Fossil analysis adds to debate over how earliest known hominin walked

A long-running and hotly contested debate over whether the first known hominid had a finger-walking gait, like chimpanzees, or walked upright, like humans, may have been settled – but not everyone is convinced.

Scott Williams of New York University and his colleagues say that reanalysis of fossil remains of Sahelanthropus chadensis shows that it possessed at least three key anatomical features that together show that our first known hominid ancestor was bipedal.

However, it took a long journey to get there.

The fossil skull, teeth and jaw of Sahelanthropus chadensis were first described in 2002, following their discovery in the desert region of Chad in north-central Africa. With a pronounced brow bone and small canines, this ancient animal was immediately recognized as being different from the great apes.

The anatomy of the skull showed that it likely lay directly above the spine, similar to the condition seen in other upright-walking bipedal hominids.

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

Since then, the debate has oscillated. For example, in 2022, a research team including Franck Guy and Guillaume Daver, both from the University of Poitiers in France, argued that certain anatomical features of the femur indicated bipedalism. In 2024, Clément Zanolli of the University of Bordeaux in France and his colleagues argued that Guy and Daver’s team was wrong, because these so-called bipedal anatomical traits could also be observed in non-bipedal great apes.

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

He and his colleagues first examined the point on the femur where the gluteus maximus muscle had once attached to the bone. They discovered that this attachment point resembled those observed on hominid femurs.

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

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

“I first identified it by touch, then confirmed it visually using 3D scans of the fossil,” says Williams. “It is a small bump that is only located 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 an attachment point to the iliofemoral ligament, the largest and strongest ligament in the human body. This ligament is loose when we sit but tightens around the femoral head to hold it in the hip joint when we stand and walk, preventing our torso from falling backward or sideways.”

Williams doubts, however, that the new research can completely settle the debate over how Sahelanthrope moved.

“We are quite convinced that Sahelanthrope At this point, I was one of the first bipedal hominids, but I would be stupid to think we ended the debate.

Guy and Daver, who claimed that hominid was bipedal in 2022, issued a joint statement to New scientist in response to the new document.

“This not only confirms our initial interpretations of early hominid adaptations and locomotion. Sahelanthrope but also puts forward new arguments in favor of its usual terrestrial bipedalism, despite an overall morphology which remains close to that of a great ape,” say the researchers.

But they also recognize that only the discovery of new remains will definitively end the debate.

John Hawks of the University of Wisconsin-Madison agrees with the new findings and says they indicate a complex origin of the hominin lineage.

“I think it can be misleading to imagine that Sahelanthrope are all hominids or apes,” Hawks says. “Our evolution began as a blurry, gradual set of changes toward more upright posture and movement, and Sahelanthrope had features that help us understand these changes.

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

“In my opinion, this new study simply confirms that Sahelanthrope its long bones resemble those of African great apes, and that it probably behaved in a manner that could fall anywhere between those of a chimpanzee and a gorilla, but clearly differed from the usual bipedalism as known in Australopithecus And Homo,” said Zanolli.

Human origins and gentle walking in prehistoric southwest England

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