A unique form of locomotion in Swartkrans hominins: An analysis of the trabecular structure of the first metatarsal

Changes in foot bone morphology within the hominin clade are crucial for reconstructing the evolution of bipedalism and a modern human-like gait. Studies of the external morphology of the first metatarsal in humans, non-human apes and fossil hominins, have documented changes in its robusticity, epiphyseal shape and its articulation with the medial cuneiform. Trabecular bone structure has been shown to reflect habitual joint positioning, and as a result offers a promising method of interpreting first metatarsal loading in extant and fossil apes. In this study, microtomography is used to quantify the trabecular structure throughout the head and base of the first metatarsal, of a comparative sample of Homo sapiens (n=11), Pan troglodytes (n=10), Gorilla gorilla (n=10), and Pongo pygmaeus (n=6). Results from these analyses are then applied to two fossil hominin first metatarsals (SKX 5017 and SK 1813), the former being attributed to Paranthropus robustus, and the latter being of unassigned taxonomic status. Results show that within the comparative sample, bone volume fraction (BV/TV) and degree of anisotropy (DA) effectively separate bipedalism from all other forms of locomotion. Specific patterns in anisotropy and trabecular bone density distribution support the hypothesis that higher BV/TV in the dorsal regions of the bone and overall higher DA are reflective of a foot adapted for bipedalism. SKX 5017 shows patterns that are different from all modern taxa, indicating a unique form of bipedalism characterized by a habitually hyperdorsiflexing metatarsophalangeal joint and retained arboreal adaptations. SK 1813 shows a trabecular distribution in the head that is different from SKX 5017 and intermediate between modern human and non-human primates, indicating habitual but less frequent bipedalism than modern Homo sapiens and greater arboreal adaptations than SKX 5017. These results suggest that Swartkrans hominins employed habitual bipedalism, but also displayed a wider range of locomotor behaviour than modern humans.