The Long Limb Bones of the StW 573 Australopithecus Skeleton from Sterkfontein Member 2: Descriptions and Proportions

Due to its completeness, the A.L. 288-1 (Lucy) skeleton has long served as the archetypal bipedal Australopithecus . However, there remains considerable debate about its limb proportions. There are three competing, but not necessarily mutually exclusive, explanations for the high humerofemoral index of A.L. 288-1: (1) a retention of proportions from an Ardipithecus -like most recent common ancestor (MRCA); (2) indication of some degree of climbing ability; (3) allometry. Recent discoveries of other partial skeletons of Australopithecus , such as those of A. sediba (MH1 and MH2) and A. afarensis (KSD-VP-1/1 and DIK-1/1), have provided new opportunities to test hypotheses of early hominin body size and limb proportions. Yet, no early hominin is as complete (>90%), as is the 3.67 Ma Little Foot (StW 573) specimen, from Sterkfontein Member 2. Here, we provide the first descriptions of its upper and lower long limb bones, as well as a comparative context of its limb proportions. As to the latter, we found that StW 573 possesses absolutely longer limb lengths than A.L. 288-1, but both skeletons show similar limb proportions. This finding seems to argue against an allometric explanation for the limb proportions of A.L. 288-1. In fact, our multivariate allometric analysis suggests that limb lengths of Australopithecus , as represented by StW 573 and A.L. 288-1, developed along a significantly different (p < 0.001) allometric scale than that which typifies modern humans and African apes. Our analyses also suggest, as have those of others, that hominin limb evolution occurred in two stages with: (1) a modest increase in lower limb length and a concurrent shortening of the antebrachium between Ardipithecus and Australopithecus , followed by (2) considerable lengthening of the lower limb along with a decrease of both upper limb elements occurring between Australopithecus and Homo sapiens .