The earliest human ancestors probably walked on two legs

The earliest human ancestors probably walked on two legs

By Cody Cottier Edited by Claire Cameron

Apart from our large brains, the characteristic that distinguishes humans from other animals is our ability to walk completely upright on two legs, a phenomenon that has no parallel in the animal kingdom. But exactly when this trait evolved in our ancient ancestors – it was a mystery until now. A new fossil analysis Turns out the earliest known hominins began to develop adaptations for bipedalism.

Sahelanthropus tchadensis Lived in north-central Africa seven million years ago, just about the time the hominin lineage split from our closest animal relatives, chimpanzees and bonobos. When anthropologists first discovered it sahelanthropus Skull fragments in Chad in 2001, they immediately surprised Was it supposed to walk on two legs – the hole at the base of its skull where the spinal cord entered, like the other two legs, seemed well-positioned to carry its head. But with only a partial skull, there was not much to go on.

Researchers later realized that the femur found with the skull fragments belonged to a hominin, but when it was first analyzed, researchers found no evidence of bipedalism. Those findings, published in 2020This refuted earlier hypotheses and cast doubt on whether this species should be considered a hominin. “The field is still divided on how to interpret these fossils,” says Scott Williams, a paleontologist at New York University who co-authored the new analysis but who was not involved in the 2020 study.

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The work of Williams and his team, published today science advancementTurns the narrative upside down once again. Using three-dimensional geometric morphometrics – a method that allows anthropologists to measure the shape of fossils – he and his colleagues identified rudimentary forms of several physical characteristics that are important for bipedalism in later hominins. australopithecus For modern humans.

Two of these features were reported in previous work: the femur is curved inward, and there is a small bulge where the gluteus maximus would have attached to it. In 2022, a team led by paleontologists Guillaume Davor and Frank Guy of the University of Poitiers in France used these characteristics as a basis. to debate that sahelanthropus He was a “habitual” two-legged person(We, as “obligate” bipeds, have no choice but to walk upright,)

But Williams found a subtle third clue. While rubbing his thumb on his femur one day, he felt a small lump exactly where the iliofemoral ligament – ​​a major stabilizer for walking on two legs – attaches to that bone in humans. “I was very excited about it,” he says. “It’s there; it’s hard to see.” Williams informed Dever and Guy, who independently confirmed the existence of this femoral tubercle.

Not everyone is convinced. Marine Cazenave, a paleontologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who co-authored a rebuttal Last year, Dever and Guy’s 2022 paper said the new study offered only “weak evidence” for bipedalism. Some non-bipedal primates have thighs that curve inward, she says. As for the femoral tubercle, Cazenaway says its function is poorly understood, adding that the fossil’s “badly preserved conditions” make it impossible to know the true extent of this feature.

In any case, Williams says, sahelanthropus “Definitely depended on the trees.” This is where he would have eaten, slept and sought safety. But on land, Williams is sure it walked on two legs and used its hands to carry food. Given the sparse fossil record, this is difficult to determine. Dever and Guy plan to return to the original field site later this year in hopes of finding something else that others may have missed. “Closing the debate will require the discovery of new remains,” they said in a joint statement.

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