The oldest known specimen of a fossil hominid, or member of the human evolutionary lineage after it diverged from the ape lineage, has been found in Kenya. It is a fragment of jawbone with two molar teeth and could be 1 million years older than the previous record holder -- bones from Laetoli, Tanzania that date back 3.7 million years.

There is little question that the new find is a hominid, but its exact age has not been pinned down. Various lines of evidence suggest that it could have lived anywhere from 4.15 million to 5 million years ago.

The specimen was found by Kiptalam Cheboi, a Kenyan working on an expedition led by Andrew Hill of Harvard University's Peabody Museum. It came from a hilly area west of Lake Baringo in western Kenya.

The specimen is too small to reveal much that is new about the nature of human evolution, but its antiquity pushes back the hominid lineage very close to the time when some scientists think it began. Comparisons of genes from living humans and apes suggest that it would have taken roughly 5 million years for the observed differences to arise.

Genetic comparisons are not universally accepted as a valid means of establishing the time of human-ape divergence. The more traditionally accepted times of divergence date back more than 10 million years. Still, many traditionalists are accepting the more recent date.

The new specimen, like most fossils, cannot be dated directly. Its age is inferred on the basis of two facts. It was found above a layer of volcanic ash known to have formed close to 5 million years ago; thus, the specimen must have lived after the ash fell. And the fossil was found in the same layer as bones of animal species that disappeared from other known sites about 4.15 million years ago; thus, the specimen probably lived more than 4.15 million years ago.