Controversial femur could belong to ancient human relative

Few scientists have had access to a thigh bone kept in a French collection for over a decade.
Fossilised remains on the ground in the Chadian Sahara, 2001.

A fossilized skeleton was discovered in Chad in 2001. Researchers have raised questions about its femur (long bone, centre right). Credit: Alain Beauvilain

When anthropologists meet in France at the end of January, one of the most provocative fossils in the study of human evolution will not feature on the agenda. The approximately 7-million-year-old femur1 was examined more than a decade ago by scientists in the French city of Poitiers, but has yet to be thoroughly described in a published scientific paper.

The fossil may belong to the earliest known hominin, the group that includes humans and their extinct relatives. Few people have had access to it, but two scientists who analysed the bone briefly in 2004 have prepared a preliminary description of it. They had hoped to present their analysis at the meeting, which is organized by the Anthropological Society of Paris and is taking place in Poitiers. But the proposal by Roberto Macchiarelli, a palaeoanthropologist at the University of Poitiers and France's National Museum of Natural History in Paris, and Aude Bergeret, director of the Museum of Natural History Victor-Brun in Montauban, France, was rejected by the conference organizers.

“This specimen is really important. It’s critical,” says Macchiarelli, who has shared his unpublished report with Nature’s news team. The femur probably belongs to a species called Sahelanthropus tchadensis, he says. The bone is important because it could settle whether the species is the earliest hominin yet found, as its discoverers have claimed after analysing the skull2. “This is a fantastic occasion to finally tell people what we have, and what we know about this specimen.”

The Anthropological Society of Paris told Nature that it had rejected 6 out of 65 abstracts. It said: “This work is conducted by an independent and impartial scientific committee, which is sovereign in its decision. Hence, any accusation about this would not be founded”.

A blockbuster controversy

The Sahelanthropus femur was discovered early on the morning of 19 July 2001 beside a battered skull and other bones at a site in the Djurab Desert in northern Chad, says Alain Beauvilain, a retired geographer who led the field team that made the discovery and who was present at the time.

Michel Brunet, a palaeontologist at the University of Poitiers, who headed the Chadian expedition that discovered the Sahelanthropus remains, argues that the species is the earliest known representative of the hominin lineage.

His team described the skull — dubbed Toumaï, which means 'hope of life' in the Chadian Daza language — in a 2002 Nature paper2 that became a scientific blockbuster. A subsequent analysis of the skull and other fragments by Brunet and his team suggests that Toumaï probably walked upright on two legs3. Brunet declined to comment on the analysis of the thigh bone or on Macchiarelli’s and Bergeret's efforts to describe it at the Poitiers meeting. “Our studies are still in progress,” he wrote in a brief e-mail. “Nothing to say before publishing.”

Other researchers have questioned whether Toumaï was indeed part of the lineage that led to humans, pointing to recently discovered fossils from Ethiopia and Kenya as better contenders for the earliest hominin. But Brunet’s team has stood by Toumaï’s hominin status in response to the controversy4 and in a subsequent publication that described a lower jaw and teeth3. Despite the debate, the discovery of the fossils made Brunet famous in France, and especially in Poitiers, where a street is named after him.

Beauvilain says that the femur and other material remained in Chad until they were eventually shipped to Poitiers in 2003, where they were stored in a collection of animal bone fragments from the trip. In 2004 Bergeret, who was then a graduate student at the University of Poitiers, came across the blackened and badly damaged bone while analysing other animal bones in the collection. “I discovered the femur by chance,” says Bergeret.

Exciting find

Brunet and other members of his team were back in Chad when Bergeret found the femur. So she asked Macchiarelli, who studies human evolution and who was then head of the department of geosciences at the University of Poitiers, for help in analysing it. She says that she examined it closely for several days, comparing it to other hominin fossils. “I remember joking with another student, who told me, ‘You found Toumaï's femur!’,” Bergeret says. “I realized when I saw Roberto Macchiarelli that this joke was probably based on reality.”

David Pilbeam, a palaeoanthropologist at Harvard University in Cambridge, Massachusetts, who was a co-author on the 2002 and 2005 papers describing Toumaï’s skull and jaw, says that he saw the femur briefly while visiting Brunet in Poitiers more than a decade ago, but that he is not involved in any analysis. “All I can recall is that it lacked ends and was very black,” he says.

In their short description of the femur, Macchiarelli and Bergeret contend that the bone differs greatly from that of a roughly 6-million-year old potential hominin found in Kenya in 2000 that is thought to have walked on two feet, called Orrorin tugenensis. Macchiarelli doubts that Sahelanthropus is a hominin, but thinks a conclusion should be made only after more careful study of all its remains, including the femur.

The femur and other Sahelanthropus remains are crucial to determining the status of the species, because individual anatomical parts can often be misleading about evolutionary history, says Bernard Wood, a palaeoanthropologist at George Washington University in Washington DC. And even if the species turns out not to be a hominin, he says, it is an incredibly important fossil because it might identify a now-extinct lineage of great ape that once roamed Africa.

A paper describing the femur is “long overdue”, says palaeoanthropologist Bill Jungers, at Stony Brook University in New York. “We don’t know why it’s been kept secret. Maybe it’s not even a hominin. Who the hell knows until someone can expose it.”

Nature 553, 391-392 (2018)

doi: 10.1038/d41586-018-00972-z
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