A long-standing debate among scholars of human evolution centers on the number of hominid species that existed in the past. Whereas some paleoanthropologists favor a sleek family tree, others view the known fossil record of humans as indicative of a tangled bush. The latter view has gained popularity in recent years, but a new fossil from Tanzania suggests that a bit of pruning might be in order. Researchers writing today in the journal Science report that a specimen unearthed from Olduvai Gorge--a site made famous several decades ago by Louis and Mary Leakey--bridges two previously established species, indicating that they are instead one and the same.

A team led by Rutgers University paleoanthropologist Robert Blumenschine discovered the fossil--an upper jaw with all of its teeth and the lower face. It dates to between 1.84 and 1.79 million years ago, around the time that our genus, Homo, was starting to come into its own in terms of evolving a larger brain, making tools and exploiting larger animals for food. According to the report, the new find, dubbed OH 65, strongly resembles a fossil from Kenya attributed by some workers to the species H. rudolfensis. At the same time, however, key features link OH 65 to the type specimen of the earliest known member of Homo, H. habilis. "OH 65 allows us to reshuffle the specimens that belong in the ancestral genus and tie together rudolfensis and habilis," Blumenschine comments. "It shows that all three specimens are likely to be members of the same species--Homo habilis."

OH 65 and the stone tools and butchered animal remains that were found with it turned up in the drier, western part of the gorge. Based on the patterns of toolmaking and butchery evident in the stones and bones, Blumenschine and his colleagues posit that hominids like OH 65 may have made "irregular, seasonal forays to the western basin streams from the ecologically more productive southeastern basin," during wetter periods. If so, that would furnish additional support for the notion that even the earliest members of our genus were behaviorally flexible, migrating in response to seasonal shifts in resource and shelter availability.