Ancient bones confirm that the oldest human ancestor walked upright

Ancient bones confirm that the oldest human ancestor walked upright

What could be the oldest known human ancestor, an ape-man called Sahelanthropus tchadensis who lived in Africa about 7 million years ago, walked upright most of the time, according to a new study.

The findings suggest that the ability to walk upright – known as bipedalism – occurred very early on in the human family tree and reinforces the idea that it may be an evolutionary hallmark of our lineage.

“Our conclusion is that we have, most likely, features related to bipedal locomotion in Sahelanthropus” said Franck Guy, a paleoanthropologist at the University of Poitiers and a researcher at the French science agency CNRS, one of the authors of the study.

The study by Guy and his colleagues, published Wednesday in the journal Nature, is based on a reassessment of three fossilized limb bones — a femur from a thigh and two ulna from a forearm — found in Chad’s Djurab Desert at the edge south of the Sahara. than 20 years ago.

a single skull Sahelanthropus individual, nicknamed Toumaï – which means “hope of life” in the local Daza language – was found at the same location, and since then there has been debate as to whether it was our ancestor. But the new study reinforces the suggestion that it does.

The researchers think Sahelanthropus it lived just a few million years after the last common ancestor of modern humans – who also walk upright – and chimpanzees, which don’t.

While the reason why our ancestors started to walk on two legs is hotly debated by scientists, it is likely that bipedalism led to larger brains better controlling the now freed forelimbs, which developed in human hands.

Image: Sahelanthropus (Franck Guy/ PALEVOPRIM/ CNRS/ University of Poitiers)

Image: Sahelanthropus (Franck Guy/ PALEVOPRIM/ CNRS/ University of Poitiers)

It has also been suggested that walking upright is more energy efficient than climbing, and that early hominids faced a changing climate in which they had to be flexible to find food.

Advanced intellectual skills such as using tools, language and abstract thinking are believed to have emerged much later.

“All we know at this point is that bipedality evolved long before brain enlargement and tool use,” said paleoanthropologist Yohannes Haile-Selassie, director of the Institute of Human Origins at Arizona State University, who was not involved in the study. most recent study.

One of the distinguishing features of Toumaï’s skull is that the hole for the spinal cord is placed in front of similar holes in monkeys that did not walk upright, which suggests that his skull was on top of the spine rather than in front of it.

Some previous assessments of the bones of the site members – Guy emphasizes that they may be from other individuals – have suggested Sahelanthropus may not have walked upright after all.

But the latest study rejects that idea based on a battery of scientific tests that included biometric measurements and internal X-ray scans.

When comparing the Sahelanthropus bones with those of other extinct apes and modern humans, as well as those of chimpanzees, gorillas and orangutans – our closest living relatives – the researchers determined that the ancient species likely walked upright most of the time.

The arm bones, however, indicate Sahelanthropus it could also climb trees, both bipedally – using its arms for balance, like modern humans – and quadrupedally, with its forelimbs helping to carry its weight.

Humanity split off from the chimpanzee group during the late Miocene, probably between 10 and 7 million years before the present.  This divergence resulted in very different morphologies: limb bones, for example, show differences notably linked to quadrupedal locomotion for chimpanzees and bipedal locomotion for existing humans.  (Franck Guy/ PALEVOPRIM/ CNRS/ University of Poitiers)

Humanity split off from the chimpanzee group during the late Miocene, probably between 10 and 7 million years before the present. This divergence resulted in very different morphologies: limb bones, for example, show differences notably linked to quadrupedal locomotion for chimpanzees and bipedal locomotion for existing humans. (Franck Guy/ PALEVOPRIM/ CNRS/ University of Poitiers)

The study indicates Sahelanthropus it is in fact the oldest known human ancestor, although it is possible that there are even older ancestral species that have yet to be found, said Guillaume Daver, assistant professor of paleoanthropology at the University of Poitiers and lead author of the study.

“In the future, we may find older hominids [human ancestors] remains that show forms of bipedalism…but we can also find remains of older hominids that do not show bipedalism,” he said.

The findings also suggest that Sahelanthropus likely lived in an environment where both ground bipedalism and tree climbing were useful, such as mixed grasslands, forests and palm trees, the researchers wrote — although the site in northern Chad where the fossils were found is barren desert today.

an indication that Sahelanthropus was a human ancestor is that Toumaï’s skull has relatively small canine teeth.

This is something seen in other human ancestors and modern humans, but not other modern apes, and scientists think it could be a sign of reduced aggression.

The study suggests that both upright gait and smaller canines evolved at roughly the same time, said Gen Suwa, a professor of paleoanthropology at the University of Tokyo who was also not involved in the study.

And this may be because walking upright evolved from the need to bring food to mates and relatives, which in itself was a sociological adaptation to lower levels of aggression between individuals. “This may have been early on in our origins,” Suwa said in an email.

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