This finding sheds new light on the earliest hominid fossils, all of which have been found in Africa over the past 15 years. The fossils have puzzled scientists with their inconsistent and unusual blend of human and chimpanzee characteristics. The scientists, working at Harvard University and the Massachusetts Institute of Technology, completed an exhaustive analysis of human, chimpanzee and gorilla genomes to find out the evolutionary history of each.

"The genome analysis revealed big surprises, with major implications for human evolution," said the paper's co-author, Harvard biologist Eric Lander. "First, human-chimp speciation occurred more recently than previous estimates. Second, the speciation itself occurred in a more unusual manner that left a striking impact across (the female chromosome) chromosome X." The paper says "the two species split no more than 6.3 million years ago and probably less than 5.4 million years ago". The finding casts new light on the famous Toumai skull, found in Chad in 2002, which was dated to 6.5 to 7.4 million years. Toumai was believed to be the earliest hominid skull. Because Toumai now seems to be older than the final split between chimpanzees and hominids, it is probably neither chimp nor hominid but a common ancestor of both.

"It is possible that the Toumai fossil is more recent than previously thought," said the paper's lead author, Nick Patterson of Harvard University. "But if the dating is correct, the Toumai fossil would precede the human-chimp split. The fact that it has human-like features suggest that human-chimp speciation may have occurred over a long period with episodes of hybridisation between the emerging species." Australian National University anthropologist Colin Groves said the idea of humans and chimpanzees swapping genes had been around for decades but the Nature paper was the first hard evidence.

Different species interbred in the wild "quite often", said Dr Groves, and the paper made a very strong case for hominids and chimpanzees doing the same. "It's very interesting and I can't see any other option from their evidence," he told The Age. He said the paper allowed scientists to place the three earliest hominid fossils - Toumai and the later specimens Orrorin tugenensis and Ardipithecus kadabba - more accurately on the human family tree, although debate would continue.

Toumai "represents something that lived before the human and chimp lines speciated. The other two . . . I've accepted that they're on the human line." Dr Groves said that even today it could be possible for humans and chimps to have sex and produce offspring, although there would be ethical problems.

theage.com.au