AFTER more than a decade in limbo, a crucial fossil of an early human relative has finally been scientifically described. The leg bone suggests that Sahelanthropus tchadensis, the earliest species generally regarded as an early human, or hominin, didn’t walk on two legs, and therefore may not have been a hominin at all, but rather was more closely related to other apes like chimps.
A paper from a rival group, not yet peer-reviewed, disputes this. The studies are the latest twist in a bitter saga that has seen the fossil held back from publication and its existence ignored.
“We have been anxiously awaiting the publication of this femur for many years,” says Kelsey Pugh at the American Museum of Natural History in New York.
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“7 million
The age in years of bones of Sahelanthropus tchadensis”
Michel Brunet from the University of Poitiers in France and his colleagues discovered the remains of Sahelanthropus tchadensis in Chad in 2001. The team described a skull, dubbed Toumaï, plus fragments of lower jaw and some teeth (, ).
Brunet and his colleagues have always maintained that Sahelanthropus habitually walked on two legs – like modern humans but unlike chimpanzees and other apes. This was based on an , suggesting that the spine was held upright. Many other researchers have that this isn’t sufficient evidence for bipedality.
Resolving this is key, because the Sahelanthropus bones are believed to be , far older than other human relatives like Australopithecus. If it was a biped, that would make it the oldest known hominin. If not, it may not be that closely related to us.
The researchers found a femur, or thigh bone, along with two ulnas, or forearm bones, that would help clarify the matter, but they published nothing about them for almost two decades, . Brunet didn’t respond to a request for comment from New Scientist.
The bones were brought to the University of Poitiers. There, Aude Bergeret-Medina, who discussed the bones with one of her tutors, Roberto Macchiarelli, identified a long, unlabelled bone as a femur, probably from a primate, in 2004.
Bergeret-Medina had been given permission by her superiors to cut the femur into pieces, but she became uneasy about doing this. Macchiarelli examined it and advised her to wait until this could be checked with Brunet and his team, most of whom were in Chad.
Later, Bergeret-Medina was unable to find the femur. Neither she nor Macchiarelli ever saw it again. However, when Brunet’s team didn’t describe the femur, she and Macchiarelli prepared a study using her photos and measurements.
She and her colleagues first tried to present their findings at a 2018 conference in Poitiers, but the presentation was In late 2019, they submitted a paper that has now been published (, ).
Bergeret-Medina’s team argues that the femur isn’t that of a bipedal animal. “There are a lot of indicators which deeply discourage bipedal gait,” says Macchiarelli. In particular, the bone is curved, not straight, typical of apes like chimps.
No bones about it
However, a second study, posted on a Nature Research journals preprint server, disputes this, though it has not yet passed peer review (, ). Its lead author is Franck Guy at the University of Poitiers, a co-author on the original Sahelanthropus paper, who declined to comment.
Guy and his colleagues say the femur does show signs of bipedality. For instance, it has a hard ridge near the top, which they say would support an upright body. Macchiarelli declined to comment on the paper, but shared with New Scientist a copy of a letter he sent to Nature detailing claimed inaccuracies.
Other palaeoanthropologists agree with the analysis by Bergeret-Medina’s team. “The shape of the femur and general morphology doesn’t look like a biped to me,” says Brigitte Senut at the National Museum of Natural History in Paris, France.
And Madelaine Böhme at the University of Tübingen in Germany says: “I saw the pictures 10 or 12 years ago, and it was clear to me that it’s more similar to a chimp than to any other hominin.”
It remains unclear when and where bipedalism first evolved, says Böhme. Another African species, Orrorin tugenensis, lived 6 million years ago and has clear signs of bipedality. But prior to that, most apes lived in Eurasia, not Africa, and she has found tentative evidence that bipedality emerged there.
