Ancient human ancestor could walk on two legs
A fossil belonging to an ancient hominin that lived seven million years ago bears signs of walking upright, according to a new study.
Williams et al., Sci. Adv. 12, EADV0130
Besides our large brains, the trait that most distinguishes humans from other animals is our ability to walk upright on two legs, a style of locomotion that has no parallel in the animal kingdom. But just when our ancient ancestors Whether this trait evolved was still a mystery. A new fossil analysis suggests that the earliest known hominids began to develop adaptations for upright walking.
Sahelanthropus chadensis lived in north-central Africa seven million years ago, just as the hominin lineage diverged from our closest animal relatives, chimpanzees and bonobos. When did anthropologists discover the first Sahelanthropus skull fragments in Chad in 2001, they I immediately wondered regardless of whether he was bipedal, the hole at the base of his skull where the spinal cord would enter seemed well positioned to support his head, just like other bipeds. But with only a partial skull, there wasn't much to do.
Researchers later realized that the femur found next to the skull fragments belonged to apebut when it was first analyzed, researchers saw no evidence of bipedalism. These findings, published in 2020contradicted an earlier hypothesis and raised doubts as to whether this species should be considered ape at all. “There's sort of a split right now about how to interpret these fossils,” says Scott Williams, a paleoanthropologist at New York University who co-authored the new analysis but was not involved in the 2020 study.
About supporting science journalism
If you enjoyed this article, please consider supporting our award-winning journalism. subscription. By purchasing a subscription, you help ensure a future of influential stories about the discoveries and ideas shaping our world today.
Williams and his team's work, published today in Achievements of sciencechanges the narrative again. Using three-dimensional geometric morphometry—a technique that allows anthropologists to quantify the shape of fossils—he and his colleagues identified vestigial forms of several anatomical features that are critical for bipedalism in later hominins, from Australopithecus to modern man.
Two of these features have been reported in previous work: the femur is twisted inward, and there is a small protrusion where the gluteus maximus attaches to it. In 2022, a team led by Guillaume Daver and Frank Guy, paleoanthropologists at the University of Poitiers in France, used these features as a basis. assert that Sahelanthropus was a “habitual” biped. (We, as “obligatory” bipeds, have no choice but to walk upright.)
But Williams found a subtle third clue. One day, while rubbing his thumb over his femur, he felt a small lump where the iliofemoral ligament—a key stabilizer of bipedal movement—would attach to the bone in humans. “I was very excited about it,” he says. “It's here, it's just hard to see.” Williams reported this to Daver and Guy, who independently confirmed the existence of this femoral tubercle.
Williams et al., Sci. Adv. 12, EADV0130
Not everyone is convinced of this. Marine Cazenave, paleoanthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, co-author refutation Last year, a 2022 paper by Daver and Guy said the new study offered only “weak evidence” for bipedalism. Some nonbipedal primates have femurs that twist inward, she said. As for the femoral tubercle, Cazenave says its function is poorly understood, adding that the “poorly preserved conditions” of the fossil make it “impossible to know the true extent of this feature.”
Either way, Williams says, Sahelanthropus “Definitely depended on the trees.” There he collected food, slept and sought safety. But on the ground, Williams is convinced he walked on two legs and used his arms to carry food. Given the paucity of fossils, it is difficult to be sure. Daver and Guy plan to return to the original field later this year in hopes of finding something else that others may have missed. “Closing the debate,” they said in a joint statement, “will require the discovery of new remains.”
It's time to stand up for science
If you liked this article, I would like to ask for your support. Scientific American has been a champion of science and industry for 180 years, and now may be the most critical moment in that two-century history.
I was Scientific American I have been a subscriber since I was 12, and it has helped shape my view of the world. science always educates and delights me, instills a sense of awe in front of our vast and beautiful universe. I hope it does the same for you.
If you subscribe to Scientific Americanyou help ensure our coverage focuses on meaningful research and discovery; that we have the resources to report on decisions that threaten laboratories across the US; and that we support both aspiring and working scientists at a time when the value of science itself too often goes unrecognized.
In return you receive important news, fascinating podcastsbrilliant infographics, newsletters you can't missvideos worth watching challenging gamesand the world's best scientific articles and reporting. You can even give someone a subscription.
There has never been a more important time for us to stand up and show why science matters. I hope you will support us in this mission.
