Endless forms most beau-toe-ful: evolution of the human hallux

Abstract

BackgroundThe adduction of the first pedal ray in humans, such that the hallux is incapable of functional opposability, is a major feature of the evolution of the hominin foot (e.g. Darwin 1872, Haeckel 1879, Latimer and Lovejoy 1990). While hallucal adduction facilitates obligate bipedalism, it inhibits but does not eliminate arboreal grasping ability. The angle of the hallux (as determined by the longitudinal axis of the first metatarsal) relative to the rest of the foot (as determined by the longitudinal axis of the second metatarsal) is a product of both hard-and soft-tissues. Given the failure of soft tissue to fossilize, direct evidence for hallucal angle evolution is scanty, and consensus has not emerged as to when and how the modern human condition of hallucal non-opposability evolved.\n\nMethodology/Principal FindingsAnalysis of a large sample (n = 331) of radiographs taken from the dorsal aspect of living human feet elucidates the relationship between osteological measures and the magnitude of hallucal adduction, which is resultant of both hard-and soft-tissue anatomy (Lovejoy et al. 2009). I describe the correlation of hallucal convergence with first metatarsal posterior articular facet morphology, which allows inference of hallucal convergence in the absence of the medial cuneiform. I report parameters of modern human hallucal convergence variation and offer insight into the hominin fossil record. I infer that the hallucal convergence of the recently reported specimen from Burtele (BRT-VP-2/73) falls within the range of living human variation, inconsistent with the interpretation that this hominin retained the ability to oppose its great toe for the purpose of arboreal locomotion (Haile-Selassie et al. 2012). Of the fossil hominin first metatarsals surveyed, all fall within the range of living human variation, consistent with previous research on medial cuneiform (McHenry and Jones 2006) and first metatarsal (Berillon 1999) morphology.\n\nConclusions/SignificanceThis study finds that the modern human condition of hallucal non-opposability was present in the genus Australopithecus. However, while the Burtele specimen (BRT-VP-2/73) falls within the range of living human variation, it displays a markedly divergent hallux compared to most living humans. This study suggests that, as in humans today, there was variation in hallucal divergence in Plio-Pleistocene hominins. Rather than using the terms opposability and the ambiguously synonymous grasping, I suggest here the term clamping (as in the motion performed by a clamp) be used to describe the less powerful and less dexterous ability of modern humans to place and hold a thin, narrow object between the great toe and second toe ray in a limited, albeit functional, manner.