The African ape-like foot of <i>Ardipithecus ramidus</i> and its implications for the origin of bipedalism.

Abstract

The ancestral condition from which humans evolved is critical for understanding the adaptive origin of bipedal locomotion. The 4.4 million-year-old hominin partial skeleton attributed to <i>Ardipithecus ramidus</i> preserves a foot that purportedly shares morphometric affinities with monkeys, but this interpretation remains controversial. Here I show that the foot of <i>Ar. ramidus</i> is most similar to living chimpanzee and gorilla species among a large sample of anthropoid primates. The foot morphology of <i>Ar. ramidus</i> suggests that the evolutionary precursor of hominin bipedalism was African ape-like terrestrial quadrupedalism and climbing. The elongation of the midfoot and phalangeal reduction in <i>Ar. ramidus</i> relative to the African apes is consistent with hypotheses of increased propulsive capabilities associated with an early form of bipedalism. This study provides evidence that the modern human foot was derived from an ancestral form adapted to terrestrial plantigrade quadrupedalism.