The earliest known ancestors of humans already walked on two legs, according to a new study.

The results are based on the analysis of fossil bones found in Chad more than 20 years ago and only now analyzed.

According to the researchers, the remains belonged to a hominid they named Sahelanthropus tchadensis, which lived between six and seven million years ago.

It would be the earliest known member of the hominin lineage, the evolutionary branch extending from the common ancestor of humans and chimpanzees to modern humans.

When the skull, one of the main bones found, was analyzed in 2002, scientists named its owner Toumaï, which means “hope of life” in Chadian.

The new study, now published in the journal Nature based on analysis of other bones, including the femur.

It has been debated for years whether Sahelanthropus tchadensis was bipedal. But the authors of the new study, from institutions in France and Chad, say they finally have an answer.

When were the fossils found?

The bones were discovered by researchers from France and Chad during an expedition in the Djourab desert in the north of the African country in July 2001.

When the first study of the skull was published in 2002, scientists led by paleoanthropologist Michel Brunet of the University of Poitiers in France pointed out that Toumaï’s brain, despite its small size, similar to that of a chimpanzee, has other brains. later hominin features such as teeth and face.

They also noted that the direction of the hole in the base of Toumaï’s skull indicates that he walked upright.

During the same 2001 expedition, the researchers collected hundreds of other fossils.

The new study is based on limb remains, including “partial left femur (thigh bone) and two left and right ulnas (ulna, or ulna, is one of the two forearm bones with radius; it forms our elbow”). ”, note the authors of the work in a popular article on the site Speech.

The femur was first analyzed in 2004 by University of Poitiers student Aude Bergeret-Medina.

Roberto Macchiarelli, a paleoanthropologist and student’s advisor, later agreed that the bone probably belonged to S. tchadensis. However, it has not been proven that the femur, ulna and skull belong to the same person.

Why did it take so long for scientists to analyze other remains found in Chad?

Extremity studies did not gain momentum until 2017.

“That wasn’t our priority,” said Poitiers paleoanthropologist Franck Guy, who led the latest research, the journal said. Nature.

in his article entitled Speechthe authors of the new study note that analysis of the material “has been slowed significantly for several reasons, including the priority given to field research and other studies of other postcranial remains, and the difficulty of analyzing this piecemeal material requires additional information. Restarted in 2017, it took five years to complete.” .

What evidence is there that Toumai walked upright?

The oldest bipedal hominid fossil ever identified belonged to Orrorin tugenensis, which was found in Kenya and was six million years old.

But the authors of the new study point out that the features of the femur and ulna found in Chad indicate that their owners were capable of walking upright.

Scientists compared these bones with bones from a wide variety of primates and other human ancestors.

“Because the preservation of these long bones is not very good (for example, the femur has lost both limbs), a concise analysis does not provide reliable interpretations,” the authors write.

“Therefore, we examine them from all angles, both in their external morphology and internal structure.”

The researchers say they used more than 20 criteria to compare the remains with other bones from existing and fossil specimens.

“Taken separately, neither of these criteria can be used to recommend a categorical interpretation of the material.”

However, as a whole, they add that the characters found show “Sahelanthropus habitually practiced bipedalism.”

The authors relied on the features of the femur, which they noted were closer to humans than to great apes.

Spanish scientist Pablo Pelaez-Campomanes is a researcher at the Museum of Natural Sciences in Madrid and co-author of the 2002 study on the Toumaï skull.

“The piece of femur they have presents a number of morphological features, such as the cross-section of the bone, the thickness of the cortical layer of the bone, and the shape of the different muscle attachments. Different movements make the most of this species, which is likely to be interpreted as bipedal and is stingy,” explained scientist BBC Mundo’ either.

«As for the ulna, it does not offer bipedal features. The article further draws the conclusion that, based on different morphological characters (curvature, insertions, etc.), it probably belongs to a species with arms adapted to a life with an important tree component».

“Therefore, the combination of the two bones, along with previous studies of the skull, would indicate that this is a bipedal species that retains osteological adaptations associated with arboreal behavior.”

Sahelanthropus could thus both walk on two legs and climb trees easily while on the ground.

Frank Guy says, “On the ground, they would rather act like bipeds. But sometimes they chose to climb. All the features point to this type of behavior.”

Pelaez-Campomanes pointed out that according to the study, Sahelanthropus would form part of the so-called hominins, a subfamily of hominids.

“In simple terms, and according to the comments of this article, Sahelanthropus will be the first representative of hominins comprising clades containing the genus Homo,” said the Spanish scientist.

“One of the traits that members of this hominin clan must possess is the acquisition of bipedalism, and it is therefore important for this species to have this in order to be considered a hominin.”

Why is the study controversial?

in one try Nature Daniel Liebarman, a paleoanthropologist at Harvard University who was not involved in the study, states that the Sahelanthropus thigh bone that accompanied the new study “looked more like a bipedal hominid than a quadrupedal ape.”

However, the debate continues.

Already in 2020, Aude Bergeret-Medina and Roberto Macchiarelli published a study based on measurements and photographs, in which they argue that the owner of the famous thighbone does not walk upright.

The new study doesn’t convince Macchiarelli, according to an article in the journal Nature.

The researcher argues that the features of the femur, described as indicative of bipedalism, may be the result of compression caused by being under sediment for millions of years.

Fred Spoor of the Natural History Museum in London told the magazine: New Scientist The authors of the new study “make a good case” for bipedalism, but the debate is likely to continue.

“It will be vital that independent teams of paleoanthropologists examine these fascinating fossils in the coming months,” Kelsey Pugh of the Natural History Museum in New York told the same publication.