Despite the general acceptance of a diversion in
the lines between the ancestors of humans and their chimpanzee
cousins around 7 million years ago, the final split may have taken
place far more recently than was previously thought.
A detailed analysis of human and chimp DNA suggests
that the lines finally diverged less than 5.4 million years ago.
The finding, which has been published in the journal Nature, is about
1-2 million years later than the fossils themselves have indicated.
A US team says that its results hint at the
possibility that interbreeding occurred between the two lines for
thousands, and even millions, of years. This hybridisation would
have been important in swapping genes for traits which allowed the
emerging species to survive in their environments, explained the
scientists, who are affiliated to the Broad Institute of MIT and
Harvard and the Harvard Medical School.
And it underlines, they believe, just how complex
human evolution has been.
'This is a hypothesis; we haven't proved it but
it would explain multiple features of our data,' said David Reich,
assistant professor of genetics at the Harvard Medical School and
an author on the Nature paper. 'The hypothesis is that there was
gene flow between the ancestors of humans and chimpanzees after
their original divergence.'
'So, there might have been an original divergence
and a separation for long enough that the species became differentiated
- for example, we might have adapted features such as upright walking
- and then there was a re-mixture event quite a while after; a
hybridisation event,' he told the Science in Action programme on the
BBC World Service.
Humans and chimps contain DNA sequences which are
very similar to each other; the differences are due to mutations, or
errors, in the genetic code that have occurred since these animals
diverged onto separate evolutionary paths.
Shown here is the cranium of Sahelanthropus tchadensis,
one of perhaps several species on the human side of the
human-chimpanzee divide which may still have interbred with an
early chimpanzee species at a time at which the two lines were
still very similar - perhaps no more different than modern
humans and Neanderthals who certainly did interbreed (a, facial
view. b, lateral view. c, dorsal view. d, basal view)