IMPLICATIONS OF LUMBOSACRAL VERTEBRAL MORPHOLOGY IN CATARRHINE PRIMATES FOR AUSTRALOPITHECINE SPINAL FUNCTION AND POSITIONAL BEHAVIOR

There is considerable debate about the capabilities of australopithecines for sustained bipedal posture and locomotion. Most work examining the form and commitment of bipedality in these Pliocene hominids has focused on the appendicular skeleton. Among mammals, however, variation in axial postcranial morphology is also strongly correlated with differences in positional behavior. This is especially true of the lumbosacral region of the vertebral column. Indeed, modern humans exhibit numerous musculoskeletal adaptations in the lumbosacral region for columnar stability and resisting stresses associated with habitual bipedality, suggesting that this anatomical region is particularly well suited for further investigation of early hominid positional behavior.

This paper presents the results of comparative allometric and morphological analyses of lumbar vertebrae and sacra from Australopithecus afarensis, A. africanus, and a large sample of modern catarrhine primates (cercopithecoid monkeys, apes, and humans). The fossil sample includes new, previously unstudied, A. africanus material (Stw-431) from Sterkfontein, South Africa. The results of the analyses reveal that australopithecines and humans uniquely share features of the neural arch and spinal curvature critical for frequent bipedal walking and standing. However, these early hominids have a high incidence of centrum pathology and differ from humans in relative size and proportions of lumbosacral centra, suggesting dissimilarities between modern and early hominids in spinal mechanics. These differences also indicate that evolutionary transformation to habitual bipedality occurred in a mosaic fashion, with lumbar lordosis and vertical stability initially more important than orthograde weight bearing.