Earliest evidence of hominin bipedalism in <i>Sahelanthropus tchadensis</i>.

Abstract

Bipedalism is a key adaptation that differentiates hominins (humans and our extinct relatives) from living and fossil apes. The earliest putative hominin, <i>Sahelanthropus tchadensis</i> (~7 million years old), was originally represented by a cranium, the reconstruction of which suggested to its discoverers that <i>Sahelanthropus</i> carried its head in a manner similar to known bipedal hominins. Recently, two partial ulnae and a femur shaft were announced as evidence in support of the contention that <i>Sahelanthropus</i> was an early biped, but those interpretations have been challenged. Here, while we find that both limb bones are most similar in size and geometric morphometric shape to chimpanzees (genus <i>Pan</i>), we demonstrate that their relative proportion is more hominin-like. Furthermore, we confirm two features linked to hominin-like hip and knee function and identify a femoral tubercle, a feature only found in bipedal hominins. Our results suggest that <i>Sahelanthropus</i> was an early biped that evolved from a <i>Pan</i>-like Miocene ape ancestor.