Ancient bones confirm oldest known 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 results suggest that the ability to walk upright – known as bipedalism – occurred very early in the human family tree and reinforces the idea that it may be an evolutionary feature of our lineage.

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

The study by Guy and his colleagues, published Wednesday in the journal Natureis based on a reassessment of three fossilized limb bones – a thigh femur and two forearm ulnae – found in Chad’s Djurab Desert, on the southern edge of the Sahara, more than 20 years ago .

A single skull of a sahelanthrope An individual, nicknamed Toumaï – which means “hope of life” in the local Daza language – was found at the same site, and there has been debate since whether it was our ancestor. But the new study strengthens the suggestion that this was the case.

Researchers think sahelanthrope 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 to walk 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
3D map of cortical thickness variation for the femurs of (from left to right) Sahelanthropus, an extant human, a chimpanzee, and a gorilla (in posterior view). 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.

It is believed that advanced intellectual abilities, such as tool use, language and abstract thinking, came much later.

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

One of the distinguishing features of Toumai’s skull is that the hole for his spinal cord is placed in front of similar holes in apes that did not walk upright, suggesting that his skull was above his spine, rather than ‘in front.

Some earlier assessments of the site’s member bones – Guy points out they could be from other people – have suggested sahelanthrope might not have walked upright after all.

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

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

The arm bones, however, indicate sahelanthrope could also climb trees, both bipedally – using its arms to stabilize itself, like modern humans – and quadrupedally, its forelimbs helping to support its weight.

Humanity separated from the chimpanzee group during the Late Miocene, most likely between 10 and 7 million years before present.  This divergence has resulted in very distinct morphologies: the bones of the limbs, for example, present differences linked in particular to a quadrupedal locomotion for chimpanzees and a bipedal locomotion for modern humans.
Humanity separated from the chimpanzee group during the Late Miocene, most likely between 10 and 7 million years before present. This divergence has resulted in very distinct morphologies: the bones of the limbs, for example, present differences linked in particular to a quadrupedal locomotion for chimpanzees and a bipedal locomotion for modern humans.
Franck Guy/ PALEVOPRIM/ CNRS / University of Poitiers

The study indicates sahelanthrope is indeed the first known human ancestor, although it is possible that there are still older ancestor species that have not yet been 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 might also find older hominid remains that don’t show bipedalism,” he said.

The results also suggest that sahelanthrope probably lived in an environment where ground bipedalism and climbing trees were helpful, such as mixed grasslands, forests and palm groves, the researchers wrote – although the site in northern Chad where the fossils were found is today today an arid desert.

A hint that sahelanthrope was a human ancestor is that Toumaï’s skull has relatively small canines.

It’s something seen in other human ancestors and in modern humans, but not in other modern apes, and scientists believe it could be a sign of reduced aggression.

According to Gen Suwa, a professor of paleoanthropology at the University of Tokyo, who was also not involved in the study, the study suggests that upright walking and smaller canines evolved around the same time.

And that could be because walking upright evolved from the need to carry food to friends and loved ones, which was itself a sociological adaptation to lower levels of aggression in individuals. “It could have been at the dawn of our origins,” Suwa said in an email.


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