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Hallucial Tarsometatarsal Joint Morphology and Function: A Three-dimensional Analysis of Medial Cuneiforms in Extant and Fossil Hominids Open Access

The human foot reflects an evolutionary commitment to obligate bipedalism and exhibits structural and functional characteristics that differentiate modern humans (Homo sapiens) from the extant great apes (Pongo, Gorilla, and Pan). This study reinvestigates anatomical change in the medial cuneiform, which forms the base of the grasping hallux in primates, using three-dimensional quantitative methods. The sample includes extant humans and great apes, KNM-RU 1656 (Proconsul), and various fossil hominins (Stw 573, OH 8, LB1, Tabun 1, Qafzeh 8 and 9, Skhul IV). Previous research has concluded, based on qualitative analyses, that modern human medial cuneiforms differ from those of the great apes due to the reorientation of the human hallux to an anatomical arrangement for obligate bipedalism. The main objective of this study is to quantitatively test the hypothesis that modern humans differ significantly from the great apes in medial cuneiform anatomy. Specifically, this study predicts that human and great ape medial cuneiforms differ significantly in the angles between the first metatarsal articular facet and all other articular facets, reflecting a more divergent great ape hallux versus a more adducted hallux in humans. This study also predicts significant differences between great apes and humans in joint angles not involving the facet for the first metatarsal, reflecting changes to the human medial cuneiform that have altered the anatomical and functional relationships this bone shares with the more proximally and laterally-placed bones in the foot. Finally, this study predicts that articular surface areas are significantly smaller in modern humans than in great apes due to the need for reduced range of motion among medial cuneiform joint surfaces in humans during bipedal locomotion. 3D medial cuneiform models were generated from laser and CT scanning methodologies, and the relative articular and non-articular surface areas, along with the angles between articular surfaces, were quantified and compared for each bone using previously published methods (Tocheri et al., 2003, 2005, 2007; Tocheri, 2007). Multivariate analyses of selected shape variables demonstrate that modern humans are clearly and significantly differentiated from the great apes and that the great ape genera are also differ significantly from each other. The observed differences exhibited by great apes and humans are consistent with hypotheses of pedal functional morphology that predict adaptation for extreme arboreality in Pongo, for a compromise in morphology that facilitates both arboreality and plantigrade walking in Pan and Gorilla, and for obligate bipedalism in Homo. Observed differences within Gorilla also reveal interesting morphological patterns which may indicate differing levels of adaptation to terrestriality among species in this genus. The data presented in this study establish a robust 3D comparative dataset for medial cuneiform shape patterns among great apes and humans which allows for comparisons with fossil hominin morphology, thereby providing an opportunity to address hallucial tarsometatarsal joint evolution within the hominin lineage specifically.

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