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, researchers said yesterday.
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 H. erectus, 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 the journal Nature. "How likely is it that one would give rise to the other?"
Co-author Maeve G. 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.
Homo 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 from 1962 to 1964. Short and with disproportionately long arms, it was the least similar to humans.
Homo erectus - "upright man" - dates from about 2 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 - Homo sapiens, or "knowing man" - originated in Africa about 200,000 years ago and are characterized 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 N. Leakey, also of Stony Brook. The Leakeys are both explorers in residence at the National Geographic Society, which partially funded the research.
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, which was remarkably well-preserved because it was almost fully embedded in sandstone, was found nearby and dated to 1.55 million years ago.
Thomas H. Maugh II writes for the Los Angeles Times.