6-Million-Year-Old Human Ancestor 1st to Walk Upright?

Brian Handwerk
for National Geographic News
March 20, 2008

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.

(Related news: "Fossil Pushes Upright Walking Back 2 Million Years, Study Says" [September 2, 2004].)

Bones Like "Lucy's"

To figure out if the species was bipedal, Richmond and co-author William Jungers of Stony Brook University in New York measured telltale indicators of bipedalism, such as joint size and thighbone shaft strength, and compared them to other early hominin fossils, living apes, and bones from about 130 modern humans from around the world.

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.

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