Fossil analysis adds to debate over how earliest known hominin walked

A long-running and bitterly fought dispute over whether the earliest known hominin had a knuckle-walking gait, like chimpanzees, or walked upright, like humans, may have been settled – but not everyone is convinced.

Scott Williams at New York University and his colleagues say that reanalysis of fossil remains of Sahelanthropus tchadensis shows it had at least three key anatomical features that together show that it was our earliest known hominin ancestor to be bipedal.

It has, however, been a lengthy journey to reach this point.

The 7-million-year-old fossil skull, teeth and a jawbone of Sahelanthropus tchadensis were first described in 2002, following their discovery in the desert region of Chad, in north-central Africa. With a pronounced brow ridge and small canines, the ancient animal was immediately recognised as being unlike great apes.

The anatomy of the skull showed that it likely sat directly on top of the spine, similar to the condition seen in other upright-walking, bipedal hominins.

Then, in 2004, French scientists identified a femur – a thigh bone – and ulna – a forearm bone – that had also been found alongside the skull fossils in Chad as belonging to Sahelanthropus. However, it wasn’t until 2020 that the researchers published their findings, arguing that the femur was curved in a way that resembled that of a non-bipedal great ape.

The debate has swung back and forth ever since. For example, in 2022, a research team including Franck Guy and Guillaume Daver, both at the University of Poitiers in France, argued that certain anatomical traits on the femur indicated bipedality. In 2024, meanwhile, Clément Zanolli at the University of Bordeaux in France and his colleagues argued that Guy and Daver’s team was wrong, because those supposedly bipedal anatomical traits could also be seen in non-bipedal great apes.

Williams, the lead author of the most recent study, says he entered this scientific fray with a “fairly ambiguous” opinion of Sahelanthropus.

He and his colleagues first looked at the point on the femur where the gluteus maximus muscle had once attached to the bone. They found that this attachment point looked similar to those seen on hominin thighbones.

They also analysed the size and shape of the femur and ulna. Although both are similar in size to the equivalent bones in chimpanzees, their proportions were more aligned with those of a hominin.

Finally, they identified a previously overlooked feature in the Sahelanthropus femur called a “femoral tubercle”.

“I actually initially identified it by touch, then confirmed it visually using 3D scans of the fossil,” says Williams. “It’s a small bump that sits only where the femoral tubercle would be; the area is smooth in apes and other non-bipeds, but it has a major function.

“It serves as the attachment point of the iliofemoral ligament, the largest and most powerful ligament in the human body. That ligament is loose when we’re sitting but tightens around the femoral head to hold it in the hip joint when we stand and walk, preventing our torso from falling backwards or sideways.”

Williams, however, doubts the new research will fully settle the argument about how Sahelanthropus moved.

“We’re fairly convinced that Sahelanthropus was an early bipedal hominin at this point, but I’d be foolish to think we ended the debate.”

Guy and Daver, who argued that the hominin was bipedal in 2022, released a joint statement to New Scientist in response to the new paper.

“It not only confirms our initial interpretations of the adaptations and locomotion of the earliest hominin Sahelanthropus but also puts forward new arguments supporting its terrestrial habitual bipedalism, despite an overall morphology that remains close to that of a great ape,” the researchers say.

But they also acknowledge that only the discovery of new remains will conclusively end the debate.

John Hawks at the University of Wisconsin-Madison says he agrees with the new findings and says they point to a complicated origin for the hominin lineage.

“I think it may be misleading to imagine that Sahelanthropus are all hominin or all ape,” Hawks says. “Our evolution started as a fuzzy, gradual set of changes towards more upright posture and movement, and Sahelanthropus had features that help us to understand those changes.”

Zanolli, who has strongly argued that Sahelanthropus was not bipedal, disputes the new paper’s findings, saying that “most, if not all, of the results point toward similarities with the African great apes”.

“In my view, this new study simply confirms that Sahelanthropus long bones resemble those of the African great apes, and that it was probably behaving in ways that could range anywhere in between those of a chimpanzee and a gorilla, but clearly differed from the habitual bipedalism as known in Australopithecus and Homo,” says Zanolli.

Human origins and gentle walking in prehistoric south-west England

Immerse yourself in the early human periods of the Neolithic, Bronze Age and Iron Age on this gentle walking tour.