Lucy's Flat Feet: The Relationship between the Ankle and Rearfoot Arching in Early Hominins

Abstract

BackgroundIn the Plio-Pleistocene, the hominin foot evolved from a grasping appendage to a stiff, propulsive lever. Central to this transition was the development of the longitudinal arch, a structure that helps store elastic energy and stiffen the foot during bipedal locomotion. Direct evidence for arch evolution, however, has been somewhat elusive given the failure of soft-tissue to fossilize. Paleoanthropologists have relied on footprints and bony correlates of arch development, though little consensus has emerged as to when the arch evolved.Methodology/Principal FindingsHere, we present evidence from radiographs of modern humans (n = 261) that the set of the distal tibia in the sagittal plane, henceforth referred to as the tibial arch angle, is related to rearfoot arching. Non-human primates have a posteriorly directed tibial arch angle, while most humans have an anteriorly directed tibial arch angle. Those humans with a posteriorly directed tibial arch angle (8%) have significantly lower talocalcaneal and talar declination angles, both measures of an asymptomatic flatfoot. Application of these results to the hominin fossil record reveals that a well developed rearfoot arch had evolved in Australopithecus afarensis. However, as in humans today, Australopithecus populations exhibited individual variation in foot morphology and arch development, and “Lucy” (A.L. 288-1), a 3.18 Myr-old female Australopithecus, likely possessed asymptomatic flat feet. Additional distal tibiae from the Plio-Pleistocene show variation in tibial arch angles, including two early Homo tibiae that also have slightly posteriorly directed tibial arch angles.Conclusions/SignificanceThis study finds that the rearfoot arch was present in the genus Australopithecus. However, the female Australopithecus afarensis “Lucy” has an ankle morphology consistent with non-pathological flat-footedness. This study suggests that, as in humans today, there was variation in arch development in Plio-Pleistocene hominins.

A complete second metatarsal (StW 89) from Sterkfontein Member 4, South Africa

“…This reconstruction of the foot would be consistent with seemingly contradictory studies finding evidence for an arch in australopiths using bones found in the lateral column (e.g., DeSilva, 2010;Ward et al, 2011), but the absence of an arch in the medial column (e.g., Berillon, 2003;Harcourt-Smith and Aiello, 2004). However, we caution about over-generalizing the arch given the range of variation in arch development in humans today and evidence that such variation may have existed in australopiths as well (DeSilva and Throckmorton, 2010). Thus, the low medial arch may characterize the foot of StW 89, but not necessarily australopiths in general.…”

Human-like external function of the foot, and fully upright gait, confirmed in the 3.66 million year old Laetoli hominin footprints by topographic statistics, experimental footprint-formation and computer simulation

“…However, a recent paper [51] argues that the form of a new fourth metatarsal of A. afarensis indicates, contrariwise, that this species did indeed have a medial longitudinal arch and lacked a mid-tarsal break. Equally, however, another recent contribution [57] argues that the available footbones of the 'Lucy' AL-288-1 individual of A. afarensis suggest that this individual was asymptomatically flat-footed, noting correctly that modern humans are highly variable in medial arch height. We may legitimately conclude from this debate that we do not yet understand the relationship Early hominin foot function at Laetoli R. H. Crompton et al 715 between the bony morphology of the midfoot and its internal kinematics and external function [1].…”

Reevaluating the functional implications of Australopithecus afarensis navicular morphology

“…The human foot is uniquely characterized by an energy saving, spring-like longitudinal arch that reflects an adaptation to terrestrial walking and running (Ker et al, 1987;Bramble and Lieberman, 2004;DeSilva and Throckmorton, 2010;Ward et al, 2011;Prang, 2015b;Stearne et al, 2016). The presence of a longitudinal arch is reflected in the geometric relationships among the bones of the human foot and ankle (DeSilva and Throckmorton, 2010;Ward et al, 2011;Prang, 2015b), including the declination of the talar head relative to the plane of the talocrural joint (Day and Wood, 1968;Peeters et al, 2013) and the declination of the calcaneocuboid joint relative to its proximodistal axis (Aiello and Dean, 1990;Prang, 2015b).…”

“…The presence of a longitudinal arch is reflected in the geometric relationships among the bones of the human foot and ankle (DeSilva and Throckmorton, 2010;Ward et al, 2011;Prang, 2015b), including the declination of the talar head relative to the plane of the talocrural joint (Day and Wood, 1968;Peeters et al, 2013) and the declination of the calcaneocuboid joint relative to its proximodistal axis (Aiello and Dean, 1990;Prang, 2015b). Humans also possess soft tissue specializations associated with the longitudinal arch, such as a well-developed plantar aponeurosis, calcaneonavicular ligament, and long plantar ligament (Gomberg, 1981(Gomberg, , 1985, which all contribute to midtarsal stabilization via the 'windlass mechanism' (Hicks, 1954; reviewed by Griffin et al, 2015).…”