When Did Human Ancestors First Walk Upright?

In 2001, researchers unearthed a scattering of fossils beneath the windswept dunes of the Djurab Desert of northern Chad. The remains were later identified as belonging to an extinct species, Sahelanthropus tchadensis, believed to have lived about 7 million years ago, roughly when the hominin lineage split from those of chimpanzees and gorillas.

The discovery ignited decades of heated debate. Did Sahelanthropus primarily walk on two legs, making it the earliest known hominin or human ancestor? Or did it amble on all fours, like other apes?

A 2026 analysis of Sahelanthropus arm and leg bone fragments led by Scott A. Williams, an evolutionary morphologist at New York University, has tipped the debate toward bipedalism. If true, it would shift the evidence of early hominins walking upright by roughly a million years.

“Bipedalism is the primary feature we look for in identifying early hominins, because we think it may have been an adaptation that evolved early in our lineage and made us distinct from our closest living relatives, chimpanzees and bonobos,” Williams explains. 

Walking upright, the thinking goes, marked a turning point in our ancestors’ evolution, freeing the forelimbs to turn into hands and paving the way for everything that followed, including sophisticated tool use, larger brains and language development.

“By demonstrating that Sahelanthropus shows adaptations to bipedalism, we are providing evidence that it was the earliest known hominin,” says Williams, the lead author of the study. However, he maintains other scenarios might exist, including that Sahelanthropus could have belonged to a separate, non-hominin ape lineage that independently evolved bipedalism. Another theory is the anatomical signs of upright walking have been misinterpreted. “It would be foolish to suggest that the case is closed,” adds Williams. 

French paleontologist Michel Brunet, whose team recovered the Sahelanthropus fossils in 2001, first argued that the extinct species had walked upright by the way it carried its head. He called the skull “Toumaï,” meaning “hope of life” in the Chadian Daza language, and hailed Sahelanthropus as the “ancestor of the whole of humankind.” But without postcrania, bones from the neck down, other scientists weren’t convinced. One team even suggested renaming it to Sahelpithecus or “Sahel Ape.”

Researchers later realized that other bone fragments found near the Toumaï skull—including pieces of a femur and two forearm bones—likely belonged to the same species. Additional studies, however, reached differing conclusions over whether the fossils point to bipedalism.

In subsequent analyses of Sahelanthropus, Williams and his team applied three-dimensional geometric morphometrics—a method for quantifying and comparing shape—alongside other analytical techniques, using casts of the original bone fragments. They identified two anatomical features in the femur: an inward twist of the shaft and a small protrusion where the butt muscles would have attached. Both are associated with upright walking, confirming results of an earlier study. While a slight inward twist occurs elsewhere in the animal kingdom, Williams notes the degree observed in Sahelanthropus has been documented only in hominins. Although similar protrusions are present among many mammals, Williams says the location and structure found in Sahelanthropus are distinctly human-like.

They also found that although the limbs were similar in overall size and shape to those of chimpanzees, their proportions relative to one another fell somewhere in between those of bonobos and Australopithecus (a later hominin exemplified by “Lucy”). “Sahelanthropus shows signs of shifting away from the long arms of apes to the hominin condition of longer legs,” Williams explains.

The evidence for bipedalism was strengthened by the presence of “an important bump” on the femur, which Williams identified by running his thumb along its surface. “[The fossils] hadn’t been seen by many individuals, and it takes holding the real thing (or less ideally, a replica) in one’s hand, rather than seeing a flat image,” Williams says.

That bump, called the femoral tubercle, was the attachment point of a major hip ligament. In humans and early hominins, this ligament tightens when we stand and walk, helping keep the thigh bone firmly in the hip socket and stabilizing the body. It stops the torso from tipping backward or side to side—something that is essential for walking upright on two legs. “The same function is not necessary for non-bipeds, so apes lack the femoral tubercle,” Williams explains.

The research suggests Sahelanthropus looked most like a “chimpanzee-like ape,” Williams says, that was “likely bipedal most of the time” while on the ground but still “heavily reliant on tree life for foraging, sleeping and safety.”

Other scientists raise concern about these conclusions. Marine Cazenave of the Max Planck Institute for Evolutionary Anthropology in Germany argues the existing fossils do not show clear evidence of bipedalism. She describes Williams’ case as weak, adding that the femoral tubercle is not directly linked to upright walking.

For other researchers, the findings advance new arguments for Sahelanthropus’ habitual terrestrial bipedalism, “despite an overall morphology that remains close to that of a great ape,” University of Poitiers researcher Franck Guy says.

Still, Guy cautions the question cannot be settled without the discovery of new remains: “As is often the case in our discipline, the evidence comes from the fossils.” As for what might put the debate to rest once and for all, he says “more complete postcranial elements or even a pelvis would be amazing.”