A controversial hominid that lived 7 million years ago may have walked on two legs after all, according to a new analysis of its fossilized bones.
After its discovery in 2001, Sahelanthropus tchadensis (nicknamed Toumai) was considered one of the earliest human ancestors, but some scientists argue that it's a more distant cousin, not a direct human antecedent.
Much of the debate centers on whether this primate habitually strolled around on two legs, or if it walked with the help of its hands as modern chimps and gorillas do.
Now, a study led by scientists at New York University claims to have settled the argument: Give your great, great (x infinity) grandmother S. tchadensis a kiss.
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"Our analysis of these fossils offers direct evidence that Sahelanthropus tchadensis could walk on two legs, demonstrating that bipedalism evolved early in our lineage and from an ancestor that looked most similar to today's chimpanzees and bonobos," says Scott Williams, anthropologist at New York University.
The researchers reached this conclusion by conducting 3D geometric analyses of the arm and leg bones of the creature, and comparing them to the same bones in related species, both living and extinct.
They claim to have found three key features that indicate bipedalism. For one, they found a twist in the femur that helps the legs point forward and makes walking easier. Second, S. tchadensis seems to have had prominent buttock muscles, important for keeping the hips stable.
Both of these features have been identified in previous work by other scientists. But the smoking gun of the new work, according to the team, was the discovery of a femoral tubercle.
This is a kind of anchor point for a powerful ligament that connects the pelvis and the femur – vital for bipedalism and known only in hominins.
That's not to say that S. tchadensis had completely given up its tree-climbing heritage, however.
"Sahelanthropus tchadensis was essentially a bipedal ape that possessed a chimpanzee-sized brain and likely spent a significant portion of its time in trees, foraging and seeking safety," says Williams.
"Despite its superficial appearance, Sahelanthropus was adapted to using bipedal posture and movement on the ground."
The research was published in the journal Science Advances.