Fossil analysis adds to debate over how earliest known hominin walked

The long-running and bitter debate over whether the earliest known hominids walked on their knuckles like chimpanzees or walked upright like humans may have been settled, but not everyone is convinced.

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

However, it has it was a long journey to reach this point.

A 7-million-year-old fossil skull, teeth and jawbone. Sahelanthropus chadensis were the first described in 2002following their discovery in the desert region of Chad, northeast central Africa. Thanks to the pronounced brow ridges and small fangs, the ancient animal was immediately recognized as unlike great apes.

The anatomy of the skull showed that it was likely positioned just above the spine, similar to the condition seen in other upright bipedal hominins.

Then, in 2004, French scientists identified the femur (thigh bone) and ulna (forearm bone), which were also found alongside skull fossils in Chad, as belonging to Sahelanthropus. However, this was not until 2020 that the researchers published their findings, claiming that the femur was curved in a manner similar to that of a non-bipedal ape.

Since then the debate has gone back and forth. For example, in 2022, a research group consisting of Frank Guy And Guillaume Daverboth from the University of Poitiers in France, argued that some the anatomical features of the femur indicated bipedality.. Meanwhile in 2024 Clement Zanolli from the University of Bordeaux in France and his colleagues argued that Guy and Daver's team were wrongbecause these presumably bipedal anatomical features could also be seen in non-bipedal apes.

Williams, the lead author of the latest study, says he entered this scientific battle with a “pretty ambivalent” view of Sahelanthropus.

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

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

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

“I identified it first by touch and then confirmed it visually with a 3D scan of the fossil,” Williams says. “It's a small bump that's just where the femoral tubercle should be; in monkeys and other non-bipedal animals this area is smooth, but it serves an important function.

“It serves as the attachment point for the iliofemoral ligament, the largest and most powerful ligament in the human body. This ligament is loose when we sit, but tightens around the head of the femur, holding it in the hip joint when we stand and walk, preventing our torso from falling backward or to the side.”

Williams, however, doubts that the new study will completely resolve the debate about how Sahelanthropus excited.

“We are quite convinced that Sahelanthropus was at that time one of the first bipedal apes, but it would be foolish to think that we were finished with the debate.”

Guy and Daver, who argued that the hominin was bipedal in 2022, issued a joint statement to New scientist in response to the new newspaper.

“This not only confirms our original interpretations of the adaptation and movement of the earliest hominins. Sahelanthropus but also puts forward new arguments in support of its habitual terrestrial upright posture, despite an overall morphology that remains close to that of great apes,” the researchers say.

But they also acknowledge that only the discovery of new remains will finally put an end to the debate.

John Hawkes from the University of Wisconsin-Madison says he agrees with the new findings and argues that they indicate a complex origin for the hominin lineage.

“I think it would be misleading to imagine that Sahelanthropus They’re all apes or they’re all apes,” Hawkes says. “Our evolution began as a loose, gradual set of changes towards more upright posture and movement, and Sahelanthropus had features that help us understand these changes.”

Zanolli, who strongly argued that Sahelanthropus was not bipedal, disputes the new paper's findings, saying that “most, if not all, of the results indicate similarities to African apes.”

“In my opinion, this new study simply confirms that Sahelanthropus long bones resembled those of African apes, and that its behavior could probably vary between that of a chimpanzee and a gorilla, but was clearly different from the habitual upright posture known among Australopithecus And Homo,says Zanolli.

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