Humans and chimpanzees diverged from a single ancestral population through a complex process that took 4 million years, according to a new study comparing DNA from the two species.
By analyzing about 800 times more DNA than previous studies of the human-chimp split, researchers from the Broad Institute of MIT and Harvard were able to learn not just when, but a little bit about how the sister species arose.
"For the first time, we're able to see the details written out in the DNA," said Eric Lander, founding director of the Broad Institute. "What they tell us at the least is that the human-chimp speciation was very unusual."
The researchers hypothesize that an ancestral ape species split into two isolated populations about 10 million years ago, then got back together after a few thousand millennia.
At that time the two groups, though somewhat genetically different, would have mated to form a third, hybrid population. That population could have interbred with one or both of its parent populations.
Then, at some point after 6.3 million years ago, two distinct lines arose.
Some experts in human evolution are skeptical of that precise scenario, but nevertheless impressed with the study.
"It's a totally cool and extremely clever analysis," said Daniel Lieberman, a professor of biological anthropology at Harvard. "My problem is imagining what it would be like to have a bipedal hominid and a chimpanzee viewing each other as appropriate mates, not to put it too crudely."
Past studies that compared human and chimp DNA could only offer a point estimate of how long ago the two species split by averaging the amount of divergence in their genes. Generally, those studies come up with a figure of about 7 million years ago.
But since the completion of the chimpanzee genome project in September it is possible to look at how specific sections of the genetic code have evolved. The Broad Institute study, which will be published in a future issue of the journal Nature, is one of the first to do that.
"There are a lot of big surprises here," Lander said.
For one thing, the new data suggest the human-chimp split was much closer to the present than the 7-million-year date that fossils and previous studies indicate — certainly no earlier than 6.3 million years ago, and more likely in the neighborhood of 5.4 million.
The data also show that the human-chimp split probably took millions of years.
That's because in some parts of the DNA sequence, the genetic difference between humans and chimps is so large that those genes must have been isolated from each other nearly 10 million years ago.
But in other places, the human and chimp lines are so close that they appear to have still been swapping genetic material at least until 6.3 million years ago.
One of those areas is the X-chromosome, which is intriguing.
"The genes that are a barrier to speciation tend to be on the X-chromosome," said David Reich, the main author of the study.