Cross-breeding between species has long been reported for horses and donkeys, which gives rise to mules and more recently for tigers and lions (‘Liger’) and bovines and antelopes (‘Beefelo’), but is cross-breeding between great apes such as chimpanzees and humans possible? Is the link between the two species close enough? What actually determines the possibility of cross-breeding? Is it the similarity of DNA between species or the number of chromosomes, or is it rather dependent on evolutionary history?
Great apes are obviously different in body posture, strength, communication, behavior and (in most cases, according to human assessment) intelligence, but more than 95% of their whole DNA sequence is identical to ours, with even 98.8% similarity for their coding DNA (1). This is definitely more than other pairs of species that can produce offspring, like horses and donkeys that have an estimated similarity of 94%, so it surely cannot be the answer to the problem.
A major factor in determining the possibility of fertilizing another species’ eggs is the amount of time that has elapsed since the last individual that both species descend from, referred to as the last common ancestor. While the last common ancestor of humans and chimpanzees is often dated back to 13 million years ago, a recent analysis observed more recent gene flow in chromosomes: humans and chimpanzees still had sex and produced offspring until ‘as recent’ as 4 million years ago (2). This is similar to the dating of the last common ancestor of modern horses and donkeys which is dated at 4 to 4.5 million years ago (3). So, it appears that it is more complicated than a simple date of last ‘gene distribution’.
During my research, I found many people arguing that chimpanzees and humans can surely not produce viable offspring due to the different number of chromosomes, namely 48 chromosomes for chimpanzees and 46 chromosomes for humans. This is simply not true. Again, my example is based on horses and donkeys (but it is also true for bulls and antelopes): they can successfully crossbreed, even though their number of chromosomes is different (62 and 64 chromosomes for horses and donkeys, respectively). The offspring with its 63 chromosomes is, however, infertile. Whether this is caused by the uneven number of chromosomes as this lonely number 63 cannot pair with another chromosome and segregate correctly or rather by an effect more downstream of pairing, namely the stop of spermatogenesis at a certain point with unknown reasons is still under debate.
You might say to yourself that this issue of cross-breeding is just a big thought experiment and while we can continue to pontificate until the cows come home we’ll never be able to test it. This type of research is of course precluded for ethical considerations in our current times, but in the 1920s, the Russian biologist Ilya Ivanovich Ivanov, a specialist in the field of artificial insemination and hybridization, attempted to ‘build a living super-warrior’ under the leadership of Stalin. The idea was to generate a pain- and cold-insensitive creature with superior strength to humans.
Source: WETA/Twentieth Century Fox
Fortunately, the method of artificial insemination was already developed at that time, so no humans had to have sex with an ape. Instead, Ivanov tried to fertilize female chimpanzees with human sperm. All three attempts at fertilization failed. Only as a next step did Dr. Ivanov’s experiments become more ‘exceptional’: he selected human ‘volunteers’, in this case Soviet women, to be inseminated with chimpanzee sperm. Fortunately (or unfortunately if you’re into that sort of thing) the story ends here. It is still unknown whether this experiment has been conducted but did not result in pregnancy or whether this could not be done because either the male ape died too soon or Ivanov was arrested as information about the experiments started to leak out just before he could conduct them.
Approximately 50 years later, at Cornell University in New York, Michael J. Bedford discovered that human sperm can adhere to and penetrate the egg of a gibbon, both in vivo and in vitro (4). This was, however, not possible for other lower primates (like baboons, rhesus monkeys), probably due to an altered egg and sperm surface (4). Rumor has it Bedford expressed his confidence that this penetration would work in greater apes as well, although this was not tested. However, exactly this argument that the sperm surface with their specific adherence to the complementary surface is the strongest argument of those arguing against the possibility of human-chimpanzee crossbreeding.
Around the same time, a creature called ‘Oliver’ has incited uproar in the United States. Oliver was considered a humanzee, the missing link between the two species: his face and behavior were human-like, he walked on two feet and apparently had a un-chimp-like self-awareness. The owners stated that Oliver had 47 chromosomes, which has never been proven by official reports, but it brought fame and glory to him. Eventually, after 25 years of Oliver being in the spotlight, scientists disenchanted Oliver’s story by providing a DNA analysis confirming that Oliver is just a normal chimpanzee with 48 chromosomes (5).
Oliver the Chimpanzee
Original publication: unknown
Even though no human-chimpanzee hybrid has been reported in history so far, I am more than confident that sperm and eggs of humans and chimpanzees can create a zygote (the technical name for a fertilized egg) but whether this can actually lead to the proper formation and development of an embryo is a whole other story. Similar to the non-functional spermatogenesis in mules, proper development could arrest at a certain point, leading to a maybe horribly deformed or just retarded embryo that is not at all similar to a chimpanzee nor a human being. But if the embryo developed correctly, what would the hybrid be like? Would it be a hybrid with the intelligence of a human, and the strength of a chimpanzee, as desired by Stalin, or maybe the other way around? And, more importantly, how would you classify a humanzee? Is it a human and can therefore not be owned, or is it an animal? What does it mean to be a human? How would it change the classification or shift the boundary between human and animal? These and more are questions that I do not want to answer as they by far exceed my expertise.
Stefanie Schuster is a PhD candidate in the Hertie Institute for Clinical Brain Research in the Clinical Neurogenetics lab of Prof. Dr. Ludger Schöls in Tübingen, Germany.
- The Chimpanzee Sequencing and Analysis Consortium. Initial sequence of the chimpanzee genome and comparison with the human genome. Nature. 2015;437:69-86.
- Patterson N, Richter D, Gnerre S et al. Genetic evidence for complex speciation of humans and chimpanzees. Nature. 2005;441:1103-8.
- Orlando L, Ginolhac A, Zhang G et al. Recalibrating Equus evolution using the genome sequence of an early Middle Peistocene horse. Nature. 2013;499:74-8.
- Bedford JM. Sperm/egg interaction: the specificity of human spermatozoa. Anat Rec. 1977;188:477-87.
- Ely JJ, Leland M, Martino M, Swett W, Moore CM. Technical note: chromosomal and mtDNA analysis of Oliver. Am J Phys Anthropol. 1998;105:395-403.