This 7-Million-Year-Old Fossil May Reveal When Ancient Humans Started Walking Upright

How one of the oldest candidates for human ancestor moved in its environment remains an open question. A seven-million-year-old fossil from central Africa has been interpreted as either an early upright walker or an ape that still supported itself on four limbs.

A new study now adds weight to this question. Published in Scientific advancesresearch presents strongest anatomical evidence to date Sahelanthropus chadensis — a species discovered in Chad in the early 2000s — was capable of walking on two legs. Using high-resolution 3D analysis, the team identified a structure in the thigh bone previously found only in bipedal hominins, as well as other features that support upright posture and movement. Since walking on two legs is a defining characteristic of the human lineage, evidence of bipedalism at this age would place the species at or near the earliest known point of human ancestry.

“Our analysis of these fossils provides [evidence] that Sahelanthropus chadensis could walk on two legs, demonstrating that bipedalism evolved very early in our lineage and from an ancestor that most closely resembled today’s chimpanzees and bonobos,” said study author Scott Williams, in a press release.

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Why this ancient human fossil was difficult to classify

Researchers discovered the fossil in Chad’s Djurab Desert in the early 2000s, but early studies focused primarily on its skull. Some features at the base of the skull suggest that the head might have been positioned for an upright posture, but the anatomy of the skull alone offered only limited insight into how the animal actually moved.

Interpretations became more complex as other bones from the original find, including parts of the forearm and a femur, were examined. Analyzes of these limb bones varied, with some researchers emphasizing features related to climbing, and others identifying features consistent with bipedal movement.

If the species could walk on two legs, the implications would extend beyond a single fossil. Upright walking is thought to have emerged around the time humans and chimpanzees diverged, well before later evolutionary changes such as larger brains or tool use.

The new findings also challenge the idea that bipedalism only emerged after major changes in body size or brain anatomy, suggesting instead that vertical movement evolved in a species that still retained many ape-like features.

Reveal the first signs of walking upright

In the new study, researchers re-examined the forearm and thigh bone using detailed comparisons with living apes and fossil hominins, as well as high-resolution 3D shape analysis. Together, these approaches allowed the team to distinguish traits related to upright walking from those associated with climbing or more generalized movement.

The most distinctive feature they identified was a femoral tubercle – a small projection on the thigh bone that serves as an attachment point for the iliofemoral ligament. This ligament, the strongest in the human body, plays a key role in stabilizing the hip when standing and walking. Until now, the femoral tubercle had only been documented in bipedal hominins.

The analysis also confirmed two additional features associated with bipedal locomotion: a natural twist of the femur that helps orient the legs forward, and a gluteal muscle attachment pattern comparable to that seen in early human ancestors such as Australopithecus. Together, these features contribute to hip stability when standing and walking.

Although the species probably still spends time in trees and has much shorter legs than modern humans, the proportions of its limbs differ from those of apes. The relatively long thigh bone compared to the forearm indicates an early evolution toward bipedal movement, suggesting that upright walking was already part of its behavioral repertoire.

“Sahelanthropus chadensis “It was essentially a bipedal ape that had a brain the size of a chimpanzee and probably spent a significant portion of its time in the trees, foraging for food and seeking safety,” Williams said. “Despite its superficial appearance, Sahelanthrope has been adapted to the use of bipedal posture and ground movements.

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