A recent paper in Current Opinion in Genetics and Development gives a concise overview of hybridization in primates, including humans.
Let’s start with some numbers. The genetic contribution of one primate species to the genome of another by means of ancient gene flow:
- Tibetan Macaque genomes contain 1-8% DNA from Rhesus Macaques
- Chimpanzee genomes harbor about 1% Bonobo DNA
- Non-African humans have 1 to 5% of Neanderthal DNA (Africans never came into contact with Neanderthals and hence to not have their DNA)
- Modern-day people from Oceania have 4-6% Denisovian (an extinct group of archaic humans) ancestry in their genomes
These figures clearly indicate that gene flow has occurred during primate evolution. The obvious question is: did it matter? Did ancient hybridization influence the evolutionary history of primates? For Neanderthal genes it has been shown that they contribute to depression and the immune system. For non-human primates, the question remains unanswered for now. But the advent of more genomic data hold promise to tackle this issue.
An interesting observation by the authors (Jenny Tung and Luis Barreiro) is absence of sterility or low viability of primate hybrids. Most hybrids, including those involving modern humans are alive and kicking (i.e. fertile). Some exceptions are hybrids in captivity, such as a Baboon x Rhesus Macaque cross, and hybrids between Black Howler Monkeys and Mantled Howler Monkeys in Mexico. In the latter case, only female hybrids have been reported, suggesting that male offspring are not viable. This is in line with the predictions of Haldane’s Rule.
Another striking discovery is the existence of so-called ‘ghost lineages’. By sequencing ancient DNA researchers have found indications of gene flow in Africa between anatomically modern humans and older lineages that are now extinct. A similar patters was uncovered for African Baboons of the genus Papio. This seems to suggest that the African continent is home to several primate ghosts lineages. Scary! But seriously, this ancient DNA approach could lead to the discovery of unknown extinct species.
The popular press has focused most on ancient gene flow between humans and Neanderthals. For some reason, intercourse between these two closely related species captivates the attention of the general public. Commercial companies that offer personal genome sequencing, such as 23andMe, even provide you with an estimate of Neanderthal ancestry in your own genome. I had my genome sequences this year. The result: 3.0% Neanderthal!
Tung, J. and L. B. Barreiro (2017). The contribution of admixture to primate evolution. Current Opinion in Genetics & Development 47: 61-68.