Ardipithecus ramidus ankle provides evidence for African ape-like vertical climbing in the earliest hominins

Abstract

The origin of the human lineage was catalyzed by bipedalism, but how this locomotor mode evolved is debated. We investigated the evolutionary context of human bipedalism by analyzing the morphology of the 4.4 million-year-old hominin talus attributed to Ardipithecus ramidus (ARA-VP-6/500-023). Our results demonstrate that ARA-VP-6/500-023 bears similarities to the tali of chimpanzees and gorillas, who are adapted to vertical climbing and terrestrial plantigrade quadrupedalism. Additionally, we identify the presence of derived features in ARA-VP-6/500-023 consistent with previous suggestions of an enhanced push-off mechanism in the foot of Ar. ramidus. Our observations of the human and ape fossil record are inconsistent with recently proposed models of human origins, which envision the last common ancestor of humans and chimpanzees as a generalized arboreal ape. Instead, our results strongly imply that humans evolved from an African ape-like ancestor, which directly narrows the range of explanations for the origin of our lineage.