A comparative analysis of femoral morphology in Australopithecus afarensis: Implications for the evolution of bipedal locomotion

This study is a comparative analysis of the early hominin proximal femur that aims to detect morphological variation in this element and apply the findings to hypotheses of skeletal changes that accompany the evolution of bipedal locomotion. The particular emphasis is Australopithecus afarensis and several recently recovered femora of this species are formally described. With these new data, three research avenues related to femoral morphology are pursued. The first is to assess the degree of size and shape variation in the femoral sample of A. afarensis. The second is to compare the femoral morphology of Plio-Pleistocene hominin taxa. The third is to evaluate the morphology of the proximal femur that has been considered adaptive for bipedal locomotion to determine if it is associated with the evolutionary emergence of this locomotor pattern.; This study is based on linear measurements and three dimensional landmarks of the proximal femur obtained from Pongo, Gorilla, Pan, recent Homo and original fossils of A. afarensis, A. africanus, A/P. robustus, A/P. boisei, H. habilis and H. erectus. The collected data are analyzed using univariate, traditional multivariate, and geometric morphometric methods in order to characterize femoral morphology and its intra- and interspecific variation. Phylogenetic methods are used to consider the evolution of femoral morphology.; It is concluded that femora of A. afarensis vary considerably in size, but not in shape, and are similar in this regard to highly sexually dimorphic gorillas and orangutans. Significant variation in some aspects of anatomy is found among early hominins, yet all bipedal taxa share morphology, such as an elongated and tall femoral neck relative to the non-bipeds in this study. The identification of uniquely bipedal morphology helps to understand the morphological change in the proximal femur that led to the emergence of bipedality. In fact, as determined by a phylogenetic test, femoral morphology such as a superior position of the femoral head relative to the greater trochanter is first expressed in the initial bipedal ancestor. This may indicate that this morphology evolved in response to selection for bipedal locomotion.