Sunday, March 23, 2008

ANOTHER MAYBE HOMININ WALKED UPRIGHT 6 MYA

An analysis of six-million-year-old bones from an early human ancestor that lived in what is now Kenya suggests that the species was the earliest known hominin to walk, a new study says.

"This provides really solid evidence that these fossils actually belong to an upright-walking early human ancestor," said study lead author Brian Richmond, a biological anthropologist at George Washington University in Washington, D.C.

Orrorin tugenensis, known by only a handful of bones, has generated controversy since its discovery in the hills of northwest Kenya in 2000. The species existed during a critical period in the human evolutionary timeline. The genetic differences between human and chimpanzee lineages point to divergence from a common ancestor that lived somewhere between five
and eight million years ago.

Scientists have hotly debated whether or not O. tugenensis was an upright-walking human ancestor or an ape, since bipedalism-or walking on two legs-is often considered a first fundamental step in human evolution.

O. tugenensis's thighbone, or femur, was different from that of modern humans and living apes but surprisingly similar to species that lived three to four million years later. "It really closely resembles the thighbone structure of early hominids like Australopithecus, the species that [the well-known female specimen] 'Lucy' belongs to," Richmond said.

Donald Johanson, director of the Institute of Human Origins at Arizona State University and Lucy's discoverer, agreed. "I had occasion to see the material about five years ago in Nairobi, and I was struck by the similarities-particularly between the femur and Lucy's
femur," said Johanson, who was unaffiliated with the research. O. tugenensis also had a walking style shared by hominins, including Lucy, until early members of our own genus Homo developed a more modern gait about two million years ago.

Richmond's research also weighs in on another long-standing debate-where exactly does O. tugenensis reside on the human family tree? The fossil's discoverers, Martin Pickford and Brigitte Senut of the Coll├Ęge de France, have suggested that the species was a direct ancestor of the genus Homo, even though that genus doesn't appear in the fossil record until about two million years ago. If they are correct, hominins that lived from six million to two million years ago-including Lucy and the Australopithicines-were not ancestors of modern humans but merely a now-extinct branch of our family tree.

But Richmond's results, published in the journal Science March 21, 2008, contradict this claim. "Our analysis shows that these fossils resemble early hominin fossils more than they resemble Homo at two million years ago," he said.

Ian Tattersall, curator of the division of anthropology at the American Museum of Natural History, said that such a conclusion was not a surprise. "If you were going to predict what an early hominid would look like six million years ago, you'd say [it looks] much more like the
Australopithecines than like Homo," said Tattersall, who was unaffiliated with the research.

Another researcher believes that a comparison of O. tugenensis with older Miocene hominids could reveal that it's actually more like those older species-and was thus tree-dwelling. Tattersall of the American Museum of Natural History noted that this
adaptation would also be expected in any of the earliest human ancestors to walk.

"The early bipeds, like Australopithecines, were bipedal when they were on
the ground ... They were adept at climbing trees as well," he said.
http://news.nationalgeographic.com/news/2008/03/080320-biped-ancestor.html

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