Image: COURTESY OF NATURE

Scholars of human evolution have long debated just how many branches and twigs make up the human family tree. Some place the known human fossils into numerous genera and species, creating a bushy tree. Others tend to see more similarities between fossils and opt for trees with fewer offshoots. Now newly described fossils from Ethiopia indicate that for at least one part of the human pedigree, less is more.

Announcing their findings today in the journal Nature, a team of researchers led by Ethiopian paleoanthropologist Berhane Asfaw reports that the million-year-old fossils, which belong to Homo erectus, lay to rest the idea that that species should be split in two. Some experts have argued that earlier African representatives of the group, which dates back as far as 1.7 million years ago, look so different from later examples from Asia that they warrant their own species, H. ergaster. In this scenario, Asian Homo erectus went extinct and H. ergaster spawned modern humans. But the newly recovered remains--including a complete skull cap (see image)--exhibit an intermediate anatomy that, according to Asfaw's group, bridges the gap between the two proposed species.

"Before this time, we really haven't had a good comparison between African and Asian forms from the same time window," says team member W. Henry Gilbert, a graduate student at the University of California, Berkeley. "We've had early African forms and late Asian forms, and people have used the differences between them to generalize about all African and Asian specimens. Now that we have a later African form for comparison, we are finding that they are very similar in a lot of the features that people were formerly using to separate early African from late Asian ones."

"The anthropological splitting common today is giving the wrong impression about the biology of these early human ancestors," asserts Berkeley paleoanthropologist Tim White, another study co-author. "The different names indicate an apparent diversity that is not real." Homo erectus, he says, was a biologically successful organism, "not a whole series of different human ancestors, all but one of which went extinct."

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