The possibility of human–ape hybrids has been entertained since at least the medieval period; Peter Damian (11th century) claimed to have been shown the monstrous offspring of a human woman who had mated with an ape. Linnaeus (1758) used Homo troglodytes as the taxonomical name for a hypothetical human and orangutan hybrid.
Humans have one pair fewer chromosomes than other apes, with ape chromosomes 2 and 4 fused in the human genome into a large chromosome (which contains remnants of the centromere and telomeres of the ancestral 2 and 4). Having different numbers of chromosomes is not an absolute barrier to hybridization; similar mismatches are relatively common in existing species, a phenomenon known as chromosomal polymorphism.
All great apes have similar genetic structure. Chromosomes 6, 13, 19, 21, 22, and X are structurally the same in all great apes. Chromosomes 3, 11, 14, 15, 18, and 20 match between gorillas, chimpanzees, and humans. Chimps and humans match on 1, 2p, 2q, 5, 7–10, 12, 16, and Y as well. Some older references include Y as a match between gorillas, chimps, and humans, but chimpanzees, bonobos, and humans have recently been found to share a large transposition from chromosome 1 to Y not found in other apes.
This degree of chromosomal similarity is roughly equivalent to that found in equines. Interfertility of horses and donkeys is common, although sterility of the offspring (mules) is nearly universal (with only around 60 exceptions recorded in equine history). Similar complexities and prevalent sterility pertain to horse–zebra hybrids, or zorses, whose chromosomal disparity is very wide, with horses typically having 32 chromosome pairs and zebras between 16 and 23 depending on species. In a direct parallel to the chimp–human case, the Przewalski's horse (Equus ferus przewalskii) with 33 chromosome pairs, and the domestic horse (E. f. caballus) with 32 pairs, have been found to be interfertile, and produce semi-fertile offspring: male hybrids can breed with female domestic horses.
In 1977, researcher J. Michael Bedford discovered that human sperm could penetrate the protective outer membranes of a gibbon egg. Bedford's paper also stated that human spermatozoa would not even attach to the zona surface of non-hominoid primates (baboon, rhesus monkey, and squirrel monkey), concluding that although the specificity of human spermatozoa is not confined to Homo sapiens sapiens alone, it is probably restricted to the Hominoidea.
Reports of attempted hybridization
There have been no scientifically verified specimens of a human–chimp hybrid, but there have been substantiated reports of unsuccessful attempts at human/chimpanzee hybridization in the Soviet Union in the 1920s, and various unsubstantiated reports on similar attempts during the second half of the 20th century.
Ilya Ivanovich Ivanov was the first person to attempt to create a human–chimp hybrid by artificial insemination. Ivanov outlined his idea as early as 1910 in a presentation to the World Congress of Zoologists in Graz. In the 1920s, Ivanov carried out a series of experiments, working with human sperm and female chimpanzees, but he failed to achieve a pregnancy. In 1929 he organized a set of experiments involving nonhuman ape sperm and human volunteers, but was delayed by the death of his last orangutan. The next year he fell under political criticism from the Soviet government and was sentenced to exile in the Kazakh SSR; he worked there at the Kazakh Veterinary-Zootechnical Institute and died of a stroke two years later.
In the 1970s, a performing chimp named Oliver was popularized as a possible "mutant" or even a human–chimpanzee hybrid. An examination of Oliver's chromosomes at the University of Chicago in 1996 revealed that Oliver had forty-eight—not forty-seven—chromosomes, thus disproving an earlier claim that he did not have a normal chromosome count for a chimpanzee. Oliver's cranial morphology, ear shape, freckles, and baldness fall within the range of variability exhibited by the common chimpanzee. Scientists performed further studies with Oliver, the results of which were published in the American Journal of Physical Anthropology.
In the 1980s, there were reports on an experiment in human–chimpanzee crossbreeding conducted in the People's Republic of China in 1967, and on the planned resumption of such experiments. In 1981, Ji Yongxiang, head of a hospital in Shengyang, was reported as claiming to have been part of a 1967 experiment in Shengyang in which a chimpanzee female had been impregnated with human sperm. According to this account, the experiment came to nothing because it was cut short by the Cultural Revolution, with the responsible scientists sent off to farm labour and the chimpanzee dying from neglect. According to Timothy McNulty of Chicago Tribune, the report was based on an article in the Wenhui Bao paper of Shanghai. Li Guong of the genetics research bureau at the Chinese Academy of Sciences was cited as confirming both the existence of the experiment prior to the Cultural Revolution and the plans to resume testing.
Evidence for early hominin hybridization
There is evidence for a complex speciation process for the Pan–Homo split. This concerns times pre-dating the emergence of Homo and would concern hybridization between Pan and Ardipithecus or Orrorin, not Homo. Different chromosomes appear to have split at different times, suggesting that large-scale hybridization may have taken place over a period of as much as four million years leading up to the two emerging ("human" and "chimp") lineages as late as six million years ago. The similarity of the X chromosome in humans and chimpanzees might suggest hybridization taking place as late as four million years ago. This latter conclusion should be regarded as uncertain, with alternative proposals available to explain the apparent short divergence time in the X chromosome.
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