Rethinking the evolution of the human foot: insights from experimental research

Abstract

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.

Assessing patterns of variation in BV/TV in the calcaneus and C2 vertebra of Gorilla gorilla, Pan troglodytes, and populations of Homo sapiens from the Pleistocene and Holocene that differ in physical activity levels

“…Although great apes sometimes land with the heel and forefoot simultaneously when walking, they often heel strike like humans when walking producing an impact peak (Schmitt & Larson, 1995; Vereecke, D'Août, De Clercq, Van Elsacker, & Aerts, 2003). As a result, in terms of impact, chimpanzees and gorillas almost certainly experience generally lower impact forces in the heel than humans (Holowka & Lieberman, 2018).…”

Modeling uniquely human gene regulatory function in humanized mice

“…In humans, this is due in part to the shortening of digits 2-5, which correlates with opposability and dexterity of the hand (Almécija et al, 2015). Differences between human and great ape feet include shorter digits with a non-opposable hallux, longer heel bones, and unique transverse and longitudinal arches defined by relative metatarsal-tarsal joint orientation in humans (Holowka and Lieberman, 2018;Holowka et al, 2017). The changes in limb development that confer human-specific morphological outcomes therefore involve discrete modifications to the proportions and positioning of ancestral skeletal structures.…”

Functional Anatomy, Biomechanical Performance Capabilities and Potential Niche of StW 573: an Australopithecus Skeleton (circa 3.67 Ma) From Sterkfontein Member 2, and its significance for The Last Common Ancestor of the African Apes and for Hominin Origins

“…Human feet as a whole are highly plastic and functionally degenerate, and as shown by Venkataraman et al (2013b) and Kraft et al (2014), they are perfectly capable of functioning efficiently in climbing as well as terrestrial bipedal walking and running, having unquestionably retained a prehensile (if relatively adducted) hallux (see e.g. Figures 25 and 26), contra Holowka and Lieberman (2018). The high human death rates from falls from trees of less than 20 m. quoted by Venkataraman et al (2013b) are a clear indication that, even were plasticity and degeneracy insufficient, selection would certainly favour retention of hallucal prehension in any human population engaging in barefoot climbing (common in human childhood).…”