The oldest and most mysterious member of the primate lineage that gave rise to humans was already able to walk upright 7 million years ago, say French researchers who analyzed the femur (thigh bone) of the species. At the same time, their arms still had adaptations to climb trees efficiently, much like today’s chimpanzees.
These conclusions, presented in this week’s edition of the scientific journal Nature, help to dispel some doubts that surround the Sahelanthropus tchadensis, discovered in Chad (North-Central Africa) and revealed to the public for the first time 20 years ago. At the time, the international team responsible for the discovery, led by scientists from the University of Poitiers, had identified only the primate’s skull, which was quite complete, but with a distorted shape.
Reconstructions of the original shape of the skull, made with the help of CT scans, indicated that the individual, nicknamed Toumaï (“hope of life” in one of the native languages ​​of Chad), had a brain of similar size to that of the great African apes. of today. But the most important information coming from the analyzes had to do with the position of the cranial opening that connects to the vertebrae in the neck.
In quadrupedal animals such as chimpanzees, this opening is at the back of the skull. But in Toumaï’s case, it is positioned right under the skull, in the middle. That is, he would have his head “straight” in an upright position, just like human beings. It would be a strong indication that the S. tchadensis would be a bipedal animal like us. Since, among known primates, only hominins (a group of humans and their close extinct relatives) are bipedal, this would indicate that the creature is the oldest member of this lineage.
Many people, however, were not convinced by this analysis, and the lack of more fossils of the species made a more complete reconstruction of the hominin difficult. Now, the Poitiers team is presenting details about the species’ locomotion based on three new bones: a femur (the left one) and two ulnas (one of the arm bones that connect with the hands), the right and the left.
“Our study shows that the oldest representative of our primate group was able to move bipedally in trees as well as on the ground. Sahelanthropus able to move like a quadruped in the trees,” he told Sheet researcher Guillaume Daver, first author of the study.
The bipedal side of the species would be linked, among other things, to the fact that the upper part of the femur is facing forward and is relatively flat, as in other hominins. The musculature of the glutes (the rear) also seems to have been robust enough to sustain this type of movement.
The arm bones, on the other hand, present a typical curvature of animals that supported their weight on them during frequent movement through the trees, with the elbows flexed. According to the researchers, the combination would make sense in an environment where there were areas with gallery forest (around rivers and lakes), palm trees and more open areas, with grasses, allowing the animals to explore resources both on the ground and high in the trees. .
“We consider that hominin bipedalism may have arisen shortly after the separation of the human and chimpanzee lineages or was inherited from the last common ancestor of both. At this point, it is difficult to say which of the hypotheses is correct”, says the French scientist. .
For paleontologist Daniel Lieberman, from Harvard University, who commented on the study at the request of Nature, the femur data are not yet definitive, but it makes more sense to imagine that the Sahelanthropus it was bipedal when they are considered along with the details of the skull. Only more information about the anatomy of the species will be able to solve the dilemma for good, he concludes. Are more fossils of the species still hiding in the sands of Chad? “To be honest, we don’t know, but we believe it is possible and we will continue our fieldwork efforts,” says Daver.
