Metatarsophalangeal joint function and positional behavior in Australopithecus afarensis

Recent discussions of the pedal morphology of Australopithecus afarensis have led to conflicting interpretations of australopithecine locomotor behavior. We report the results of a study using computer aided design (CAD) software that provides a quantitative assessment of the functional morphology of australopithecine metatarsophalangeal joints. The sample includes A. afarensis, Homo sapiens, Pan troglodytes, Gorilla gorilla, and Pongo pygmaeus. Angular measurements of the articular surfaces relative to the long axes of the metatarsals and phalanges were taken to determine whether the articular surfaces are plantarly or dorsally oriented. Humans have the most dorsally oriented articular surfaces of the proximal pedal phalanges. This trait appears to be functionally associated with dorsiflexion during bipedal stride. Pongo has the most plantarly oriented articular surfaces of the proximal pedal phalanges, probably reflecting an emphasis on plantarflexion in arboreal positional behaviors, while the African hominoids are intermediate between Pongo and Homo for this characteristic. A. afarensis falls midway between the African apes and humans. Results from an analysis of metatarsal heads are inconclusive with regard to the functional morphology of A. afarensis. Overall, the results are consistent with other evidence indicating that A. afarensis was a capable climber.