Three‐dimensional Functional Morphology of the Australopithecus afarensis Fourth Metatarsal

Abstract

The presence of an inflexible foot with human‐like arches has been debated for Australopithecus afarensis based on fossilized footprints, and has been recently inferred from the first complete fourth metatarsal discovered for this species (A.L. 333‐160, Ward et al. 2011. Science 331,750‐753). Initial interpretation of this fossil was made on a limited comparative sample using two‐dimensional metrics that did not capture key functionally relevant aspects of metatarsal form. To further explore the functional morphology of this fossil, we compared articular surface orientation and surface topography of A.L. 333‐160 with that of a larger sample Pan troglodytes (n=26), Gorilla gorilla (n=17), and Homo sapiens (n=31) using novel three‐dimensional measurements of in silico polygonal models of fourth metatarsals. Humans differ from both chimpanzees and gorillas in torsion, convexity and orientation of the base, as well as convexity of dorsal and plantar portions of the head. A.L. 333‐160 falls within the range of humans in all measures. Discriminant function analysis cleanly separates humans from apes, and demonstrates that Australopithecus afarensis groups only with humans. These data support the hypothesis that the Australopithecus afarensis foot shared the stiff midfoot and lateral arch characteristic of later hominins.