Bipedal Foot Morphology

Humans differ from all other primates in having nonopposable big toes (halluces). This greatly compromises arboreal abilities, since the foot cannot be used to grasp supports effectively. However, by aligning the hallux with the rest of the digits the connective tissues, tendons, ligaments and muscles of the foot are aligned to permit a longitudinal arch of the foot, integral for shock absorption during bipedal walking and running. Humans also have an osteoligamentous structure of the foot that tightens during loading to stiffen and stabilize the midfoot so that it becomes a rigid lever during push off in gait, whereas feet of apes can extend here, referred to as a ‘mid-tarsal break’, again compromising the ability of the foot to grasp variably oriented arboreal supports. Lastly, the human talocrural joint is characterized by the tibia being oriented orthogonal to the talocrural joint surface, whereas great apes have a verus angle in which the tibia is angled laterally, permitting the foot to be inverted so that the sole contacts the substrate when the animal is climbing. All hominins share this talocrural joint morphology, indicating that they have been selected to walk bipedally and sacrifice climbing ability, and furthermore that they would not have been able to climb trees like apes do. This is also evidenced by the non-divergent big toe. The humanlike pattern was clearly present by 1.5 million years ago as seen by a footprint trail from Ileret, Kenya. There are claims for a slightly divergent toe and less well-developed arches in the Laetoli 3.6 million year old Australopithecus afarensis footprints, and in the A. africanus bones of Stw 542 from Sterkfontein. However, closer analysis of both of these specimens indicates a fundamentally human pattern. Still, even with very slightly longer toes, they would not have been able to use the foot effectively in climbing, which would impact the ability to move in trees while hunting or fleeing predators, especially when carrying their young. If there there were differences, they were minor and do not undermine the fundamental fact that the foot of all hominins was dramatically reshaped by selection for bipedal performance at the expense of arboreal agility, showing the relative importance of these locomotor modes for the animals’ survival and reproductive success.