P. t. koolokamba individuals. In 1992, a molecular genetics paper by Morin et al. (381.7, 293) referred to the "little-known gorilla-like koolokamba from Congo."
There seems to be no valid reason for considering Hill’s form "koolokamba" a separate subspecies. Specimens of this supposedly distinct form occur sporadically in captivity, but the explanation that this is a ‘rare species’ (Hill 1967) does not follow. It could be a rare variant, just as albinos are a rare variant in man.

The crucial point is that these "variants" are not reported in chimpanzee populations where the opportunity to hybridize with gorillas is lacking. All of these intermediate apes — at least those of known origin — come from regions where not only chimpanzees, but also gorillas, are present (see Table 8.1). Throughout the length of the west coast of equatorial Africa, from Senegal to the Congo, chimpanzee collection has been intense for many years (Wolfheim 1983, pp. 709-710). And yet, the vast majority of these intermediate apes have been reported in this region in the area Hill (1969, p. 41) calls "Lower Guinea," which is the portion of the west coast where gorillas are present (see map above). None have been reported from other west coast countries where there are no gorillas, such as Sierra Leone, where chimpanzees have been heavily exported for more than a century. For example, in one five-year period (January 1973 - April 1978), 1395 chimpanzees were shipped from Sierra Leone (Short 1980, p. 6). Among so many specimens, why no "variants"?

Koolokamba
Karl Ammann with Mzee, one of his intermediate apes, a male Image: Karl Ammann, used with permission

Chimpanzee-gorilla intermediates are still being seen today. In recent email correspondence with wildlife photographer and anti-bushmeat activist Karl Ammann, I have received photos of two such apes, now in his possession, but originally from the northern forests of Democratic Republic of Congo. He also has pictures of a crested "chimpanzee" skull on his website. Ammann also says that he knows of at least four such apes currently in captivity in Africa.

Koolokamba
Mzee, showing large canines Image: Karl Ammann, used with permission

Koolokamba
Bili, another of Ammann's chimpanzee-gorilla intermediates, a female, which Ammann says, came "from just outside Bili" (northern D. R. Congo) Image: Karl Ammann, used with permission

The only reports of intermediate apes outside Lower Guinea are from areas in or adjacent to the eastern range of the gorilla in inland central Africa (Frechkop and Marit 1968; see also Hartmann 1886, pp. 223, 240, and Karl Ammann's intermediate apes, pictured in the box at right). But in comparison with coastal Lower Guinea, relatively few of these "chimpanzee variants" have been reported from this region — probably because, as Wolfheim (1983, p. 710) points out, few chimpanzees of any kind have been collected for export from the inaccessible reaches of the northeastern part of the Democratic Republic of Congo. Hartmann (1886, pp. 223, 240) does, however, mention a possible case from the eastern range, an ape found by Livingston in what is now the Maniéma district of modern Zaire west of Lake Tanganyika. In the 1960s, Marit did capture two such specimens, "chimpanzees" with gorilla-like sagittal crests, in the eastern range of the gorilla near Kampene, in the Maniéma district (Frechkop and Marit 1968, p. 32). He and Frechkop then searched the extensive collection of chimpanzee skulls in the Musée Royale d’Afrique Centrale at Tervuren, Belgium. They found the skulls of ten more "chimpanzees" with heavy jaws and sagittal crests. Nine were of known origin — all from gorilla country, one from the western range, and eight from the eastern. The eastern range preponderates because the Tervuren specimens are mostly from what is now the Dem. Rep of Congo, which was formerly a Belgian colony, a region that does not include any part of the eastern range of the gorilla. The western skull was obtained in 1928 at Bomongo, just over the Oubangui River from the eastern limit (Cousins 1978) of the lowland gorilla’s range (Impfondo) and may have been carried over the river to Bomongo by humans. The six crested chimpanzee skulls mentioned by Cramer (1997, p. 43) are probably among the nine listed by Frechkop and Marit since Cramer (1997, p. 8) selected his specimens from the same collection.

If the numerous intermediate apes that have been reported are only chimpanzee variants and not hybrids of gorilla and chimpanzee, why are there no accounts of them from other parts of the chimpanzee’s extensive range where gorillas are absent? Why is it that "one finds chimpanzees with gorilla-like noses," as Groves (1970a, p. 21) observes, "quite commonly in Gabon and Cameroun [i.e., Lower Guinea]" and not elsewhere? Indeed, why are these "chimpanzee" variants consistently described as "gorilla-like" if gorillas are not in some way involved? The vast majority of genetic variants have only a single gene mutated. This is the typical case with albinos, the example given by Reynolds and Luscombe. It is unlikely that mutation of a single gene would alter enough characteristics to make a chimpanzee "gorilla-like."

Shea (1984, p. 10) suggests that these intermediate individuals simply reflect "an important biological reality, i.e., gorillas and chimpanzees are very closely related animals with patterns of morphological development which coincide and overlap." But again, this explanation does not account for the fact that these so-called variants are found only in regions where chimpanzees have the opportunity to hybridize with gorillas, which only represents about a third of the entire chimpanzee range. Although he attempts to account for koolokambas in terms of individual variation within, and overlap between gorilla and chimpanzee populations with respect to their physical traits, Shea himself points out (ibid) that the "congruence [between chimp and gorilla] is so great that the production of viable hybrids remains a real possibility." Indeed, as documented elsewhere on this website, the pygmy chimpanze (bonobo) and chimpanzee do in fact hybridize.

In the case of animals less closely related to humans, hybridization is the first explanation that springs to the naturalist’s mind when intermediate specimens are observed. Nothing is more common than for a biologist to diagnose specimens as hybrid when they (1) have intermediate traits between two putative parents and (2) the specimens were obtained in a geographic region where the two proposed parents come into contact. Such diagnoses commonly occur where even a single trait (such as coat color or body size) indicates hybridity. For example, in considering specimens from a region where the Cotton Deer Mouse (Peromyscus gossypinus) and the White-footed Deer Mouse (P. leucopus) come into contact, Howell (1921, p. 50) diagnosed them as hybrids between the two on the sole basis of the fact that they had "decidedly larger skulls." In other respects, the specimens were similar only to P. leucopus, but in the present case, that of putative hybrids between the chimpanzee and gorilla, it is not merely the two facts that 1) the specimens come only from regions where both chimpanzees and gorillas are present and (2) the skulls are larger than those of ordinary chimpanzees. It is also the case that a variety of other traits are similar to those of gorillas in these supposed chimpanzees.

The evidence suggests that these intermediate apes actually are hybrids; if they are, the whole thrust of the debate has gone astray. Under such circumstances it would not be germane to debate whether a rare subspecies of chimpanzee exists in Lower Guinea and whether this unusual type deserves a special name. More than a hundred years ago, von Koppenfels wrote from Africa that he had "positive proof" that the koolokamba was a hybrid and "not a distinct species," and went on to say (von Koppenfels 1881, p. 448) that this "mongrel progeny of the male gorilla and female chimpanzee discovered by me, is found, but in individual cases, and as such, deserves no special name." If natural hybridization does in fact occur between gorillas and chimpanzees, von Koppenfel’s attitude is the only sensible one to take. Any further discussion of species and subspecies would be pointless.

And there is no reason to think that nucleotide sequence data will be particularly helpful in resolving this question. There is good reason to suppose, as there is in humans (see next section), that gorillas are backcross hybrids

Conclusion

We have seen evidence that the gorilla is of probable hybrid origin. A specific cross was proposed as a likely object of further investigation (chimpanzee × giant forest hog). The existence of apes intermediate between the gorilla and chimpanzee suggests that the northern chimpanzee population continues to hybridize with the gorilla even today. In the typical hybrid zone, at least those where the hybrids are partially fertile, the influence of hybridization spreads outward into adjacent populations until a barrier of some type is reached. In the present case, the barrier would be the Congo, Zaire, and Lualaba rivers. Beyond this barrier, insulated from the genetic influence of hybridization, is the pygmy chimpanzee (bonobo), which is therefore expected to be closer to the primitive chimpanzee type. Hence, what we call Pan troglodytes seems itself to be a hybrid population of sorts: a population composed of individuals quite similar to the pygmy chimpanzee, but somewhat influenced by interbreeding with the gorilla. And the traits in question seem to derive, ultimately, from H. meinertzhageni, since it appears likely that the gorilla is a hybrid of crossing between the chimpanzee and giant forest hog. So the picture that emerges is that humans, the common chimpanzee, and the gorilla all appear to be products of hybridization. Only the pygmy chimpanzee, is the "true" and original chimpanzee, in the sense that it must have predated its three hybrid offspring (humans, gorillas, and common chimpanzees). Also, if the conclusions reached thus far are correct, the giant forest hog and the ordinary pig must also have existed before these offspring first came into being. Therefore, whatever the origin of the gorilla might be, we should use only the pygmy chimpanzee as a standard of comparison in identifying human traits of porcine origin (at least when the trait in question differs between the pygmy chimpanzee and the common). This is a useful point to keep in mind as we fine-tune our analysis of human origins.

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TABLE 8.1 Reported Intermediate Apes of Known Origin

Examples from western range of the gorilla:

1855

Duvernoy's Troglodytes tschégo

Gabon

1861

Du Chaillu's koolokamba

Gabon

1866

Gratiolet and Alix (T. aubryi)

Gabon

1877

Alix and Bouvier (Gorilla mayêma)

Gabon

1877

Mafuka

Gabon

1880s

von Koppenfels' "cross-bred" animals

Gabon

1896

Garner's koolokamba

Gabon?

1898

Jena Museum ape (Duckworth)

Gabon

1898

Duckworth's specimen

Gabon

1900

Johanna (Keith)

Gabon

1913

Elliot's pseudogorilla

Gabon

1930s

Merfield's chogas

S.W. Cameroon

1939

Raingeard's koula-nguias

Gabon

1948

Rode's koolokamba skulls

Congo and Gabon

1954

de Lassaletta's koolokamba

Nsork rain forest, Equatorial Guinea

1960

Denis's koolokamba

Congo

1970

Grove's gorilla-nosed chimps

Gabon and Cameroon

1980

Cousin's "Minnie"

Cameroon

Examples from eastern range of the gorilla:

1870?

Livingston's Soko

Maniéma District, D.R. Congo

1968

Frechkop and Marit (crested "chimps")

D.R. Congo, Burundi

2012

Karl Amman's Mzee and Bili (and crested "chimp" skulls shown on his website)

northern D.R. Congo



CITATIONS:

[1]. See (242.85; 460.5), among many others.

[2]. (489.8; 489.9)

[3]. (140.1; 328.67)

[4]. (270.9,211; 140.1,33)

[5]. (602.7)

[6]. (602.7)

[7]. (140.1; 270.9; 579.4)

[8]. (450.6)

[9]. (187.54,8)

[10]. (187.54,9)

[11]. (337.5,177; 452.5,333; 492.6,300)

[12]. (492.6,324)

[13]. (170.1,82)

[14]. (180.5,152). See also (74.9,227).

[15]. (495.2; 496.6)

[16]. (180.8,48,52); See also (180.5,345; 180.8,52; 567.8,Plate XV).

[17]. (180.8,44a)

[18]. (180.5; 180.8)

[19]. (158.3,159)

[20]. (180.5,152)

[21]. (180.5,152 and Plate 3b)

[22]. (495.9, 52)

[23]. (168.7,199)

[24]. (158.3,209)

[25]. (158.3,248)

[26]. (563.3,Fig. 2)

[27]. (158.3,223,248; 223.1)

[28]. (270.9; 563.3)

[29]. (223.1)

[30]. (533.8,149)

[31]. (563.3,114)

[32]. (452.8,152,Plate 60)

[33]. (140.1,35; 495.65,125,126,Table 7-1)

[34]. (140.1,35; 495.65,125,126,Table 7-1)

[35]. (270.9,211)

[36]. (140.1,111; 290.9,137)

[37]. (270.9,214; 460.5,146; 602.7,687)

[38]. (270.9)

[39]. (140.1,115)

[40]. (140.1)

[41]. (588.4,1179)

[42]. (158.3)

[43]. (153.4)

[44]. (206.9,313)

[45]. (207.1,Table II)

[46]. (158.3,170-171; 267.8)

[47]. (158.3,Fig. 147; 267.8)

[48]. (180.5; 180.8)

[49]. (158.3,159)

[50]. (180.8,Plates 1 and 2)

[51]. (495.65,134,135,Fig. 7-6)

[52]. (495.06,435)

[53]. (495.65,127,Table 7-2; 495.2,233,Table 3; 495.65,127,Table 2)

[54]. (495.3; 495.65,131-132); See also (452.8,154-155,Plates 62 & 63).

[55]. (140.1; 495.3)

[55b]. (495.06, Figs. 16, 17; 495.65, 127, Table 2; 495.65, 135, 163)

[55c]. (495.06, 435a, 439b)

[56]. (495.06,434a; 495.2; 4955.65,130)

[57]. (533.8,136)

[58]. (452.8,153,Plate 61)

[59]. (170.1,81)

[60]. (360.8,115)

[61]. (452.8,89a). See also (194.1).

[62]. (533.8,269)

[63]. (405.9,376-377)

[64]. (452.8,89b)

[65]. (337.5,53; 495.65,142)

[66]. (398.8; 399.1)

[67]. (602.7,701)

[68]. (507.2)

[69]. (80.7, 122.3,Table V)

[70]. (205.9; 326.15; 379.7; 379.8,266; 531.8,35-39; 589.85)

[71]. (119.3; 621.9,362)

[72]. (183.6)

[73]. (28.3; 122.3,43; 188.55; 223.05,12; 337.5,52; 495.7;56)

[74]. (145.1,169)

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