Earliest Human Ancestor May Have Walked on Two Legs

Earliest Human Ancestor May Have Walked on Two Legs

By Cody Cottier edited by Claire Cameron

Aside from our massive brains, the trait that almost all distinguishes people from different animals is our potential to stroll absolutely upright on two legs, a mode of motion with out parallel within the animal kingdom. But precisely when our historical ancestors advanced this trait was a thriller—till now. A new fossil analysis means that the earliest-known hominin had begun to evolve diversifications for bipedalism.

Sahelanthropus tchadensis lived in north-central Africa seven million years in the past, proper when the hominin lineage break up off from that of our nearest animal kinfolk, chimpanzees and bonobos. When anthropologists found the primary Sahelanthropus cranium fragments in Chad in 2001, they immediately wondered whether or not it was bipedal—the outlet on the base of its cranium the place the spinal wire would have entered appeared properly positioned to hold its head, as in different bipeds. But with solely a partial skull, there wasn’t a lot to go on.

Researchers later realized {that a} femur discovered alongside the cranium fragments belonged to the hominin, however when it was first analyzed, researchers noticed no proof for bipedalism. Those findings, published in 2020, contradicted the sooner speculation and raised doubts as as to if the species must be thought-about a hominin in any respect. “The field is kind of split right now on how to interpret these fossils,” says Scott Williams, a paleoanthropologist at New York University, who co-authored the brand new evaluation however who was not concerned within the 2020 examine.

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Williams and his staff’s work, revealed right this moment in Science Advances, reverses the narrative but once more. Using three-dimensional geometric morphometrics—a technique that permits anthropologists to quantify the shapes of fossils—he and his colleagues recognized rudimentary types of a number of anatomical options which can be important for bipedalism in later hominins, from Australopithecus to trendy people.

Two of those options have been reported in earlier work: the femur is twisted inward, and there’s a small protrusion the place the gluteus maximus would have hooked up to it. In 2022 a staff led by Guillaume Daver and Franck Guy, paleoanthropologists on the University of Poitiers in France, used these options as a base to argue that Sahelanthropus was a “habitual” biped. (We, as “obligate” bipeds, don’t have any selection however to stroll upright.)

But Williams discovered a delicate third clue. Rubbing his thumb alongside the femur at some point, he felt a small bump proper the place the iliofemoral ligament—a key stabilizer for bipedal motion—would connect to that bone in people. “I was super excited about it,” he says. “It’s there; it’s just hard to see.” Williams knowledgeable Daver and Guy, who independently confirmed the existence of this femoral tubercle.

Not everyone seems to be satisfied. Marine Cazenave, a paleoanthropologist on the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who co-authored a rebuttal final 12 months to Daver and Guy’s 2022 paper, says the brand new examine affords solely “weak evidence” for bipedalism. Some nonbipedal primates have inward-twisted femurs, she says. As for the femoral tubercle, Cazenave says its operate is poorly understood, including that the fossil’s “badly preserved conditions” make it “impossible to know the real extent of this feature.”

In any case, Williams says, Sahelanthropus “was definitely reliant on trees.” That’s the place it might have foraged, slept and sought security. But on the bottom, Williams is persuaded that it walked on two legs, utilizing its fingers to hold meals. Given the sparse fossil document, it’s laborious to make sure. Daver and Guy are planning to return to the unique area web site later this 12 months in hopes of discovering one thing extra that others may need missed. “Closing the debate,” they mentioned in a joint assertion, “would require the discovery of new remains.”

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