Bone strength and athletic ability in hominids: \textit{Ardipithecus ramidus} to \textit{Homo sapiens}

The ability of the femur to resist bending stresses is determined by its midlength cross-sectional geometry, its length and the elastic properties of the mineral part of the bone. The animal's athletic ability, determined by a ``bone strength index,'' is limited by this femoral bending strength in relation to the loads on the femur. This analysis is applied to the fossil record for \textit{Homo sapiens, Homo neanderthalensis, Homo erectus, Homo habilis, Australopithecus afarensis} and \textit{ Ardipithecus ramidus}. Evidence that the femoral bone strength index of modern \textit{Homo sapiens} has weakened over the last 50,000 years is found.