'When those things change, the path to speciation may also change, or potentially even reverse.'Д«ecause there's so little bonobo DNA in the chimps, Hvilsom and her colleagues suggest that for chimps, the bonobo genes were disadvantageous. How two species form depends on whether the environment encourages their separation, whether the dividing populations are big enough to survive on their own, and other factors. Rus Hoelzel, an evolutionary biologist at Durham University in the United Kingdom. The chimp-bonobo results help 'us to understand the nature of speciation,' says A.

'It's going to be pretty common,' he predicts. 'The more we look at genomes, the more it seems to be found,' Mallet says.

These findings come on the heels of other genome analyses-such as between coyotes, dogs, and wolves-showing such gene flow between species. (Up to 4% of human DNA today came from extinct relatives.) 'Neither of them like swimming, so the Congo River is a major barrier,' Mallet says. That chimps have a trace of bonobo DNA suggests getting together was a challenge for the two species. In contrast, the western chimp subspecies has no bonobo DNA, the researchers note, suggesting that only those chimps living close to the Congo River entertained bonobo consorts. Then, just 200,000 years ago, central chimps got another boost of bonobo genes, the team reports today in Science. The genetic analysis indicates that this inbreeding happened during two time periods: 1.5В million years ago bonobo ancestors mixed with the ancestor of the eastern and central chimps. Using the same tests that uncovered hybridization among humans, she and her colleagues determined that 1% of the central chimpanzee's genome is bonobo DNA. She wanted to see whetherВ humans'В closest relatives also dallied beyond species boundaries. But Hvilsom was also intrigued by the discoveryВ in 2010 of Neandertal DNA in the human genome. They had been comparing as many great ape genomes as possible in order to help conserve the animals: They sought genetic differences that could help pinpoint the geographic origin of a confiscated ape and so identify where illegal hunting occurred. She and her colleagues uncovered this liaison when they examined the complete genomes of 75 chimps and bonobos from 10 African countries. Before the 1930s, bonobos were considered a subspecies of chimp as well and were called pygmy chimps, but researchers decided based on physical differences that these smaller apes are distinctive enough to warrant separate species status.ДЄlthough in the classical view species are not supposed to be able to interbreed successfully, that was and still is not always true of bonobos and chimpanzees, says Christina Hvilsom, a conservation geneticist at the Copenhagen Zoo. Chimps now live in fragmented populations that have become different enough to qualify as four subspecies the western, eastern, and Nigeria-Cameroon chimpanzees are much less common than the central chimpanzees.

(Chimps are bigger and have a male-dominated society instead of a female-dominated one.) Populations of both apes are shrinking because of deforestation and hunting. 'It adds to a growing body of work showing that species exchange genes,' says Peter Grant, an evolutionary biologist from Princeton University who was also not involved with the study.Д«onobos ( Pan paniscus) live in Democratic Republic of the Congo, just across the Congo River from their closest living relatives, the chimpanzeesВ ( P.В troglodytes), who are spread out between western and central Africa. 'What they found was really cool,' says Michael Arnold, an evolutionary biologist at the University of Georgia in Athens who was not involved in the work. Although it's not yet clear whether the acquired genes were ultimately beneficial or harmful, the finding strengthens the idea that such cross-species mating played an important role in the evolution of the great apes. New research reveals that chimps mixed it up with bonobosВ at least twice during the 2 million years since these great apes started evolving their own identities. Now, it turns out one of our closest living relatives, chimpanzees, also dallied with another species. Tens of thousands of years ago, modern humans slept around with Neandertals and swapped some genes.

Chimpanzee mating