7 Million Years Old – New Secrets of the Oldest Representative of Humanity Revealed

Representation of the modes of locomotion practiced by Sahelanthropus. Bipedalism was common among the earliest known representatives of humankind, probably on the ground but also in trees. It coexisted with other types of movement in a tree environment, including quadrupedal movement using firm hand grips, clearly differing from that of gorillas and chimpanzees who use the back of their phalanges for support (“knuckle-walking”). Credit: Sabine Riffaut, Guillaume Daver, Franck Guy / PALEVOPRIM / CNRS – University of Poitiers

A new study reveals that Sahelanthropus tchadensis, the oldest representative species of humanity, was bipedal.

It is believed that the development of bipedalism was a turning point in human evolution. However, there is disagreement over its modalities and age, notably due to the fact there are no fossilized remains. Researchers from the University of Poitiers, the French National Centre for Scientific Research (CNRS), and its Chadian partners analyzed three limb bones from Sahelanthropus tchadensis, the earliest known representative of the human genus. The study, which was recently published in the journal Nature, supports the hypothesis that bipedalism was developed extremely early in human history, at a time still associated with the ability to move on four limbs in trees.

Collection working session between Franck Guy (left) and Guillaume Daver (right), at the PALEVOPRIM laboratory, Poitiers (CNRS/University of Poitiers). Credit: Franck Guy / PALEVOPRIM / CNRS – University of Poitiers

Sahelanthropus tchadensis is considered the earliest representative species of humans, dating back 7 million years. Its description goes back to 2001 when the Franco-Chadian Paleoanthropological Mission (MPFT) uncovered the bones of three people at Toros-Menalla in the Djurab Desert (Chad), including a particularly well-preserved cranium. This cranium, particularly the orientation and anterior location of the occipital foramen where the spinal column is inserted, reveals a form of locomotion on two legs, implying that it was capable of bipedalism.

In addition to the cranium, nicknamed Toumaï, and fragments of jaws and teeth that have already been published, the locality of Toros-Menalla 266 (TM 266) yielded two ulnae (forearm bone) and a femur (thigh bone). These bones were also attributed to Sahelanthropus because no other large primate was found at the site; however, it is impossible to know if they belong to the same individual as the cranium. Paleontologists from the University of Poitiers, the CNRS, the University of N’Djamena, and the National Centre of Research for Development (CNRD, Chad) recently published their complete analysis in Nature.

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 ulnae, 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 enables us to understand the variations of mechanical constraints on the femur and to interpret these constraints in terms of locomotor mode. Credit: Franck Guy / PALEVOPRIM / CNRS – University of Poitiers

The femur and ulnae were subjected to a battery of measurements and analyses, concerning both their external morphology and their internal structures using microtomography imaging: biometric measurements, geometric morphometrics, biomechanical indicators, etc. These data were compared to 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 Homo, Homo sapiens).

The structure of the femur indicates that Sahelanthropus was usually bipedal on the ground, but probably also in trees. According to results from the ulnae, this bipedalism coexisted in arboreal environments with a form of quadrupedalism, that is arboreal clambering enabled by firm hand grips, clearly differing from that of gorillas and chimpanzees who lean on the back of their phalanges.

The Djurab Desert, where the fossil sites that yielded the postcranial remains of Sahelanthropus tchadensis are located. Credit: MPFT, 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 characteristics of the femur and ulnae. They are, by far, the most parsimonious interpretation of the combination of these traits. All these data reinforce the concept of very early bipedal locomotion in human history, even if at this stage other modes of locomotion were also practiced.

This work was supported by the French Ministry for Europe and Foreign Affairs, the Chadian Government, the French National Research Agency (ANR), the Nouvelle-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, precursor and inspirer of the MPFT’s work in the Djourab Desert.

Reference: “Postcranial evidence of late Miocene hominin bipedalism in Chad” by G. Daver, F. Guy, H. T. Mackaye, A. Likius, J. -R. Boisserie, A. Moussa, L. Pallas, P. Vignaud, and N. D. Clarisse, 24 August 2022, Nature.
DOI: 10.1038/s41586-022-04901-z

The study was funded by the Région Nouvelle-Aquitaine, Université de Poitiers, and the Centre National de la Recherche Scientifique.