For decades, the question of when our earliest ancestors first stood up and walked on two legs has puzzled scientists and sparked heated debate. Now, a groundbreaking study published in Science Advances on January 2, 2026, is tipping the scales in favor of an answer: bipedalism—walking upright on two feet—may have emerged as early as seven million years ago, right at the dawn of the human lineage.
The star of this scientific drama is Sahelanthropus tchadensis, a species whose fossils were first unearthed in Chad’s Djurab Desert in the early 2000s. According to Discover Magazine, these remains, including a crushed cranium, forearm bones, and a partial thigh bone, have long been at the center of a tug-of-war between researchers. Was this ancient ape-like creature a habitual upright walker, or did it still rely mostly on all fours, like its modern chimpanzee cousins?
New research led by Scott Williams, a paleoanthropologist at New York University, is providing the most compelling anatomical evidence yet that Sahelanthropus tchadensis could indeed walk on two legs. Using high-resolution 3D analysis and comparative data sets, Williams’ team identified a key structure on the femur—a femoral tubercle—never before seen outside bipedal hominins. This small projection on the thigh bone serves as the attachment point for the iliofemoral ligament, the strongest ligament in the human body, which stabilizes the hip during upright standing and walking.
“Our analysis of these fossils offers direct [evidence] that Sahelanthropus tchadensis could walk on two legs, demonstrating that bipedalism evolved early in our lineage and from an ancestor that looked most similar to today’s chimpanzees and bonobos,” Williams said in a press release reported by Discover Magazine.
But the femoral tubercle wasn’t the only clue. The researchers also found a natural inward twist in the femur, helping to orient the legs forward—another adaptation for upright movement. Additionally, the pattern of gluteal muscle attachment on the bone closely matched that seen in early human ancestors like Australopithecus, further supporting the case for bipedalism.
As Williams explained to NPR, “This animal, that otherwise looks very chimpanzee-like in its overall shape, has these bipedal adaptations hafted on top. And therefore, we’re looking at a very early biped, a very early member of our lineage.”
While these findings are significant, they don’t put all doubts to rest. The fossil record for Sahelanthropus tchadensis is tantalizingly fragmentary. Carol Ward, a paleoanthropologist at the University of Missouri not involved in the research, pointed out to NPR that the most conclusive pieces of the puzzle—like a knee joint or pelvis—are still missing. “Because there’s just not the anatomy preserved that we really need. The conclusive pieces just haven’t been found yet,” Ward said, adding, “We do not have any of those bones.”
Still, the evidence that is available paints a fascinating picture. The limb proportions of Sahelanthropus—a relatively long thigh bone compared with the forearm—hint at an early shift toward bipedal movement. Unlike modern humans, however, the species likely spent significant time in the trees, foraging and seeking safety. Its legs were much shorter than ours, and it probably retained a grasping big toe and other features associated with arboreal life.
“Sahelanthropus tchadensis was essentially a bipedal ape that possessed a chimpanzee-sized brain and likely spent a significant portion of its time in trees, foraging and seeking safety,” Williams told Discover Magazine. “Despite its superficial appearance, Sahelanthropus was adapted to using bipedal posture and movement on the ground.”
This mosaic of adaptations—part tree-dweller, part upright walker—suggests that the transition to bipedalism was not a simple leap but a gradual process. According to NPR’s Nell Greenfieldboyce, Williams’ new methods involved creating 3D models of the fossils and comparing them to a large dataset of other fossils, humans, and living apes. These sophisticated analyses allowed the team to distinguish traits linked to upright walking from those associated with climbing.
The implications of these findings ripple far beyond a single fossil. If Sahelanthropus tchadensis was already capable of walking upright seven million years ago, it means bipedalism emerged close to the time when humans and chimpanzees split from their last common ancestor. This would push the origin of upright walking further back than many scientists previously thought, challenging the idea that bipedalism only developed after other major evolutionary changes, like larger brains or tool use.
“Bipedalism allowed for the eventual evolution of large brains,” Williams told NPR, emphasizing how freeing up the hands for manipulating objects and making tools may have set the stage for later advances in human evolution.
Yet, controversy lingers. As Williams himself admitted to NPR, “Anything before [four million years ago], it starts to get controversial.” The debate over Sahelanthropus’s locomotion is unlikely to end soon, especially given the incomplete nature of the fossil record. Some researchers emphasize traits linked to climbing, while others point to features consistent with upright movement.
Ward remains optimistic that future discoveries will shed more light on this pivotal chapter in our evolutionary story. “More and more fossils are being discovered all the time,” she observed, suggesting that the secrets of our earliest upright ancestors may yet be revealed.
For now, though, the new analysis stands as the strongest anatomical evidence yet that bipedalism—one of the defining features of the human lineage—arose early and in a creature that, to modern eyes, would have looked much more like a chimpanzee than a person. The discovery reminds us that evolution is rarely straightforward, and that our own story is full of twists, turns, and unexpected walkers on the ancient African plains.
As scientists continue to unearth fossils and refine their techniques, the mystery of when and how our ancestors first stood tall remains one of the most captivating in all of science.
