The Sahelanthropus, the oldest representative of humanity, was already bipedal

The acquisition of bipedalism is considered a decisive step in human evolution. However, there is no consensus on its modalities and its antiquity, mainly due to the lack of fossil remains.

A team involving researchers from France’s National Center for Scientific Research (CNRS), the University of Poitiers and Chad, has examined three limb bones from the oldest currently identified human representative, the Sahelanthropus tchadensis.

Published in Nature on August 24, 2022, this study reinforces the idea that bipedalism was acquired very early in our history, at a time still associated with the ability to move on four limbs in trees.

The Djurab desert, where the fossil deposits that gave rise to the postcranial remains of Sahelanthropus tchadensis are located photo MPFT, PALEVOPRIM / CNRS – University of Poitiers

With 7 million years, the Sahelanthropus tchadensis It is considered the oldest representative species of humanity. Its description dates back to 2001, when the Franco-Chadian Paleoanthropological Mission (MPFT) discovered in Toros-Menalla, in the Djurab desert (Chad), the remains of several individuals, including a very well preserved skull.

This skull, and in particular the orientation and anterior position of the occipital foramen where the vertebral column inserts, indicates a mode of locomotion on two legs, suggesting that it was capable of being bipedal.

The Toumaï skull | photo GuillaumeG on Wikimedia Commons

In addition to the skull, nicknamed toumai, and of the fragments of jaws and teeth already published, in the locality of Toros-Menalla 266 (TM 266) two ulna (forearm bone) and a femur (thigh bone) were found. These bones were also attributed to sahelanthropus because no other large primates were found at the site; however, it is impossible to know if they belong to the same individual as the skull.

Paleontologists from the University of Poitiers, the CNRS, the University of N’Djamena and the National Center for Development Research (CNRD, Chad) published their full analysis in Nature on August 24, 2022.

The femur and ulna were subjected to a battery of measurements and analyses, related to both their external morphology and their internal structures using microtomography images: biometric measurements, geometric morphometry, biomechanical indicators, etc.

Left: 3D models of the postcranial material of Sahelanthropus tchadensis. From left to right: the femur, in posterior and medial view; the right and left ulna, in anterior and lateral view. Right: Example of analysis performed to interpret the locomotor mode of Sahelanthropus tchadensis. 3D cortical thickness variation map for the femurs of (from left to right) Sahelanthropus, an extant human, a chimpanzee, and a gorilla (in posterior view). This analysis makes it possible to understand the variations in the mechanical limitations of the femur and to interpret these limitations based on the locomotor mode | photo Franck Guy / PALEVOPRIM / CNRS – University of Poitiers

These data were compared with those of a relatively large sample of extant and fossil apes: chimpanzees, gorillas, orangutans, Miocene apes, and members of the human group (Orrorin, Ardipithecus, australopithecines, ancient man, Homo sapiens).

The structure of the femur indicates that sahelanthropus used to be bipedal on the ground, but probably also in trees. According to the results of the ulnae, this bipedalism coexisted in arboreal environments with a form of quadrupedalism, that is, of arboreal climbing made possible by the firm grip of the hands, clearly different from that of gorillas and chimpanzees that lean on the back of their bodies. their phalanges.

Humanity diverged from the chimpanzee group during the recent Miocene, most likely between 10 and 7 million years before present. This divergence gave rise to very different morphologies: the bones of the extremities, for example, present differences notably linked to quadrupedal locomotion for chimpanzees and bipedal locomotion for modern humans | photo Franck Guy / PALEVOPRIM / CNRS – University of Poitiers

The conclusions of this study, including the identification of habitual bipedalism, are based on the observation and comparison of more than twenty features of the femur and ulna. They are by far the most parsimonious interpretation of the combination of these traits.

All these data reinforce the concept of bipedal locomotion very early in the history of humanity, although other modes of locomotion were also practiced at this stage.

This work has been supported by the French Ministry for Europe and Foreign Affairs, the Government of Chad, the National Research Agency (ANR), the New Aquitaine Region, the CNRS, the University of Poitiers and the French representation in Chad. It is dedicated to the memory of the late Yves Coppens, forerunner and inspirer of the MPFT’s work in the Djourab desert.


Sources

CNRS | Daver, G., Guy, F., Mackaye, H.T. et al.. Postcranial evidence of late Miocene hominin bipedalism in Chad. Nature (2022). doi.org/10.1038/s41586-022-04901-z