古生物学：初期人類が二足歩行を始めた時

One of the earliest known hominins — Sahelanthropus tchadensis — was walking on two legs as far back as seven million years ago, according to the analyses of thigh and forearm fossils presented in Nature. These findings build on previous analyses that came to similar conclusions.

The discovery of numerous fossils in Toros-Menalla in Chad in 2001 led to the naming of a new species of early hominin (the taxonomic group that includes extant humans and our extinct relatives) known as Sahelanthropus tchadensis. This species was dated to around seven million years ago. Analyses of a nearly complete cranium at the time of discovery suggested that S. tchadensis may have been walking on two legs — a defining feature of hominins known as bipedalism. Previously undescribed arm and leg bones, recovered at the same time from the same site, provide an opportunity to validate this hypothesis.

Guillaume Daver, Franck Guy and colleagues present the analysis of a left thigh (femur) bone and a pair of forearm (ulnae) bones from the original site of discovery of the S. tchadensis fossils in 2001. The authors reveal that the anatomy of the femur is indicative of bipedalism of S. tchadensis on land around seven million years ago, supporting predictions from cranial evidence. Additionally, the authors highlight that features of the ulnae matched traits that are characteristic of the adaptation to climb, albeit cautiously. For example, functional patterns in the ulnae suggest that S. tchadensis could clamber up and down trees, likely with some form of grasping and irregular movement of the limbs.

The authors conclude that, taken together, this evidence suggests that early humans evolved the ability to walk on two legs soon after humans and chimpanzees diverged, at the same time as retaining bone features that permitted climbing capabilities.