OUR ancestors walked on two legs 6 million years ago, around the time the first hominids appeared.
The discovery comes from a CT scan of a femur of Orrorin tugenensis. About the size of a chimpanzee, Orrorin lived in Kenya shortly after the split between human and chimpanzee ancestors.
The scan by a team led by Robert Eckhardt of Pennsylvania State University in University Park and Brigitte Senut of the French Museum of Natural History in Paris reveals that the neck shaft of the femur was designed for upright walking.
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The neck shaft functions as a cantilevered beam in the hip joint that carries the weight of the head and body. In chimps, the outer cortex of the bone is uniformly thick to withstand the forces caused by moving on all four limbs.
Bipedal walking applies different forces, as the abductor muscles along the neck of each femur take on different functions. As a result, the outer cortex of the bone in humans is thicker on the lower side than on top (Science, vol 305, p 1450). In Orrorin the ratio is 3 to 1, close to the human ratio of at least 4 to 1, Eckhardt says.