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January 2, 2026<\/p>\n
3 min read<\/p>\n
Add Us On Google<\/span>Add SciAm<\/span><\/span><\/p>\n Earliest Human Ancestor May Have Walked on Two Legs<\/p>\n A fossil belonging to an ancient hominin that lived seven million years ago bears the hallmarks of bipedalism, according to a new study<\/p>\n By Cody Cottier edited by Claire Cameron<\/span><\/p>\n Wiliams et al., Sci. Adv. 12, eadv0130<\/p>\n Aside from our big brains, the trait that most distinguishes humans from other animals is our ability to walk fully upright on two legs, a style of movement without parallel in the animal kingdom. But exactly when our ancient ancestors evolved this trait was a mystery\u2014until now. A new fossil analysis suggests that the earliest-known hominin had begun to evolve adaptations for bipedalism.<\/p>\n Sahelanthropus tchadensis lived in north-central Africa seven million years ago, right when the hominin lineage split off from that of our nearest animal relatives, chimpanzees and bonobos. When anthropologists discovered the first Sahelanthropus skull fragments in Chad in 2001, they immediately wondered whether it was bipedal\u2014the hole at the base of its skull where the spinal cord would have entered seemed well positioned to carry its head, as in other bipeds. But with only a partial cranium, there wasn\u2019t much to go on.<\/p>\n Researchers later realized that a femur found alongside the skull fragments belonged to the hominin, but when it was first analyzed, researchers saw no evidence for bipedalism. Those findings, published in 2020, contradicted the earlier hypothesis and raised doubts as to whether the species should be considered a hominin at all. \u201cThe field is kind of split right now on how to interpret these fossils,\u201d says Scott Williams, a paleoanthropologist at New York University, who co-authored the new analysis but who was not involved in the 2020 study.<\/p>\n If you’re enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.<\/p>\n Williams and his team\u2019s work, published today in Science Advances, reverses the narrative yet again. Using three-dimensional geometric morphometrics\u2014a method that allows anthropologists to quantify the shapes of fossils\u2014he and his colleagues identified rudimentary forms of several anatomical features that are critical for bipedalism in later hominins, from Australopithecus to modern humans.<\/p>\n Two of these features were reported in previous work: the femur is twisted inward, and there\u2019s a small protrusion where the gluteus maximus would have attached to it. In 2022 a team led by Guillaume Daver and Franck Guy, paleoanthropologists at the University of Poitiers in France, used these features as a base to argue that Sahelanthropus was a \u201chabitual\u201d biped. (We, as \u201cobligate\u201d bipeds, have no choice but to walk upright.)<\/p>\n But Williams found a subtle third clue. Rubbing his thumb along the femur one day, he felt a small bump right where the iliofemoral ligament\u2014a key stabilizer for bipedal movement\u2014would attach to that bone in humans. \u201cI was super excited about it,\u201d he says. \u201cIt\u2019s there; it\u2019s just hard to see.\u201d Williams informed Daver and Guy, who independently confirmed the existence of this femoral tubercle.<\/p>\n Wiliams et al., Sci. Adv. 12, eadv0130<\/p>\n Not everyone is convinced. Marine Cazenave, a paleoanthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who co-authored a rebuttal last year to Daver and Guy\u2019s 2022 paper, says the new study offers only \u201cweak evidence\u201d for bipedalism. Some nonbipedal primates have inward-twisted femurs, she says. As for the femoral tubercle, Cazenave says its function is poorly understood, adding that the fossil\u2019s \u201cbadly preserved conditions\u201d make it \u201cimpossible to know the real extent of this feature.\u201d<\/p>\n In any case, Williams says, Sahelanthropus \u201cwas definitely reliant on trees.\u201d That\u2019s where it would have foraged, slept and sought safety. But on the ground, Williams is persuaded that it walked on two legs, using its hands to carry food. Given the sparse fossil record, it\u2019s hard to be sure. Daver and Guy are planning to return to the original field site later this year in hopes of finding something more that others might have missed. \u201cClosing the debate,\u201d they said in a joint statement, \u201cwould require the discovery of new remains.\u201d<\/p>\n If you enjoyed this article, I\u2019d like to ask for your support. 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I hope you\u2019ll support us in that mission.<\/p>\n","protected":false},"excerpt":{"rendered":" January 2, 2026 3 min read Add Us On GoogleAdd SciAm Earliest Human Ancestor May Have Walked on Two Legs A fossil belonging to an ancient hominin that lived seven million years ago bears the hallmarks of bipedalism, according to a new study By Cody Cottier edited by Claire Cameron Wiliams et al., Sci. 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