Acetabular orientation, pelvic shape, and the evolution of hominin bipedality.

Hominin pelvic form differs dramatically from that of other primates by having more laterally facing iliac blades, a wider sacrum, and a larger, transversely broad pelvic inlet. The orientation of the acetabulum may also differ, plausibly related to differences in load transmission during upright posture and habitual bipedal locomotion, which may, in turn, affect overall pelvic geometry. We compared acetabular orientation in humans, a phylogenetically broad sample of extant anthropoid primates, and fossil hominins including Australopithecus afarensis (A.L. 288-1, KSD-VP-1/1), Australopithecus africanus (Sts 14), Australopithecus sediba (MH2), and Homo neanderthalensis (Kebara 2). We measured the three-dimensional orientation of the acetabulum on in silico models of individual hipbones aligned to the median plane by registering models to landmark coordinates on articulated pelves. Humans and fossil hominins both possess significantly more ventrally opening acetabula than other extant anthropoids, which exhibit laterally facing acetabula. The orientation of the hominin acetabulum was essentially humanlike by at least 3.6 Ma, well before the appearance of other unique features in the pelvis of Homo that may be associated with long-distance walking or running, thermoregulation, parturition, and larger body size in this genus. These results suggest that the ventral orientation of the acetabulum is a key component in the suite of pelvic characteristics related to habitual bipedality in hominins and should be considered in future analyses of hominin pelvic morphology.