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.
Name: Chimpanzee (
Pan troglodytes)
Primary Classification: Hominidae (Great Apes and Humans)
Location: Equatorial Africa
Habitat: Savanna, woodlands, swamps and rainforest.
Diet: Fruits, leaves, seeds, bark, honey, ants, termites and occasionally meat.
Size: Averages 3 to 5 ft in height and 100 to 175 lbs in weight.
Description: Black or brown hair; hairless face that turns dark with age; bulging eyebrows; protrusive lips; very long arms; opposable thumbs and big toes; males have large canines; no ail.
Cool Facts: It is one of the only animals that uses tools, such as a twig that it pokes into termite nests and removes when covered in insects. It is our closest relative, sharing about 98 percent of our genes.
Conservation Status: Endangered
Major Threat(s): Habitat loss and poaching
What Can I Do?: Visit
The Jane Goodall Institute for information on how you can help.
