In the past we’ve covered debates over the hominid fossil species Sahelanthropus tchadensis, based originally upon a partial cranium discovered in Chad that has sometimes been called “Toumaï.” Many have called this fossil a human ancestor that lived at about 7 million years ago, around the time of our supposed most recent common ancestor with chimpanzees, possibly representing one of the first species in our ancestral lineage to walk upright. But this view has always been controversial. As I wrote in 2021:
In 2002, Michael Brunet of the Centre National de la Recherche Scientifique in France announced the discovery of a major hominin fossil find dubbed the “Toumaï Skull,” representing the species Sahelanthropus tchadensis. Although at that time the fossil was known only from a skull and some other bone and jaw fragments, he called Toumaï “the earliest known hominid, [which] could be consider[ed] as the ancestor of all later hominids, i.e. as the ancestor of the human lineage.”
Articles by Brunet and colleagues in the journal Nature called it “the earliest known hominid ancestor,” or more cautiously proposed it as “close to the last common ancestor of humans and chimpanzees.” Although Brunet’s technical paper at the time admitted that “There is not yet sufficient information to infer reliably whether Sahelanthropus was a habitual biped,” he and his team proposed that “such an inference would not be unreasonable given the skull’s other basicranial and facial similarities to later fossil hominids that were clearly bipedal.” To this day, the Smithsonian Institution calls it “one of the oldest known species in the human family tree.”
These claims have led to much disagreement in the paleoanthropology community. Brigitte Senut, of the Natural History Museum in Paris, called Toumaï “the skull of a female gorilla,” and co-wrote in Nature, along with Milford H. Wolpoff, Martin Pickford, and John Hawks, that “Sahelanthropus was an ape,” not bipedal, and that many features “link the specimen with chimpanzees, gorillas or both, to the exclusion of hominids.” This debate has continued.
The previous chapter in the debate came in 2020 when a paper in the Journal of Human Evolution by Macchiarelli et al. analyzed the femur of Sahelanthropus tchadensis and found that the species was a quadruped with a chimp-like body plan, and probably not a human ancestor. Now a newly published 2026 paper in Science Advances by Williams et al. adds to the story — but not without more controversy.
A Complex Form of Locomotion
This latest paper in Science Advances reanalyzed the partial ulnae and femur of Sahelanthropus and claims to “confirm two features linked to hominin-like hip and knee function and identify a femoral tubercle, a feature only found in bipedal hominins. Our results suggest that Sahelanthropus was an early biped that evolved from a Pan-like Miocene ape ancestor.” But in their view Sahelanthropus was not a full-time or “obligate” biped, because it also spent a lot of time in trees and even engaged in knuckle-walking:
Sahelanthropus was an adept navigator of the arboreal environment. … Sahelanthropus may represent an early form of habitual, but not obligate, bipedalism. In addition to terrestrial bipedalism, Sahelanthropus likely engaged in a diverse set of arboreal positional behaviors not limited to vertical climbing, below- branch forelimb suspension, arboreal quadrupedalism and bipedalism, and various forms of climbing (careful, cautious, bridging, etc.) as inferred previously from the Sahelanthropus ulnae as well as for A. ramidus, Orrorin, and other Miocene taxa. Multiple African ape–like morphologies of the Sahelanthropus ulnae also suggest an association with knuckle-walking…
Scott Williams, a paleoanthropologist at NYU who lead authored the study, framed it this way in a news release from NYU:
“Sahelanthropus tchadensis was essentially a bipedal ape that possessed a chimpanzee-sized brain and likely spent a significant portion of its time in trees, foraging and seeking safety,” says Scott Williams, an associate professor in New York University’s Department of Anthropology who led the research. “Despite its superficial appearance, Sahelanthropus was adapted to using bipedal posture and movement on the ground.”
In many ways this paper postures (no pun intended) Sahelanthropus as similar to many other hominin species: at most its form of bipedalism was different from that of living humans, and it also had a highly arboreal lifestyle and sometimes even walked on all fours. But even the possibility of some form of bipedalism in Sahelanthropus has excited the pro-evolution news media, with Smithsonian Magazine calling it “Potentially … the Oldest Known Human Ancestor,” Haaretz putting out the headline “Study identifies our likely earliest ancestor after split from chimp” and claiming it “could have been our earliest ancestor,” and the Washington Post asserting we now have “evidence that a human ancestor walked upright 7 million years ago.”
But if you think this latest paper settles the debate, think again. There are still strong critics of the paper’s claim that Sahelanthropus was a habitual biped.
Many Skeptics Remain
A recent news story in The Guardian reports “Fresh bone analysis makes case for earliest ‘ancestor of humankind’, but doubts remain.” It shows that scientists remain skeptical of the analysis which claims that Sahelanthropus was a bipedal human ancestor:
But the case is far from closed. Dr Marine Cazenave, of the Max Planck Institute for Evolutionary Anthropology, in Germany, said most of the results pointed to similarities with African great apes or extinct apes, and called the evidence for upright walking “weak”. She found the femoral tubercle unconvincing too, adding that it is not directly related to upright walking and was “very faint” in a “highly damaged” region of the thigh bone.
Dr Rhianna Drummond-Clarke, at the same institute, found some of the evidence convincing, but still had questions. “More work is needed to clarify whether walking on two feet was used to walk in the trees, or to move on the ground, the latter of which is a defining feature of the human lineage,” she said. The results could equally suggest Sahelanthropus was an early chimpanzee that became less upright, and a knuckle-walker, as it evolved, she said.
Likewise Scientific American reports on Cazenave’s views:
Not everyone is convinced. Marine Cazenave, a paleoanthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who co-authored a rebuttal last year to Daver and Guy’s 2022 paper, says the new study offers only “weak evidence” for bipedalism. Some nonbipedal primates have inward-twisted femurs, she says. As for the femoral tubercle, Cazenave says its function is poorly understood, adding that the fossil’s “badly preserved conditions” make it “impossible to know the real extent of this feature.”
And New Scientist lists yet more skeptics:
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.
Roberto Macchiarelli, a co-author of two scientific papers in the Journal of Human Evolution that have critiqued the view that Sahelanthropus was a biped, weighed in as a skeptic at the Washington Post:
But Roberto Macchiarelli, a paleoanthropologist who has argued the fossils did not come from a biped, said the femur has been too warped and damaged by time to show the twisting and tubercle that would prove Sahelanthropus regularly walked on two legs.
“Body proportions in Sahelanthropus are 100 percent apelike, certainly not ape-hominin ‘intermediate,’” Macchiarelli said in an email. “Compared to most scientific disciplines and research,” he added, “paleoanthropology is deeply affected by competition and politics.”
Clearly, even after this latest publication, there are still credible authorities who doubt that Sahelanthropus was an upright-walking ancestor of humans. And don’t skip over Macchiarelli’s comment about “competition and politics” in paleoanthropology — we’ll come back to it in a subsequent post.