Reconstructing early hominin locomotor repertoires remains critical to understanding human origins. The foot has played a prominent role in this research, and within the foot, the calcaneus is important because of its weight-bearing function. While hominid calcanei have been previously studied, entire calcaneus external shape and allometry have yet to be scrutinized. Here I analyze the relationships between the calcaneus and locomotor behavior, estimated body mass (EBM), and phylogeny in humans (n=130) and nonhuman primates (n=289). These results are used to put fossil calcanei attributed to Australopithecus sediba, A. africanus, A. afarensis, Homo naledi, and H. habilis/Paranthropus boisei into a locomotor context.

Calcanei were surface or micro-CT scanned. Linear measures and articular facet surface areas were collected on the surface models. Body mass was estimated using femoral head superoinferior breadth and established regression equations from the literature. Relationships between calcaneus dimensions and EBM were determined using phylogenetic least squares regression analyses and residuals relative to isometry were analyzed. External shape was examined using three-dimensional geometric morphometric (GM) sliding semilandmark analyses. Shape variation was summarized using principal components analysis and Blomberg’s K statistic was used to test for phylogenetic effects. Procrustes distances between taxon averages were calculated.

Most linear measures exhibit phylogenetic signal relative to EBM;​ however, phylogenetic signal is weak in GM data, indicating shape variation may be functionally driven. Humans exhibit mediolaterally wider calcanei, both relative to EBM and in comparisons of centroid-scaled calcanei. More arboreal great apes have relatively deeper cuboid facet pivot regions, which contribute to larger cuboid facet surface areas for EBM in these and serves to increase midfoot flexibility. This is also apparent within gorillas, with the most arboreal taxon exhibiting the deepest pivot region and the most terrestrial, the flattest. A. afarensis demonstrates the most human-like calcaneus, consistent with obligate bipedalism when terrestrial. H. naledi is modern human-like, but with traits suggesting a different form of bipedalism. A. africanus, A. sediba, and H. habilis/P. boisei calcanei possess unique combinations of human and nonhuman primate-like morphologies, indicating a combination of bipedal and arboreal behaviors.