Human ancestors started walking upright on two feet over seven million years ago, study suggests

Anatomy of thigh bone indicative of bipedalism of human ancestor species on land

Scientists have found that one of the earliest known human ancestors was walking on two legs as far back as seven million years ago, building on previous analyses that came to similar conclusions.

In the new research, published on Wednesday in the journal Nature, scientists assessed thigh and forearm fossils of the hominin Sahelanthropus tchadensis unearthed in Toros-Menalla in Chad in 2001.

The species of early hominin – an evolutionary group that includes modern humans and our extinct relatives – was dated to around seven million years ago.

A previous analysis of a nearly complete cranium at the time of discovery suggested that S. tchadensis may have been walking on two legs, which is considered a defining feature of hominins known as bipedalism.

Now an asessment of the arm and leg bones, recovered at the same time from the same site, have provided an opportunity to validate this hypothesis, say scientists, including Franck Guy from the Université de Poitiers in France.

In the new study, researchers present an 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.

They say the anatomy of the femur is indicative of bipedalism of S. tchadensis on land around seven million years ago, supporting earlier predictions made based on analysis of the cranial fossils.

The features of the ulnae also match traits that are characteristic of the adaptation to climb, albeit cautiously, scientists say.

They say the functional patterns in the forearm bones suggest S. tchadensis could clamber up and down trees, likely with some form of grasping and irregular movement of the limbs.

Considering the cranial and limb fossil analyses together, researchers say evidence points to these early humans developing the ability to walk on two legs soon after humans and chimpanzees diverged.

They say this was about the same time as retaining bone features that permitted climbing capabilities.