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Whether a 7-million-year-old primate nicknamed ‘Toumai’ walked on two or all fours has sparked drama among paleontologists – along with an endangered femur.
Since the discovery of Sahelanthropus tchadensis first fossil in 2001, it has often been cited as our earliest known hominin ancestor. Initial analysis suggested that Sahelanthropus regularly walked upright and had a combination of ape-like and human-like features.
These findings, however, were based on a single skull.
The skull has anatomical features that potentially indicate that this primate had an erect spine and therefore spent part of its time walking on just two legs. Its tiny teeth also look more human than those of a monkey. Subsequent reconstruction confirmed these results.
But other researchers have since argued that this alone was not enough to rank Sahelanthropus as a hominin bipedal – a primate directly ancestral to humans – rather than a related hominid, but not directly ancestral.
Around the same time and in the same place where the skull was found, in Toros-Menalla in Chad, a partial left femur was also found. The femur disappeared after another researcher began examining it in 2004, supposedly discovering it by accident.
Aude Bergeret-Medina and her supervisor, paleoanthropologist Roberto Macchiarelli from the University of Poitiers in France, finally continued their analysis based on measurements and photos. They have just published their findings, which question Sahelanthropus place in our family tree.
“Based on our analyzes, the partial femur does not exhibit any features consistent with regular episodes of bipedal land travel,” Macchiarelli and his team write in their article.
“So if there is convincing evidence that S. tchadensis is a hominin of the strain, then bipedalism can no longer be considered a requirement for inclusion in the hominid clade. “
Another article awaiting peer review from one of the original authors Sahelanthropus studies dispute this, claiming that the femur has a hard upper ridge that supports an upright position.
Meanwhile, another paleontologist, Martin Pickford of the Muséum national d’histoire naturelle, wonders if the femur even belongs to Toumai, or at least to another. Sahelanthropus.
Yet others agree with Macchiarelli’s assessment of the femur.
“I saw the photos 10 or 12 years ago, and it was clear to me that it looks more like a chimpanzee than any other hominid,” said the University of Tübingen paleontologist, Madelaine Böhme, who did not participate in any of the studies. New scientist.
Analysis of molecular differences in our DNA suggests that humans separated from chimpanzees and bonobos (our closest living relatives) about 6 to 8 million years ago. The only other fossil evidence of a possible hominin from this time comes from Orrorin tugenensis.
Macchiarelli and his team compared the femur to that of O. tugenensis and determined that there is at least one species-level difference between them.
afterwards compare them also with Australopithecus, gorillas and modern humans, they believe these differences suggest that the mode of locomotion of the two older species was also different.
They suspect Sahelanthropus may be an ancestral parent with no living descendants remaining – an extinct primate line.
They also point out that others have suggested that the small teeth found in the original study might just indicate that the primate is female. But the team agrees that fascinating questions remain, particularly around the lines we use to define what exactly makes a primate human, citing a 2017 article in their conclusion:
“Exactly where in Africa, and under what circumstances, the monkey-man demarcation began, and when, how and why the monkey-man border became irrevocably established, are significant research challenges that remain unresolved.
We’ll need a lot more fossils before we know the answers.
This research was published in the Journal of Human Evolution.
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