Although Sahelanthropus outwardly resembles modern apes, the study suggests its anatomy was better suited to walking on two legs than moving on all fours. According to the research team, the animal likely walked upright at least part of the time, marking a critical evolutionary transition.

“It would have looked much like an ape, probably closest to a chimpanzee or bonobo,” says Scott Williams, an associate professor at New York University and the study’s lead author. “But while those animals occasionally walk upright, this species shows adaptations that point toward regular bipedal movement.”

The findings revive a long-standing debate that began more than two decades ago. Fossils of Sahelanthropus were first uncovered in 2001 in Chad’s Djurab Desert, sparking excitement and controversy in equal measure. At the time, some researchers suggested the species could be the direct ancestor of modern humans, largely based on the position of the skull. Some continued to be doubtful, claiming that there was insufficient data to support upright walking.

The disagreement was not resolved by later findings of fragments of the forearm and a partial thigh bone. Scientists couldn’t agree on whether the bones belonged to an ape with a different mode of locomotion or to a bipedal hominin.

In the latest study, Williams and his colleagues re-examined those limb bones using modern imaging techniques, comparing their shape, proportions and three-dimensional structure with fossils from both known hominins and non-human apes. One anatomical detail stood out: a small projection on the thigh bone associated with a powerful ligament that stabilises the body when standing and walking.

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The researchers argue that this trait, which has only been observed in bipedal relatives within the human lineage, helps prevent the torso from swaying during upright motion. Based on the information that is currently available, the early hominin most likely walked on two feet and spent a considerable amount of time in trees for food and shelter. These behaviours lend credence to theories that claim bipedalism evolved gradually rather than in a single evolutionary leap.

Not everyone is convinced

Some scientists argue that the fossil evidence is still insufficient to draw firm conclusions and point out similarities between Sahelanthropus’s bones and those of modern African apes. Whether the animal primarily used upright walking on the ground or in trees, which is crucial in defining the human lineage, is still up for debate.

Supporters of the revised analysis agree that more fossils are needed to settle the debate, but they also value the increased examination. It is expected that excavations at the original site in Chad will resume, increasing hope that discoveries will eventually clarify when and how humanity took its first steps.