The story of early human evolution centers largely on the adaptation of bipedality. “The hallux clearly plays so fundamental a role in primate locomotion that any relevant fossil specimen would have great significance for the assessment of locomotor patterns in Pliocene hominids” (Latimer and Lovejoy, 1990: 125). The hallux, or first toe, consists of the first metatarsal and the proximal and distal phalanges (White and Folkens, 2000). The most important indication of bipedality in this digit is the hallucal tarsometatarsal joint (the joint at which the first metatarsal articulates with the medial cuneiform).
Though significant fossil evidence of Australopithecus afarensis has been documented, controversy over interpretation of the species’ anatomical adaptations, and thus its locomotor patterns, still exist. This work addresses an Australopithecus afarensis infant first metatarsal from the A.L. 333 site in Hadar, Ethiopia, never before described nor accessioned to the Hadar hominid catalogue.
The fossil was compared to the first metatarsals of modern human and chimpanzee infants, and, in measurements where these groups differed, the fossil was found to be morphologically similar to humans. This anatomy suggests the potential for fully modern bipedal locomotion, which implies profound behavioral changes by 3.5 million years ago. This provides further evidence of obligate bipedality in Australopithecus afarensis.