Limb proportions and positional behavior: Revisiting the theoretical and empirical underpinnings for locomotor reconstruction in Australopithecus africanus

Major changes in body shape occurred during human evolution, but questions remain about body shape in australopiths. The present study investigates the specifics of the presumed relationships between limb indices and positional behavior underlying prior work that compared proportions among extant hominids in order to make inferences about extinct hominins. We find that although both intermembral index or ratio of diaphyseal and articular proportions distinguish humans from great apes, neigher correlates well with variation in the degree of arboreality in the locomotor repertoire of extant hominids. Brachial index and a ratio of diaphyseal and articular dimensions from the fore- and hindlimb, however, do correlate with degree of arboreality, and scale slightly positively allometrically within species in all extant taxa. These two observations are taken into consideration in a more nuanced interpretation of a reanalysis of articular-diaphyseal limb proportions in an expanded sample of the Sterkfontein postcrania. This study confirms previous findings that  Australopithecus africanus  had larger forelimb dimensions in relation to hindlimb dimensions than modern humans and  A. afarensis , similar to the patterns seen in extant apes, particularly western gorillas. However, data presented here suggest that interpreting a particular taxon as “human-like” or “ape-like” may be overly simplistic. Instead, while both  A. africanus  and  A. afarensis  were almost certainly committed bipeds that incorporated some arboreality into their locomotor repertoire,  A. africanus  apparently used a set of locomotor behaviors that was more distinct from that of  A. afarensis  than  Pan troglodytes  and  Gorilla gorilla  are from each other.