Morphological and evolutionary insights into the keystone element of the human foot’s medial longitudinal arch

Abstract

AbstractThe evolution of the medial longitudinal arch (MLA) is one of the most impactful adaptations in the hominin foot that emerged with bipedalism. When and how it evolved in the human lineage is still unresolved. Complicating the issue, clinical definitions of flatfoot in livingHomo sapienshave not reached a consensus. Here we digitally investigate the navicular morphology ofH. sapiens(living, archaeological, and fossil), great apes, and fossil hominins and its correlation with the MLA. A distinctive navicular shape characterises livingH. sapienswith adult acquired flexible flatfoot, while the congenital flexible flatfoot exhibits a ‘normal’ navicular shape. AllH. sapiensgroups differentiate from great apes independently from variations in the MLA, likely because of bipedalism. Most australopith,H. naledi, andH. floresiensisnavicular shapes are closer to those of great apes, which is inconsistent with a human-like MLA and instead might suggest a certain degree of arboreality. Navicular shape of OH 8 and fossilH. sapiensfalls within the normal livingH. sapiensspectrum of variation of the MLA (including congenital flexible flatfoot and individuals with a well-developed MLA). At the same time,H. neanderthalensisseem to be characterised by a different expression of the MLA.