Between 1.5 and 2 million years ago, chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) evolved from a common ancestor and formed clear physical and behavioural differences. Bonobos are smaller and more slender than chimpanzees. Socially, chimpanzees live in male-dominated groups, while bonobo society is female-dominated.
Until today, no one had considered the idea that these two separate species could exchange genes, largely because of a major physical barrier that separates chimpanzees and bonobos: the Congo River. Chimpanzees live on the northern side of the river, while bonobos live on the southern side.
It has even been suggested that the formation of the Congo River, which also happened between 1.5 million to 2 million years ago, might have been a major driver in causing the two species to differentiate from a common ancestor.
But, as our study reveals, there is evidence of ancient genetic mixing across species boundaries. We now know that hundreds of thousands of years ago, chimpanzees and bonobos were able to mate and produce offspring, leaving a genetic mark on the animals that live in the wild today.
We have observed from captive populations that it is still possible for the two apes to mate today, even after more than a million years as separate species. But only now has science been able to provide robust evidence of natural occurrences in the wild.
Based on 75 complete genomes of chimpanzees and bonobos, we found that central and eastern chimpanzees share significantly more genetic material with bonobos than other chimpanzee subspecies do. This leads us to believe that the gene mixing between bonobos and chimpanzees occurred during two different episodes – the first one, 500,000 years ago and the second one 200,000 years ago.
Our next step will be to explore whether the genetic material received from bonobos has had any selective advantage in the evolution of chimpanzees.