The earliest skeletal traces of bipedalism come from the fossils from Lukeino, in the Tugen Hills of western Kenya. A single isolated molar was found here in 1975 by Martin Pickford. Renewed surveys beginning in 2000 recovered additional specimens, including three proximal femora, a distal humerus with shaft, and most of a mandible. The deposits date to approximately 6 million years ago (Senut et al., 2001).
Human and chimpanzee femora are mostly similar on their proximal ends. They both have femur necks of about the same relative length, and both have angles between the neck and the shaft of between 110 and 130 degrees (Aiello and Dean 1990). This means that fossil remains of proximal femora are not the most obvious indicators of bipedal locomotion.
But a few telling differences divide the two species. The most functionally significant difference is the distribution of cortical bone in the femur neck. The long bones have dense layers, called cortical bone, that make up the shaft and the surfaces of the proximal and distal ends. Much of the proximal and distal ends of the bones are, however, made up of thin plates of bone arranged in a spongy texture, called trabecular bone. The distribution of cortical bone compared to trabecular bone in the femur neck reflects the way that force was habitually applied to the bone during life. In a biped, the cortical bone is thicker at the bottom of the femur neck than at the topbecause while the mass of the body applies bending force to both the top and bottom of the femur neck, the bending at the top is reduced by the pull of large muscles that support the body. In non-bipeds like chimpanzees, the cortical bone is equally thick on both the top and bottom of the femur neck (Lovejoy 1988). The difference between these thicknesses on the Lukeino femora shows they were bipedal, similar to those of later known hominid fossils (Galik et al. 2004).