Calcaneal robusticity in Plio-Pleistocene hominins: implications for locomotor diversity and phylogeny.

A key pedal adaptation to bipedality is a relatively large, weight-bearing calcaneus. The earliest evidence for a human-like, robust calcaneus is at 3.2 Ma in Australopithecus afarensis (A.L. 333-8, A.L. 333-55, A.L. 333-37) from Hadar, Ethiopia. Australopithecus sediba at 1.98 Ma from Malapa, South Africa displays a unique combination of primitive australopith features and more derived Homo-like features, but surprisingly is characterized by a gracile, chimpanzee-like calcaneus. The differences in calcaneal morphology suggest that these taxa differed in the frequency of arboreality and in the manner of foot function during terrestrial bipedal locomotion. This study examines calcaneal morphology in extant hominids (i.e., great apes and humans; N = 95) and fossil hominins (N = 5) to better understand the evolutionary development of calcaneal robusticity in early hominins. In particular, this study focuses on two additional fossil hominin calcanei that have not figured prominently in previous discussions of calcaneal robusticity: StW 352 and Omo 33-74-896. A measure of calcaneal robusticity was quantified as the ratio of calcaneal tuber cross-sectional area to calcaneal tuber length, which significantly differs between humans and non-humans using a sequential Bonferroni alpha adjustment for multiple comparisons. Additional multivariate analyses using Mosimann shape variables show that StW 352 and Omo 33-74-896 are more similar to Au. sediba in calcaneal tuber morphology than to Au. afarensis, suggesting that the latter taxon is better adapted for terrestrial bipedalism than at least some later species of Australopithecus. This finding implies the possibility of several complex evolutionary scenarios involving either multiple reversals in postcranial morphology in Australopithecus or the independent acquisition of adaptations to terrestrial bipedalism in Au. afarensis and Homo.