Biomechanical inferences about the origins of bipedal locomotion from ancient African femora

Reported here are the diaphyseal cross-sectional geometrical properties of two fossilized femora from Kenya dated at 6 Ma (million years ago), designated as the taxon Orrorin tugenensis. These femora are sampled at a temporal level near the divergence of ape and human lineages. The geometric properties of the bones are compared with previous findings from SK 82 and SK 97, hominid fossils from 2 million years later, and a sample of extant humans from Pecos Pueblo, N. M. The Orrorin femoral specimens BAR 1002'00 and BAR 1003'00 are notably robust, exhibiting cross-sectional areas with relatively high percentages of cortical bone in comparison to a contemporary sample. Total cross-sectional area is markedly smaller for BAR 1002'00 and BAR 1003'00 than for either of the Swartkrans early hominid proximal femora or the Pecos Pueblo sample, but %CA of the two Kenyan fossil femurs are comparable to those from Swartkrans and one third higher than the modern sample. Despite their relatively thicker cortical areas BAR 1002'00 and BAR 1003'00 do not exhibit higher bending resistance (I_max and I_min) or torsional rigidity (J) in compared to a modern human sample. These findings extend our knowledge of internal femoral structure related to the origins of human locomotion.