Sahelanthropus tchadensis is the oldest known hominid. Its existence dates back to when our ancestors split from the modern apes . The remains of S.tchadensis , a partial skull, jaw fragments, some teeth, and leg and arm bones, were found in 2001 in Chad, Africa.
At the time, examination of the skull of this individual, whom scientists baptized with the name Toumaï , which means “hope of life” in the language of the area, revealed that S. tchadensis was a species close to a chimpanzee both in size as in structure. However, although its brain was the size of a chimpanzee, the face and teeth were more hominid. The researchers concluded that Toumaï may have been a close relative of the last common ancestor of humans and chimpanzees.
The new study, the most comprehensive to date, has been carried out by scientists from the University of Poitiers, the Center National de la Recherche Scientifique in Paris, the University of N’Djamena and the CNRD in Chad and has taken into account, for one side, the partial skull and on the other the bone of the legs and those of the arms.
Perhaps the most interesting feature that Toumaï shares with other hominids is the opening in the skull through which the spinal cord exits . In quadrupeds this opening is usually at the back of the skull and faces backwards. In S. tchadensis , however, the opening is more or less in the middle of the skull and faces downwards. This suggests that Toumaï was bipedal and that upright walking might have helped differentiate early hominids from their relatives.
In the new study, scientists have analyzed the femur (thigh bone) and two ulnae (bone located in the forearm) that are believed to correspond to S. tchadensis . They studied both the external shapes of the bones and their internal microscopic structures. They then compared this data with corresponding details of living and fossil species, including chimpanzees, gorillas, orangutans, extinct apes from the same time, modern humans, ancient humans, and hominids such as Orrorin , Ardipithecus , and australopithecines ( Australopithecus and allies).
What they saw was that the base of the femoral neck appears to be oriented slightly towards the front of the body and is also flattened. The upper part of the thigh bone is also slightly flattened, all of which have been observed in known bipedal hominids. In addition, the attachment points of the buttock muscles are robust and human-like. And the cross-sectional shape of the thigh bone suggests that it could resist the kind of lateral bending forces seen when walking on two legs.
The findings suggest that S. tchadensis would have walked its legs both on the ground and in the forest canopy.
As for the arm bones, these are similar to those of chimpanzees and are well adapted for climbing trees. The ulnae have very curved axes which would indicate the presence of strong muscles in the forearms. The shape of the elbow joints suggests that they could withstand large forces when flexed.
Referencia: Daver, G., Guy, F., Mackaye, H.T. et al. Postcranial evidence of late Miocene hominin bipedalism in Chad. Nature. 2022. DOI: https://doi.org/10.1038/s41586-022-04901-z
