It could be the oldest, not to mention the messiest, break-up in history. When humans and chimpanzees split up along the path of evolution, they carried on having sex for as long as 4m years, geneticists claim today. The revelation suggests that the history of humanity may be far more complex than scientists appreciated.
A comparison of snippets of DNA from humans, chimps and other primates shows that after parting company up to 10m years ago early humans and chimps continued to swap genes by interbreeding, until a final split much later. The study suggests the species split for good probably less than 5.4m years ago.
Geneticists believe the interbreeding theory is the best explanation of why the X chromosomes of humans and chimps remain so similar to this day.
Dr David Reich at the Broad Institute of Harvard and MIT in Boston said: "The study gave unexpected results about how we separated from our closest relatives, the chimpanzees. We found that the population structure that existed around the time of human-chimpanzee speciation was unlike any modern ape population. Something very unusual happened at the time."
The scientists hypothesise that interbreeding between our ancestral humans and early chimps created a third, infertile "hybrid" species, the human equivalent of a mule, the infertile offspring of a horse and donkey. Though incapable of breeding among its own, the hybrid is believed to have survived by mating with its parent human or chimp species, before the two separated to follow the two distinct evolutionary paths that led to modern humans and chimps.
Dr Reich, whose study appears today in the journal Nature, said: "Hybridisation is commonly observed to play a role in speciation in plants, but evolutionary biologists do not generally view it as an important way to produce a new species in animals.
"An event between human and chimpanzee ancestors could help explain both the wide range of divergence times seen across our genomes, as well as the relatively similar X chromosomes. That such evolutionary events have not been seen more often in animal species may simply be due to the fact that we have not been looking for them."
Genetic studies of humans and chimps have previously looked only at the average difference between the two species, but the latest study looked at specific variations accumulated over millions of years across the entire genome.
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