Scientists say they have identified the oldest well-documented fossil specimen of the immediate predecessors of humans, pushing back the direct evidence for this evolutionary line by 500,000 years.
The scientists said the 3-inch scrap of skull bone found 25 years ago in Kenya was 2.4-million years old and came from a member of the evolutionary group called Homo.
People are called Homo sapiens, and extinct members of the group include Homo habilis and Homo erectus.
Experts said the finding would not change scientific understanding of human evolution, because stone tools made about 2.5-million years ago already had been attributed to a member of Homo.
But until now, there had been no strong evidence that members of Homo had lived that long ago.
"Now Homo is available to have made the first known stone tools," found in Ethiopia and dated at about 2.4-million to 2.6-million years old, said Andrew Hill, a Yale University anthropologist and co-author of the study.
Another scientist cautioned that although he believed the fossil represented Homo, the identification must be considered tentative.
The work was reported in today's issue of the journal Nature.
When the bone fragment was found, researchers said they did not know whether it came from Homo or from related creatures called australopithecines. The fragment includes the hole for the right ear and the part that meets the lower jaw.
The new analysis compared the fragment to skulls from Homo and australopithecines that had been discovered since the fossil was unearthed, Hill said in a telephone interview.
The fossil was too small for scientists to assign it to a particular member of Homo, he said.
The age of 2.4-million years was determined from analyzing rocks from the site where the fossil was found. No stone tools were found along with the fossil his group studied, he said.
Scientists had cited the stone tools as evidence of Homo, although the assumption that only Homo could make them was questionable, Bernard Wood of the University of Liverpool said in a telephone interview.