New research confirms that Sahelanthropus tchadensis, one of the oldest known hominin species dating back 6.7 to 7.2 million years, was capable of walking upright. This discovery strengthens the case for an earlier origin of bipedalism in the human lineage than previously thought. The findings, published in Science Advances, focus on fossilized leg and arm bones recovered from the Toros-Menalla site in Chad.
The Debate Over Upright Walking
For over two decades, scientists have debated whether Sahelanthropus tchadensis could walk upright. Early skepticism centered on the apelike features of its bones, suggesting it was primarily tree-dwelling. However, the latest analysis uses 3D modeling and anatomical comparisons to demonstrate clear adaptations for bipedal movement on the ground.
“The key is that Sahelanthropus tchadensis was not merely an ape that occasionally stood on two legs; it was adapted to using bipedal posture as a regular part of its behavior,” explained Dr. Scott Williams of New York University, who led the research.
Key Anatomical Evidence
The study centers on a femur (thigh bone) and two forearm bones. Despite their overall resemblance to chimpanzee limbs, subtle proportions indicate a more human-like gait. Specifically, the ratio between arm and leg length falls between that of modern bonobos and early human ancestors.
Crucially, the femur exhibits a small bony prominence — the femoral tubercle — where the iliofemoral ligament attaches. This ligament is vital for stabilizing the hip during upright walking and has only been found in hominins until now. The bone also displays internal twisting (antetorsion) that aligns with the mechanics of human knees positioning under the body’s center of mass.
Gradual Evolution of Bipedalism
This research suggests that bipedalism did not appear suddenly but developed gradually over millions of years. Sahelanthropus tchadensis likely engaged in habitual, though not exclusive, bipedalism alongside arboreal (tree-dwelling) behaviors like climbing and hanging from branches.
“We consider the evolution of bipedalism to be a process rather than an event,” the researchers emphasize.
The fossils reinforce the theory that early hominins evolved from ape-like ancestors resembling modern chimpanzees and bonobos, rather than from more specialized ape species. This puts chimpanzee-like creatures near the root of the human family tree, solidifying the idea that bipedalism emerged early in our evolutionary history.
“This analysis 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.”
This finding not only rewrites the timeline of human evolution but also underscores the complexity of how and when our ancestors first stood up straight.
