Elliptical Fourier analysis of hominoid radius shape: implications for Ardipithecus ramidus

The evolution of bipedalism in the hominin lineage remains a controversial topic. The recovery of skeletal material from Aramis, the Middle Awash Project study area in Middle Awash, Afar Regional State, Ethiopia, has the potential to elucidate the transition to terrestrial bipedalism. The 4.4-million-year-old hominin Ardipithecus ramidus (ARA-VP-6/500) is represented by a relatively complete skeleton, including a complete radius. Its describers argued that it lacked features associated with suspensory behaviors, vertical climbing, and knuckle-walking. To test this hypothesis, I collected a comparative sample of radii comprising of Homo sapiens (n=27), six species of extant apes (n=96), two species of cercopithecoids (n=31), and two fossil hominins, and quantified whole bone shape using elliptical Fourier analysis (EFA). Dorsal radial morphology effectively partitions taxa by size and locomotion. The radii of knuckle-walking chimpanzees, and particularly gorillas, retain robust epiphyses and high degrees of lateral curvature, in contrast to other species. The robusticity and unique, directional curvature observed in the African ape radius may be related to knuckle-walking. The radius of ARA-VP-6/500 exhibits distinct characteristics among hominins, falling exclusively within gorilla morphospace. Although Ar. ramidus postcrania were proposed to lack features indicative of an ancestry involving knuckle-walking, vertical climbing, and suspensory behavior, this study instead contributes to growing lines of evidence suggesting that humans likely evolved from a knuckle-walking ancestor.