May 17, 2006 — Our early ancestors interbred with chimpanzees after the two species diverged millions of years ago, new research suggests.
The provocative idea is sketched by U.S. genome experts, who have discovered that hominids and chimps diverged far more recently -- and over a much longer timescale -- than previously thought.
During that period, the authors theorize, the two primate species were rather more than kissing cousins: They had sex, swapping genes before making a final split.
"The (...) analysis revealed big surprises, with major implications for human evolution," said Eric Lander, one of the paper's co-authors and founding director of the Broad Institute, a research collaboration between Harvard University, MIT and the Whitehead Institute.
Until now, the belief was that humans and chimpanzees shared a common ancestor but parted ways around 6.5-7.4 million years ago.
One basis for that idea is a carbon-dated fossil called Toumai, believed by many scientists to be the oldest known human. Others, however, dismiss Toumai as an ape.
The estimate is also backed by the molecular clock, a way of calculating evolution on the basis of the speed at which genes mutate.
Previous molecular clock studies have focussed on the average genetic difference between humans and chimps. But the new paper, published online by the British journal Nature, takes a different approach.
Exploiting the mountain of data from the human and chimpanzee genome projects, the researchers compared the genetic codes of the two species as they are today, estimating the ages of key sequences rather than the overall average age.
They believe the two species split no later than 6.3 million years ago and probably less than 5.4 million years ago, one to two million years earlier than the Toumai estimate.
Moreover, it appears that "speciation" of chimps and hominids -- the process by which they emerged as separate species -- took an extraordinarily long time, around four million years in all.
The youngest chromosome in the human genome is the X, which helps determine gender. On average,the X chromosome is about 1.2 million years more recent than the 22 non-sex chromosomes, the scientists found.
Lander describes the X chromosome's age as "an evolutionary 'smoking gun'."
Thus something unusual must have happened on the way to speciation: an initial split between humans and chimps, followed by interbreeding, and then a final separation.
"It is possible that the Toumai fossil is more recent than previously thought. But if the dating is correct, (it) would precede the human-chimp split," said lead author Nick Patterson, also at the Broad Institute.
"The fact that it has human-like features suggest that human-chimp speciation may have occurred over a long period with episodes of hybridization (inter-breeding) between the emerging species."
A gradual divergence of species through hybridization, rather than a quick break, may be far more common than scientists have suspected.
"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," said the team's senior author, David Reich.
