(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.
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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