Another View of Gorilla Relationships

Here, as at all other points on the coast, the orangs [apes] are believed by the natives to be human beings, members of their own race, degenerated.
Rev. T. S. Savage, 1847
The first - it may be called the supreme - question in regard to the gorilla is, its place in the scale of nature, and its true and precise affinities. Is it or not the nearest of kin to human kind?
Richard Owen, 1859

Several authors have favoured the idea that gorillas are more closely related to humans than are any other ape. Richard Owen, though he was not operating from an evolutionary point of view, concluded that the apes "recede from the human type in the following order, - gorilla, chimpanzee, orangs, gibbons..." (Owen 1859). Some later anatomists, such as Elliot Smith (1924), still argued that the gorilla is closest. It is now clear that it is the chimpanzee which is in fact closest to humans (although one maverick author still insists that the orangutan is the closest: Schwartz 2005), but the gorilla comes next.
The first psychological research Yerkes & Yerkes (1929) also ranked the gorilla as closest to humans, especially in cognitive traits, although chimpanzees seemed as close to humans in affective traits - although let it be noted that the Yerkes & Yerkes had been able to study only one (young) gorilla. Research since then in comparative psychology has, however, tended to rank the gorilla after the chimpanzee, and equal to (or even after) the orangutan, in similarity to humans (Johnson et al. 2002), although there has still been less testing of gorillas than there has of chimpanzees (or orangutans, for that matter).
Half a century after Yerkes & Yerkes, primatologists and comparative psychologists reacted with astonishment when it was shown that chimpanzees could recognise their own reflections in a mirror, but shook their heads in puzzlement when Suarez & Gallup (1981) reported that, although orang-utans could also recognise their mirror reflections, gorillas were unable to do so. Other studies have refuted this stark claim, but it remains true that, so far, fewer gorillas have passed the mirror self-recognition test than other great apes (about 30% of gorillas that have been tested, compared to over 40% of chimpanzees and 85% of -orangutans - Swartz et al. 1999).
It is molecular work that has now shown conclusively that chimpanzees are closest, and gorillas next (followed by orangutans, then gibbons). Wildman et al. (2003), who included the chimpanzee in the genus Homo along with humans because their ancestors separated only 5-6 million years ago, calculated that the gorilla's ancestors would have separated from the common ancestor of human and chimpanzee only a little before this, between 6 and 7 million years ago. Watson et al. (2001), pointing out that the DNA distance between gorillas and the chimpanzee/human grouping is less than that between some pairs of species customarily placed in the same genus, enlarged the genus Homo still further by including the gorilla; they have been the only authors to go this far, although it is indeed now becoming increasingly common to include the chimpanzees in Homo. More recently Raaum et al. (2005) have recalibrated the hominoid molecular clock; they accepted a chimpanzee/human separation time of 6 million years, but made the gorilla's separation older, between 7 and 9 million. This is still comparatively recent, but outside the limits which authors such as Goodman et al. (1997) would accept as those of a single genus.
What does the fossil record say about all this? The earliest convincing representative of the human lineage is Orrorin tugenensis, from the Tugen Hills in central Kenya, which is 5.9 million years old. Until recently, the gorilla had no convincing fossil antecedents, but now a case has been made that some fossil teeth from the same levels are proto-gorillas (Pickford & Senut 2005).
Even if they are not as close to humans as chimpanzees, and even if they separated 7-9 million years ago, rather than 6-7 million, gorillas are nonetheless very humanlike in many ways. So: could we and they interbreed?
Yes, cried at least three lurid movies. A pretended documentary, Ingagi (1930), said that women went off to have sex with gorillas in the jungle, and so did the 1937 Love Life of a Gorilla. The last of these "sexploitation" films was the 1948 Forbidden Adventure - claimed to be based on a filmed African expedition of 1912, but, incredibly, set in Angkor! (And, on a loftier cultural plane, did the original King Kong mean to suggest this too?)
There has actually been one attempt to breed hybrids between humans and other apes. In the late 1920s, a Soviet biologist, Ivanov, traveled to Guinea where he inseminated three female chimpanzees with human sperm. No pregnancies resulted; in two of the cases the terrified chimpanzees were held down, struggling, and there was almost no possibility that the sperm was injected far enough in to reach the uterus. Further attempts were contemplated, but apparently never realized - these included one project to inseminate women with the sperm of an orangutan (Rossiianov 2002).
The possibility of hybridisation would depend in the first place on how similar the chromosomes are. And the chromosomes of humans, chimpanzees and gorillas are very similar indeed. According to the two most detailed comparisons (Yunis & Prakash 1982; Dutrillaux & Couturier 1986), major structural changes are few; humans and gorillas have only 12 points of reorganization, although there are some minor differences as well - mainly, the greater amount of telomeric heterochromatin in the gorilla. For comparison, there are 24 chromosomal reorganizations between baboons and the blue monkey Cercopithecus mitis (Dutrillaux et al. 1986) - and these are known to hybridize (Gray 1972). Hybrids between a siamang and a Bornean gibbon have been bred (Myers & Shafer 1979); the chromosomes of these two species are so different that it has so far proved impossible to homologize them. On chromosomal grounds, therefore, it seems perfectly possible that humans and gorillas could interbreed (and of course the possibility of human-chimpanzee hybrids is even greater, given that the chromosomal differences are still less).
Considering the chromosomal data, and that humans and chimpanzees shared a common ancestor subsequent to the separation of the gorilla lineage, hybridisation between gorillas and chimpanzees would be as plausible as that between gorillas and humans, and arguably (!) less ethically fraught. For quite some time, there have been suggestions that such hybrids exist, but these have turned out to be either unusual-looking chimpanzees or adult male gorillas which have never developed a sagittal crest. The most persistent candidates for such hybrids are what have been called the koolookamba; two authors (Cousins 1980; Shea 1984) have independently examined the evidence and concluded very firmly that koolookambas are simply large, black chimpanzees with some superficially gorilla like facial features, such as large brow ridges and wide, padded nostrils.
Up to now we have no evidence of hybrids between gorillas and either chimpanzees or humans. All we can say is that it does theoretically seem possible, because on any criterion - anatomical, psychological, genetic, geological - we are very close indeed.

Colin Groves

Other contributions:

Martha Robbins

Juichi Yamagiwa

James Byamukama and Stephen Asuma

Raymond Corbey

Richard Johnstone-Scott

Kelly Stewart

Overview
 

References
Cousins, D. (1980) On the koolookamba - a legendary ape. Acta zool. pathol. Antverp. 75, 79-93
Dutrillaux, B. & Couturier, J. (1986) Principes d'analyse chromosomique appliquée à la phylogenie: l'exemple des Pongidae et des Hominidae. Mammalia, 50 (Suppl.), 56-81
Dutrillaux, B. et al. (1986) Relations chromosomiques entre sous-ordres et infra-ordres, et schéma évolutif général des Primates. Mammalia 50 (Suppl.), 108-121
Elliot Smith, G. (1924) The Evolution of Man. London: Oxford University Press
Goodman, M. et al. (1997) Toward a phylogenetic classification of Primates based on DNA evidence complemented by fossil evidence. Molecular Phylogenetics and Evolution 9, 585-598
Gray, A. P. (1972) Mammalian Hybrids: A Check-List with Bibliography (2nd edition). Farnham Royal: Commonwealth Agricultural Bureaux
Johnson, V. E. et al. (2002) Bayesian analysis of rank data with application to Primate intelligence experiments. J. Amer. Stat. Assoc. 97, 8-17
Myers, R. H. & Shafer, D. A. (1979) Hybrid apes offspring of a mating gibbon and siamang. Science 205, 308-310
Owen, R. (1859) On the classification and geographical distributions of the Mammalia. Appendix B, On the orang, chimpanzee and gorilla. Pp. 64-103 in: Lecture on Sir Robert Reade's Foundation, delivered before the University of Cambridge in the Senate-House, May 10, 1859. London: John W. Parker & Son
Pickford, M. & Senut, B. (2005) Hominoid teeth with chimpanzee- and gorilla-like features from the Miocene of Kenya. Anthrop. Sci. 113, 95-102
Raaum, R. L. et al. (2005) Catarrhine primate divergence dates estimated from complete -mitochondrial genomes. J. Hum. Evol. 48, 237-257
Rossiianov, K. (2002) Beyond species: Il'ya Ivanov and his experiments on cross breeding humans with anthropoid apes. Science in Context 15, 277-316
Savage, T. S. (1847) Notice of the external characters and habits of Troglodytes gorilla, a new species of orang from the Gaboon River. Boston J. Nat. Hist. 5, 417-426
Schwartz, J. H. (2005) The Red Ape: Orang-utans and Human Origins (2nd edition). Cambridge, Mass.: Westview Press
Shea, B. T. (1984) Between the gorilla and the chimpanzee: a history of debate concerning the existence of the kooloo-kamba or gorilla-like chimpanzee. J. Ethnobiol. 4, 1-13
Suarez, S. D. & Gallup, G. G. (1981) Self-recognition in chimpanzees and orangutans, but not gorillas. J. Hum. Evol. 10, 175-188
Swartz, K. B. et al. (1999) Comparative aspects of mirror self-recognition in great apes. Pp. 283-294 in: The Mentalities of Gorillas and Orangutans (eds. Parker, S. T. et al.). Cambridge: Cambridge University Press
Watson, E. E. et al. (2001) Homo genus: a review of the classification of humans and the great apes. Pp. 311-323 in: Humanity from African Naissance to Coming Millennia (eds. Tobias, P. V. et al.). Florence: Firenze Univ. Press
Wildman, D. E. et al. (2003) Implications of natural selection in shaping 99.4% nonsynonymous DNA identity between humans and chimpanzees: enlarging genus Homo. Proc. Natl. Acad. Sci. 100, 7181-7188
Yerkes, R. W. & Yerkes, A. W. (1929) The Great Apes. New Haven: Yale University Press
Yunis, J. J. & Prakash, O. (1982) The origin of man: a chromosomal pictorial legacy. Science 215, 1525-1530

Prof. Colin P. Groves teaches primatology and human evolution at the Australian National University, and does research on a variety of mammal species.

Gorillas in general - overview

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