Special issue: Australopithecus sediba --- Body Size and Proportions of Australopithecus sediba

Abstract

While the timing, duration, and nature of the shift is debated, human evolution ultimately involved an increase in body size relative to our Miocene ancestors, and a transition from a more arboreal to a more terrestrial way of life. This is reflected not only in the articular and muscular attachment morphology of the upper and lower limbs, but also in the relative proportions of the limb segments, in particular the relative lengths of the bones of the forearm (and to a lesser extent, those of the arm) contrasted with those of the lower limb. Specifically, from both fossil and extant hominoid morphology, it is evident that hominin evolution was characterized by an increase in lower limb length and joint size and a decrease in upper limb (particularly forearm) length and articular size. These shifts ap-pear to have been non-concurrent, with at least some of the increase in lower limb length antedating the decrease in antebrachium length. Well-preserved skeletal elements from the upper and lower limb of 1.98 Ma Australopithe-cus sediba from Malapa, South Africa, contribute to our understanding of this adaptive shift, as well as facilitating an investigation of the species’ body size. In terms of body size, Au. sediba appears to have been small for a Plio-Pleistocene hominin, with body mass estimates ranging from ca. 30–36kg. Morphology consistent with climbing and/or suspensory behavior has been documented in the upper limb and foot of Au. sediba. In addition, the lower limb does not appear to be as elongated as that of early Homo, suggesting that long-distance terrestrial bipedality was not a regular part of the species’ behavioral repertoire.