Rethinking the evolution of the human foot: insights from experimental research
Adaptive explanations for modern human foot anatomy have long fascinated evolutionary biologists because of the dramatic differences between our feet and those of our closest living relatives, the great apes. Morphological features, including hallucal opposability, toe length and the longitudinal arch, have traditionally been used to dichotomize human and great ape feet as being adapted for bipedal walking and arboreal locomotion, respectively. However, recent biomechanical models of human foot function and experimental investigations of great ape locomotion have undermined this simple dichotomy. Here, we review this research, focusing on the biomechanics of foot strike, push-off and elastic energy storage in the foot, and show that humans and great apes share some underappreciated, surprising similarities in foot function, such as use of plantigrady and ability to stiffen the midfoot. We also show that several unique features of the human foot, including a spring-like longitudinal arch and short toes, are likely adaptations to long distance running. We use this framework to interpret the fossil record and argue that the human foot passed through three evolutionary stages: first, a great ape-like foot adapted for arboreal locomotion but with some adaptations for bipedal walking; second, a foot adapted for effective bipedal walking but retaining some arboreal grasping adaptations; and third, a human-like foot adapted for enhanced economy during long-distance walking and running that had lost its prehensility. Based on this scenario, we suggest that selection for bipedal running played a major role in the loss of arboreal adaptations.