Chimps and bonobos interbred and exchanged genes

Bonobos (pictured) interbred with chimpanzees in the distant past

Terry Whittaker/FLPA/imageBROKER/REX/Shutterstock

By Brian Owens

Chimpanzees and their relatives bonobos are closer than we thought. Bonobos seem to have donated genes to chimps at least twice in the roughly two million years since they last shared an ancestor.

The two closely related apes have occasionally interbred in captivity, and bonobos are renowned for their free and easy sex life. But the finding that they interbred in the wild was unexpected.

The two species split sometime between 1.5 and 2.1 million years ago, around the same time that the Congo River system formed. Wild bonobo populations are entirely contained in that river system, separated from two nearby subspecies of chimps, the eastern and central subspecies.

Scientists assumed the river was an impenetrable barrier, says Christina Hvilsom from Copenhagen Zoo in Denmark, one of the researchers who worked on the genetic project. But it turns out that it must have been breached more than once – although it’s not clear how that happened.

Lucky find

Hvilsom and her colleagues weren’t actually looking for genetic evidence of ancient interspecies erotica. They were mapping genetic markers that could be used to determine where illegally traded chimps came from so they could be returned to their homes in the wild.

But when they compared the chimp genomes to those of bonobos, they found clear signs of bonobo genes. “We thought it was a mistake, but we kept seeing it,” says Hvilsom.

The genetic evidence shows that a little less than 1 per cent of the chimpanzee genome came from bonobos, from one contact between 200,000 and 550,000 years ago and another, more recent one less than 200,000 years ago.

It’s not yet clear whether the two episodes of gene flow happened at low levels over a long time or in discrete pulses.

A little less than 1% of the chimpanzee genome is made up of bonobo genes

Kevin Langergraber

Humans also carry evidence of ancient interbreeding in our genome, in the form of DNA from our Neanderthal relatives. Hvilsom et al.’s study shows that we are not unique among our great-ape cousins in dallying with other branches of our family tree.

“We know that interbreeding played a role in human evolution, and now we know it was important for the great apes too,” says Hvilsom.

Tony Capra of Vanderbilt University in Nashville, Tennessee, says that scientists need to pay more attention to interbreeding when studying how new species arise. “We need to make sure our models are robust enough to account for events that don’t respect the traditional tree of life,” he says.

Journal reference: Science, DOI: 10.1126/science.aag2602

Read more: Blasts from the past: The Soviet ape-man scandal; Sharing apes: what bonobos have in common with us