New fossil analysis suggests our earliest known human ancestor, Sahelanthropus tchadensis, may have walked upright on two legs seven million years ago—reversing decades of scientific doubt and rewriting the timeline of human evolution.
What makes humans unique isn’t just our intelligence—it’s our ability to walk fully upright on two legs. Now, scientists are reevaluating when this trait first emerged. A groundbreaking study published in Science Advances argues that Sahelanthropus tchadensis, a hominin species from seven million years ago, may have already begun evolving anatomical adaptations for bipedalism—far earlier than previously thought.
The discovery hinges on subtle but critical features in the femur of a fossil found alongside skull fragments in Chad. Researchers led by Scott Williams at New York University used advanced 3D geometric morphometric analysis to quantify these features—a method far more precise than prior visual inspections.
Two features previously identified as indicators of bipedalism—the inward twist of the femur and a small protrusion where the gluteus maximus muscle attaches—were already noted in earlier studies. But Williams’ team uncovered a third, previously overlooked clue: a small bump corresponding to the attachment point of the iliofemoral ligament, a key stabilizer for upright walking in modern humans. This feature was confirmed independently by colleagues who had previously challenged the bipedal hypothesis.
“I was super excited about it,” Williams said of his discovery. “It’s there; it’s just hard to see.” The finding contradicts a 2020 study that concluded the same fossils lacked evidence for bipedalism, casting doubt on whether Sahelanthropus even qualified as a hominin.
Not all experts agree. Marine Cazenave of the Max Planck Institute for Evolutionary Anthropology argues the evidence is weak. She notes that non-bipedal primates also exhibit inward-twisted femurs, and questions whether the femoral tubercle’s function can be reliably inferred from such poorly preserved fossils. “The fossil’s badly preserved conditions make it impossible to know the real extent of this feature,” she stated.
Yet Williams insists that Sahelanthropus was likely arboreal—using trees for foraging, sleeping, and protection—and only occasionally ventured onto the ground, where it may have walked upright while using its hands to carry food. “It was definitely reliant on trees,” he said. “But on the ground, I’m persuaded it walked on two legs.”
This discovery pushes back the timeline for the emergence of bipedal adaptations by nearly two million years compared to previous estimates. It also deepens the mystery surrounding exactly how and why our ancestors transitioned from tree-dwelling primates to ground-based walkers. The implications are profound—not just for understanding human origins, but for reconstructing how early hominins interacted with their environment.
While researchers like Daver and Guy plan to return to the Chad field site later this year in hopes of uncovering additional remains, Williams’ team warns that definitive proof will require more fossils. “Closing the debate,” they said in a joint statement, “would require the discovery of new remains.” Until then, this latest analysis stands as the most compelling case yet for early bipedalism in our lineage.
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