Ancient bones confirm that the earliest known human ancestor walked upright

What may be the earliest known human ancestor, called ape-man Sahelanthropus tchadensis that lived in Africa about 7 million years ago, walked upright for much of the time, according to a new study.

The findings suggest that the ability to walk upright – known as bipedalism – appeared very early in the human family tree and reinforce the idea that it may be an evolutionary trait of our ancestry.

“Our conclusion is that we have, probably, features related to bipedal locomotion Sahelanthropus“said Franck Guy, a paleoanthropologist at the University of Poitiers and researcher at the French scientific organization CNRS, who is 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 ulnae from upper arms — found in Chad’s southernmost Djurab Desert. of the Sahara more than 20 years ago.

A single skull of a Sahelanthropus individual, named Toumaï — which means “hope of life” in the local Daza language — was found at the same site, and since then there has been debate over whether he was our ancestor. But the new study reinforces the view that it was.

Researchers think Sahelanthropus lived only a few million years after the last common ancestor of modern humans – who also walk upright – and chimpanzees, who do not.

Although why our ancestors began walking on two legs is much debated by scientists, it is likely that bipedalism led to larger brains to better control the now freed forelimbs, which then evolved into human hands.

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 humans faced a changing climate in which they had to be flexible in finding food.

Advanced mental abilities, such as using tools, language, and abstract thinking, are thought to have come much later.

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

One of the distinguishing features of the Toumaï skull is that the hole for its spinal cord is positioned in front of similar holes in apes that did not walk upright, suggesting that its skull was over its spine, rather than in front of it.

Some previous estimates of limb bones from the site — Guy stresses that they could be from other people — suggested Sahelanthropus he might not be walking upright after all.

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

Comparing it Sahelanthropus bones with those of other extinct apes and modern humans, as well as those of chimpanzees, gorillas and orangutans—our closest living relatives—researchers found that the ancient species probably walked upright for most of the time.

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

Humanity split from the chimpanzee group during the late Miocene, probably between 10 and 7 million years before the present.  This divergence led to very distinct morphologies: limb bones, for example, show differences primarily associated with a quadrupedal locomotion for chimpanzees and a bipedal locomotion for extant humans.  (Franck Guy/ PALEVOPRIM/ CNRS / University of Poitiers)

Humanity split from the chimpanzee group during the late Miocene, probably between 10 and 7 million years before the present. This divergence led to very distinct morphologies: limb bones, for example, show differences primarily associated with a quadrupedal locomotion for chimpanzees and a bipedal locomotion for extant humans. (Franck Guy/ PALEVOPRIM/ CNRS / University of Poitiers)

The study shows Sahelanthropus is indeed the earliest known human ancestor, although it is possible that there were even earlier 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 an older humanoid [human ancestors] remains that show forms of bipedalism … but we may also find older hominin remains that don’t show bipedalism,” he said.

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

An indication that Sahelanthropus was an ancestor of man is that the Toumaï skull has relatively small canines.

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

The study suggests that both upright walking and smaller canines evolved around 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 could be because walking upright evolved out of the need to carry food to mates and kin, which itself was a sociological adaptation to lower levels of interpersonal aggression. “This may have been at the dawn of our ancestry,” Souva said in an email.

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