Ancestors went ape for chimps

2006-05-18 12:02

Richard Ingham

Paris - Our early ancestors interbred with chimpanzees after the two species drew apart millions of years ago, a paper published on Thursday suggests.

The provocative idea is sketched by US genome experts, who have discovered that hominids and chimps diverged far more recently - and over a much longer timescale - than anyone had thought.

During this time, the authors theorise, the two primates were rather more than kissing cousins: they had sex, swapping genes before making a final separation.

Major implications

"The (...) analysis revealed big surprises, with major implications for human evolution," says co-author Eric Lander, director of the Broad Institute of Harvard and the Massachusetts Institute of Technology (MIT).

Until now, the belief was that humans and chimpanzees shared a common ancestor but went their separate ways around 6.5-7.4 million years ago.

The basis for this is a carbon-dated fossil called Toumai, whose supporters say is the oldest known human. Its critics, though, dismiss Toumai as an ape.

Molecular clock

The estimate is also backed by the molecular clock, a method 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 human and chimp.

But the new paper, which is published online by the British journal Nature, takes a different approach.

Genetic codes explored

Exploiting the mountain of data that has come from the human and chimpanzee genome projects, the researchers compared the genetic codes of the two species as they are today, and estimated the various age of key sequences, rather than the overall average.

They believe the two species made their split no later than 6.3 million years ago and probably less than 5.4 million years ago. In other words, around one to two million years earlier than the Toumai estimate.

Moreover, "speciation" of chimp and hominid - the process by which they emerged as separate species - took an extraordinary long time: around four million years in all.

'An evolutionary smoking gun'

The youngest chromosome of all in the human genome is the X, which helps determine gender. On average, X is around 1.2 million years more recent than the 22 non-sex chromosomes, the scientists found.

Lander describes X's tender age as "an evolutionary smoking gun".

Previous studies suggest that sex chromosomes are among the most vulnerable of chromosomes when it comes to interbreeding - co-mingling places its genes under swift selective pressure.

Thus something unusual must have happened on the way to speciation: an initial split between human and chimp, followed by interbreeding, whose results show up in progressive younger genes, and then a final separation.

- AFP