All early (Pliocene-Early Pleistocene) hominins exhibit some differences in proximal femoral morphology from modern humans, including a long femoral neck and a low neck-shaft angle. In addition, australopiths (Au. afarensis, Au. africanus, Au. boisei, Paranthropus boisei), but not early Homo, have an "anteroposteriorly compressed" femoral neck and a small femoral head relative to femoral shaft breadth. Superoinferior asymmetry of cortical bone in the femoral neck has been claimed to be human-like in australopiths. In this study, we measured superior and inferior cortical thicknesses at the middle and base of the femoral neck using computed tomography in six Au. africanus and two P. robustus specimens. Cortical asymmetry in the fossils is closer overall to that of modern humans than to apes, although many values are intermediate between humans and apes, or even more ape-like in the midneck. Comparisons of external femoral neck and head dimensions were carried out for a more comprehensive sample of South and East African australopiths (n = 17) and two early Homo specimens. These show that compared with modern humans, femoral neck superoinferior, but not anteroposterior breadth, is larger relative to femoral head breadth in australopiths, but not in early Homo. Both internal and external characteristics of the australopith femoral neck indicate adaptation to relatively increased superoinferior bending loads, compared with both modern humans and early Homo. These observations, and a relatively small femoral head, are consistent with a slightly altered gait pattern in australopiths, involving more lateral deviation of the body center of mass over the stance limb.

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Past studies of human locomotor efficiency focused on movement over flat surfaces and concluded that Neandertals were less efficient than modern humans due to a truncated limb morphology, which may have developed to aid thermoregulation in cold climates. However, it is not clear whether this potential locomotor disadvantage would also exist in nonflat terrain. This issue takes on added importance since Neandertals likely spent a significant proportion of their locomotor schedule on sloped, mountainous terrains in the Eurasian landscape. Here a model is developed that determines the relationship between lower limb segment lengths, terrain slope, excursion angle at the hip, and step length. The model is applied to Neandertal and modern human lower limb reconstructions. In addition, for a further independent test that also allows more climateterrain cross comparisons, the same model is applied to bovids living in different terrains and climates. Results indicate that: (1) Neandertals, despite exhibiting shorter lower limbs, would have been able to use similar stride frequencies per speed as longer-limbed modern humans on sloped terrain, due to their lower crural indices; and (2) shortened distal limb segments are characteristic of bovids that inhabit more rugged terrains, regardless of climate. These results suggest that the shortened distal lower limb segments of Neandertals were not a locomotor disadvantage within more rugged environments.