When did primates first walk on two legs? Was it 4 million years ago, when Australopithecus afarensis waddled through the mud in eastern Africa? Or was it 6 million years ago, when a chimpanzee-size primate, Orrorin tugenensis, roamed the Tugen Hills of what is now Kenya?
Paleontologists Martin Pickford and Brigitte Senut of the National Museum of Natural History in Paris have long believed the shape of three fragmented femur fossils from Orrorin skeletons shows that the animals walked upright, a trait strongly associated with primate ancestors of modern humans. Their argument was backed up this year by CT scans by Penn State’s Karol Galik, a mechanical engineer, and Robert Eckhardt, an evolutionary biologist. Eckhardt maintains that a thinner bone at the top of one femur provides evidence Orrorin had adapted to the physical stress of upright posture.
Some scientists contend the scans were at too low a resolution to push back the first appearance of bipedalism 2 million years. If upheld, however, the findings could shake the human family tree to its roots. Pickford and Senut believe that Orrorin had already evolved more humanlike femurs and teeth than A. afarensis, a species that includes the famous skeleton Lucy. And that might make Lucy just an evolutionary side branch, not a human ancestor.
Even so, Eckhardt says it may be premature to disown Lucy and others like her. “Many of these hominids may have been part of some widely distributed group of early humans that were not separate lineages,” he says. “In all likelihood they were capable of exchanging genes.”