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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.

(Related facts: "What Was 'Lucy'? Fast Facts on an Early Human Ancestor" [September 20, 2006].)

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.

"The overall theme of upright walking seems to have stayed fairly consistent as a successful strategy for about four million years, which is most of our evolutionary history," lead author Richmond said.

Arizona State University's Johanson added: "This suggests that you don't immediately become a modern, efficient biped all at once. "As is so often the case in the evolution of any mammal, it happens in stages, and it's interesting to see the sequence of those events."

Choosing a Branch

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 Collège 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.

(Explore an interactive map of the human evolutionary highway.)

But Richmond's results, published in the journal Science tomorrow, 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.

"It is likely to be ancestral to these early forms, not requiring a ghost lineage that goes undiscovered for four million years [until Homo's appearance]."

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.

Aloft After All?

Terry Harrison, a biological anthropologist of the Center for the Study of Human Origins at New York University, studies hominins that predated O. tugenensis during the Miocene, 23.8 to 5.3 million years ago.

While he praised Richmond's thorough analysis, he 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.

"It does not make sense [to] interpret the anatomical features of O. tugenensis as a biped that could climb trees," he said.

"I see it as a good arboreal quadruped that has a package of features like [those found in] Australopithecus."

(Related story: "Upright Walking Started in Trees, Ape Study Suggests" [May 31, 2007].)

While Richmond is confident of his conclusions, he does agree that O. tugenensis, like other early human ancestors, was certainly capable of climbing.

"The upper limb looks very much like a chimp's does today," he said.

"That points to the idea that O. tugenensis still had a powerful upper limb used in climbing trees," he said. "It probably did that regularly to access food, to sleep at night, or to escape predators."

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.

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