Anthropologists can spend a lot of time arguing over a single bone – and if that bone is one of the few known from an early ancestor, the arguments will be all the fiercer.

When a “big question” is at issue, such as when our ancestors evolved the upright stance that sets us apart from the knuckle-walking apes, the stakes get higher still. The latest ding-dong in the anthropology world centres around the oldest thigh bone in the human lineage, that of the six-million-year-old Orrorin tugenensis.

Discovered in 2000, initial reports claimed Orrorin was bipedal, but not surprisingly, many anthropologists were not convinced by an argument advanced on the basis of a single, incomplete bone.

Now Brian Richmond of George Washington University in Washington, DC and William Jungers of Stony Brook University in New York have gained access to the Orrorin fossils in Kenya, and measured the shape of the thigh bone, which reveals posture. Comparisons with thigh bones of other fossils, and of modern great apes, suggest that Orrorin was bipedal, they say.


Its thigh bone strongly resembles those of ancestors of the Homo lineage, Australopithecus and Paranthropus, upright walkers which lived between about 4 and 2 million years ago, but differs from those of modern great apes and the genus Homo. This contradicts earlier reports that said Orrorin was more closely related to Homo than to Australopithecus, but supports the idea that it was bipedal.

“I had expected Orrorin to look more primitive,” says Richmond.

Orrorin retained chimp-like arms and powerful fingers that let it climb easily into trees. But its bipedal gait “made possible a new way of living in the environment, not used by other apes, which might have led to the great success of our lineage,” says Richmond.

He thinks Orrorin‘s particular walking style remained dominant for 4 million years, until the genus Homo evolved a stride that was better for long-distance walking and running.

Lining up with Richmond, anthropologist Dan Lieberman, of Harvard University, agrees that Orrorin was bipedal. “This is a great paper,” he says.

The discoverers of Orrorin had suggested it was ancestral to Homo, but not to the australopithecines, which were thought to have evolved from a separate lineage. But Richmond puts Orrorin near the base of a single lineage that led first to the australopithecines and then Homo.

Yet Tim White, an anthropologist from the University of California, Berkeley, says that although Orrorin‘s thigh bone is “the best current evidence” for bipedalism six million years ago, even the new study gives only a hazy picture of how Orrorin walked compared to what we know about its descendants from their more abundant fossils.

Journal reference: Science, vol 319, p 1662

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