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.
Highlights
The longitudinal arch is a unique human structure that helps store elastic energy [1] and maintains the structural rigor of the foot during the push-off stage of bipedal locomotion [2]
Fossil footprints from Ileret, Kenya provide strong evidence for a human-like arched foot by 1.53 Myrago [14]. These footprints are consistent with analysis of the pedal remains of Homo from Dmanisi suggesting, based on metatarsal torsion, the presence of a well-developed longitudinal arch [18]. It is in this context that we suggest another potential skeletal correlate of rearfoot arching in the hominin lineage: the tibial arch angle (Figure 1)
The most arboreal of the great apes, the orangutan (Pongo pygmaeus) has a statistically distinct tibial arch angle from the African apes (t = 4.80; p,0.001). It is in the opposite direction as expected, producing a less posteriorly directed set than that found in African apes (Figure 3)
Summary
The longitudinal arch is a unique human structure that helps store elastic energy [1] and maintains the structural rigor of the foot during the push-off stage of bipedal locomotion [2]. The longitudinal arch acts as a shock absorber, mitigating ground reaction forces generated during the foot flat stage of the gait cycle. Tibialis anterior, and the deep digital flexors. In 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. Paleoanthropologists have relied on footprints and bony correlates of arch development, though little consensus has emerged as to when the arch evolved
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