| Habitual bipedal striding locomotion is a distinctive feature of hominins. The human fossil record, however, shows diversity in lower limb anatomy suggesting functional-mechanical differences in locomotion across these fossil taxa. Different forms of bipedality appear in the skeleton of australopithecines such as Australopithecus africanus, Australopithecus afarensis, and Australopithecus sediba. This study seeks to test the hypothesis that australopithecine lower limbs exhibit a mechanically different form of bipedality relative to modern humans, a character that is evident in lower limb flexibility; it is likely that leg flexibility allowed australopithecines to climb trees as well as walk bipedally. In order to test this hypothesis, eight skeletal indicators of lower limb flexibility were quantified in the following taxa: Australopithecus sediba, Australopithecus afarensis, Australopithecus africanus, Australopithecus robustus, Homo erectus, Homo habilis, Homo sapiens, Gorilla, Pan, Pongo, Hylobates, Ateles, and Cercopithecus. Taxa were scored on a leg flexibility gradient, giving an estimate of the arboreal/bipedal tendency of an individual. Each trait indicating lower limb flexibility (arboreality) received a value of "1"; each trait indicating lower limb stiffness (bipedality) received a value of "0"; and each trait that was neutral with regard to the two poles received value "0.5." Scores were added up and divided by the number of traits measured. The result, multiplied by 100, provides a measure of the estimated lower limb flexibility/arboreal tendency, as a percentage. Results indicate that A. sediba possessed greater lower limb flexibility and utilized a mechanically different form of bipedalism compared to humans. The knee morphology of A. sediba is particularly unique compared to both extant primates and other australopithecines, and likely acted as a stabilizing mechanism during hyperpronation while walking bipedally. Understanding whether australopithecines retained a significant amount of arboreal lower leg use while practicing bipedality has implications for inferring the selective pressures involved in the initial evolution of bipedality. |
