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Human ancestors were walking upright 7 million years ago, ancient limb bone suggests

An ancient leg bone found near the famed skull of a human ancestor is providing new evidence that our lineage may have been walking upright 7 million years ago. A partial thighbone and two lower arm bones from Chad’s Djurab Desert suggest a species known as Sahelanthropus tchadensis ambled along on two legs, as well as spending some time in the trees, according to a new study.

If the creature did walk upright, it’s a “huge” discovery, says Dan Lieberman, a Harvard University paleoanthropologist who was not involved with the new analysis. It would mean upright walking was one of the first traits to distinguish the human lineage, which diverged from chimps at about this time, he says. But, “It’s not a slam dunk,” Lieberman says, because the partial fossil thighbone lacks both ends.

Sahelanthropus first became known to the world when researchers discovered its stunningly intact skull in 2001. Paleontologist Michel Brunet of the University of Poitiers dubbed the creature Toumaï, a name from the local Daza language meaning “hope of life.”

The chimp-size skull looked most like an African ape. But its teeth suggested something different—its canines resembled those of later hominins, or members of the human family. The shape of the opening at the base of its skull—the foramen magnum—suggested Sahelanthropus balanced its head on top of a vertical neck, similar to upright walkers such as today’s humans.

But a skull alone can’t prove a species walked upright. Toumaï needed legs. It turns out it had one that had been overlooked: In 2004, a then–master’s student at Poitiers, Aude Bergeret, spotted the shaft of a thighbone and a lower arm bone in a drawer full of fossils of animals labeled “indeterminate” by the team that found the skull. She alerted her adviser, who identified it as the thighbone of a primate and looped in Brunet. But Brunet’s team didn’t give it a serious analysis until 2017 when a specialist in postcranial bones, Guillaume Daver, began to study it with Poitiers paleoanthropologist Franck Guy.

The current team members, who were not part of the analysis initially, say there were other priorities at the time to analyze the skull and find more fossils in Chad. But others say it was curious that it was such a low priority.

In the new study, this team of French and Chadian researchers used a battery of methods to analyze the internal and external structure of more than 20 traits in the bones. When they compared those traits and measurements with living and fossil apes and hominins, they found the orientation of the base of the femur’s neck and the flattening of the upper part of the bone resembled those of hominins, or members of the human lineage, but not the thighbones of African apes. In addition, the density of bone seen in a cross-section of the interior of the femur fell within the range of hominins, suggesting it underwent loading forces compatible with upright walking, the team reports today in Nature.

Two arm bones and a leg bone belonging to Sahelanthropus tchadensisFranck Guy/Palevoprim/CNRS/University of Poitiers

There was no single “magic trait,” Guy says. It was the “total pattern of features” that all point toward bipedalism, he says. If true, this species would have walked upright through the forests that rimmed the ancient Lake Chad more than 1 million years earlier than the earliest known bipedal hominins—Orrorin tugenensis and Ardipithecus.

Even if Sahelanthropus was bipedal, it still spent a fair amount of time in the trees. The two lower arm bones reveal the species was also arboreal, where it probably was safest from predators.

Still, experts say more work is needed to show Sahelanthropus was truly an upright walker. “The evidence is compatible with bipedality” but does not prove it, says functional morphologist Chris Ruff of Johns Hopkins University.

Others think the bones are so chimplike that it’s unlikely the species walked upright like later hominins. It was not a chimp, but it also “was not an upright, ground-living ape of the kind that was likely to have been our earliest ancestors,” says Bernard Wood, a paleoanthropologist at George Washington University.

Regardless, the shaft of the femur is tantalizing and important because it dates to a time when our earliest ancestors looked a lot like African apes but had diverged from the lineage leading to chimps. “At some point hominins become bipedal, but exactly how and when that transition took place is one of the most interesting, important questions about our lineage,” says paleoanthropologist Carol Ward of the University of Missouri, Columbia. “Any fossils that speak to this are really important.” But sadly, she says, “These don’t provide the conclusive information we hoped for.”

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