What may be the earliest-known human ancestor, an ape-man called Sahelanthropus tchadensis who lived in Africa roughly 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 — occurred very early on the human family tree and reinforce 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 for the French scientific agency 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 ulnas from forearms — found in Chad’s Djurab Desert on the southern edge of the Sahara more than 20 years ago.
A single skull of a Sahelanthropus individual, dubbed Toumaï — meaning “hope of life” in the local Daza language — was found at the same site, and there’s been debate since over whether it was our ancestor. But the new study reinforces the suggestion that it was.
The 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 started walking on two legs is much debated by scientists, it’s likely that bipedalism led to bigger brains to better control now freed-up forelimbs, which then developed into human hands.
It’s also been suggested that walking upright is more energy efficient than climbing, and that early hominins faced a changing climate in which they had to be flexible about finding food.
Advanced intellectual abilities, like the use of tools, language and abstract thought, are thought to have come much later.
“All we know at this point is that bipedality evolved long before brain enlargement and tool use,” said paleoanthropologist Yohannes Haile-Selassie, the director of the Institute of Human Origins at Arizona State University who wasn’t involved in the latest study.
One of the distinctive features of the Toumaï skull is that the hole for its spinal cord is placed forward of similar holes in apes that didn’t walk upright, which suggests its skull was on top of its spine, rather than in front of it.
Some earlier assessments of the limb bones from the site — Guy stresses that they could be from other individuals — suggested Sahelanthropus might 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 scans with X-rays.
By comparing the Sahelanthropus bones with those of other extinct apes and modern humans, as well as those of chimpanzees, gorillas and orangutans — our nearest living relatives — the researchers determined that the ancient species probably walked upright for most of the time.
The arm bones, however, indicate Sahelanthropus could also climb trees, both in a bipedal fashion — using its arms to steady itself, like modern humans — and in a quadrupedal fashion, with its forelimbs helping to carry its weight.
The study indicates Sahelanthropus is indeed the earliest-known human ancestor, although it’s possible there were even earlier ancestor species that haven’t yet been found, said Guillaume Daver, an assistant professor of paleoanthropology at the University of Poitiers and the study’s lead author.
“In the future we might find older hominin [human ancestors] remains that show forms of bipedalism … but we might also find older hominin remains that do not show bipedalism,” he said.
The findings also suggest that Sahelanthropus probably lived in an environment where both bipedalism on the ground and climbing trees 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.
One indication that Sahelanthropus was a human ancestor is that the Toumaï skull has relatively small canine teeth.
That’s something seen in other human ancestors and modern humans but not in other modern apes, and scientists think it may be a sign of reduced aggression.
The study suggests that both walking upright 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 that could be because walking upright evolved from the need to carry food to mates and kin, which itself was a sociological adaptation to lower levels of aggression among individuals. “This might have been at the dawn of our origins,” Suwa said in an email.