KENYA:A 1.5-million-year-old skull and an equally old jaw found in Kenya are helping rewrite the history of early man, eliminating one reputed ancestor from the human lineage and suggesting that another was much more primitive than previously believed.
The jawbone shows that Homo habilis, previously believed to be a direct ancestor of Homo erectus and thus of humans, lived side by side with the latter, making them sister species rather than mother and daughter.
"They co-existed at the same time and in the same place for half a million years," said anthropologist Fred Spoor of University College London, a co-author of the paper appearing in this week's edition of Nature. "How likely is it that one would give rise to the other?"
Co-author Maeve Leakey of Stony Brook University in New York added, "the fact that they stayed separate as individual species for a long time suggests that they had their own ecological niche, thus avoiding direct competition."
The situation is similar to modern Homo sapiens and Neanderthals living side by side in Europe 50,000 years ago, said anthropologist William Kimbel of the Institute of Human Origins at Arizona State University, who was not involved in the research.
Researchers once thought that Neanderthals were a predecessor of modern humans, but it eventually became clear that they were an evolutionary dead end. Now it seems the same is true of H. habilis, he said.
The finds "are consistent with a growing consensus" that the evolutionary tree of humans is highly branched rather than a single linear trunk, he said.
H. habilis - "handy man" - is the oldest representative of the genus Homo, dating from about 2.5 million years ago. The species was defined by Mary and Louis Leakey based on fossils found in Tanzania between 1962 and 1964. Short and with disproportionately long arms, it was the least similar to humans.
H. erectus - "upright man" - dates from about two million years ago. It was originally described in the 1890s, and specimens have been found throughout Africa, Europe and Asia. Specimens bear a striking resemblance to modern humans, but the brain is about one-quarter smaller. H. erectus made the first known tools from stone.
Modern humans - H. sapiens, or "knowing man" - originated in Africa about 200,000 years ago and are characterised primarily by a larger brain capacity.
The two new fossils were found in 2000 east of Lake Turkana by a team headed by Spoor, Leakey and anthropologist Louise Leakey, also of Stony Brook.
The jawbone was identified as H. habilis because of the distinctive pattern of its teeth. Analysis of the rock around it by geologists Frank Brown and Patrick Gathogo of the University of Utah dated the jawbone to 1.44 million years ago, making it by far the youngest ever found.
The most recent previous specimen dated from about 1.62 million years ago.
The skull was found nearby and dated to 1.55 million years ago. The skull was so well preserved that researchers could tell that its growth plates had fused, indicating that it was from an adult, Spoor said.
Its physical characteristics indicate that it is H. erectus, but it is the smallest such skull ever found, he said.
"The direct importance of these is that they both come from the same site," Kimbel said. Researchers had previously found similar overlapping specimens at different locations, but it was possible to argue that they were geographical variants.
Spoor thinks the two species co-existed by occupying separate ecological niches. The smaller teeth and jaws of H. erectus, for example, indicate that H. habilis had a tougher diet, perhaps including more fruits, berries and leaves. The evidence suggests both species descended from a common ancestor somewhere between two million and three million years ago.