The first human ancestor could have walked on two legs

Fossil belonging to ancient hominid that lived seven million years ago bears signs of bipedalism, new study finds

By Cody Cottier edited by Claire Cameron

Besides our large brains, the trait that most distinguishes humans from other animals is our ability to walk entirely upright on two legs, a style of movement without parallel in the animal kingdom. But exactly when our ancient ancestors The evolution of this trait has been a mystery – until now. A new analysis of fossils suggests that the first known hominid had begun to develop adaptations for bipedalism.

Sahelanthropus tchadensis lived in north-central Africa seven million years ago, just as the hominid lineage split from that of our closest animal relatives, chimpanzees and bonobos. When anthropologists discovered the first Sahelanthrope skull fragments in Chad in 2001, they I immediately wondered if it was bipedal, the hole at the base of its skull where the spinal cord would have entered seemed well placed to support its head, as in other bipeds. But with only a partial skull, there wasn’t much that could be done.

Researchers later realized that a femur found next to the skull fragments belonged to the hominidbut when it was first analyzed, researchers found no evidence of bipedalism. These results, published in 2020contradicted the previous hypothesis and raised doubts whether the species should be considered a species hominid at all. “The world is currently divided on how to interpret these fossils,” says Scott Williams, a paleoanthropologist at New York University who co-authored the new analysis but was not involved in the 2020 study.

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Williams and his team’s work, published today in Scientific advancesonce again reverses the narrative. Using three-dimensional geometric morphometry, a method that allows anthropologists to quantify the shape of fossils, he and his colleagues identified the rudimentary shapes of several anatomical features essential for bipedalism in later hominids, since Australopithecus to modern humans.

Two of these features have been reported in previous work: the femur is twisted inward and there is a small protrusion where the gluteus maximus would have attached. In 2022, a team led by Guillaume Daver and Franck Guy, paleoanthropologists from the University of Poitiers in France, used these characteristics as a basis. argue that Sahelanthrope was a “usual” biped. (We, as “obligate” bipeds, have no choice but to walk upright.)

But Williams found a third, subtle clue. One day, while rubbing his thumb along the femur, he felt a small bump where the iliofemoral ligament – ​​a key stabilizer of bipedal movement – ​​would attach to this bone in humans. “I was super excited about it,” he says. “It’s there; it’s just hard to see.” Williams informed Daver and Guy, who independently confirmed the existence of this femoral tubercle.

Not everyone is convinced. Marine Cazenave, paleoanthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, co-author a rebuttal last year in Daver and Guy’s 2022 paper, says the new study offers only “weak evidence” for bipedalism. Some non-bipedal primates have femurs twisted inward, she says. As for the femoral tubercle, Cazenave affirms that its function is poorly known, adding that the “poorly preserved conditions” of the fossil make it “impossible to know the real extent of this particularity”.

Regardless, Williams says, Sahelanthrope “definitely depended on trees.” This is where he would have looked for food, slept and sought refuge. But on the field, Williams is convinced he walked on two legs, using his hands to carry food. Given the few fossils available, it is difficult to be sure. Daver and Guy plan to return to the original site later this year in hopes of finding something extra that others might have missed. “Closing the debate,” they said in a joint statement, “would require the discovery of new remains.”