Sahelanthropus Tchadensis and Bipedalism Study

Author: French National Centre for Scientific Research
Published: 2022/08/24 - Updated: 2023/01/04
Category Topic: Anthropology - Related Publications

Page Content: Synopsis - Introduction - Main

Synopsis: This study reinforces the idea of bipedalism being acquired very early in our history, at a time still associated with the ability to move on four limbs in trees. At 7 million years old, Sahelanthropus tchadensis is considered the oldest representative species of humanity. Its description dates back to 2001, when the Franco-Chadian Paleoanthropological Mission (MPFT) discovered the remains of several individuals at Toros-Menalla in the Djurab Desert (Chad), including a remarkably well-preserved skull.

Defining Bipedalism

Bipedalism

Bipedalism is a form of terrestrial locomotion where an organism moves utilizing its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped meaning two feet. There are several states of movement commonly associated with bipedalism:

Jumping/hopping: Moving by a series of jumps with both feet moving together.
Walking: One foot in front of another, with at least one foot on the ground at any time.
Running: One foot in front of another, with periods where both feet are off the ground.
Standing: Staying still on both legs. This is an active process in most bipeds, requiring constant balance adjustment.

Introduction

Sahelanthropus, the oldest representative of humanity, was indeed bipedal... but that's not all!

The acquisition of bipedalism is considered a decisive step in human evolution. Nevertheless, there is no consensus on its modalities and age, notably due to the lack of fossil remains. A research team involving researchers from the CNRS, the University of Poitiers(1), and their Chadian partners examined three limb bones from the oldest human representative currently identified, Sahelanthropus tchadensis. Published in Nature on August 24, 2022, this study reinforces the idea of bipedalism being acquired very early in our history, at a time still associated with the ability to move on four limbs in trees.

Main Content

At 7 million years old, Sahelanthropus tchadensis is considered the oldest representative species of humanity. Its description dates back to 2001 when the Franco-Chadian Paleoanthropological Mission (MPFT) discovered the remains of several individuals at Toros-Menalla in the Djurab Desert (Chad), including a very well-preserved skull. This skull, in particular the orientation and anterior position of the occipital foramen where the vertebral column is inserted, indicates a mode of locomotion on two legs, suggesting that it was capable of bipedalism(2).

In addition to the skull, 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 skull. Paleontologists from the University of Poitiers, the CNRS, the University of N'Djamena, and the National Centre of Research for Development (CNRD, Chad) published their complete analysis in Nature on August 24, 2022.

Figure 1. 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 interpret these constraints in locomotor mode - Image Credit: Franck Guy / PALEVOPRIM / CNRS - University of Poitiers.

The femur and ulnae were subjected to a battery of measurements and analyses concerning their external morphology and 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 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 other modes of locomotion were also practiced at this stage.

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.

References:

(1) - At the PALEVOPRIM laboratory (CNRS / University of Poitiers).

(2) - See these two articles:

A new hominid from the Upper Miocene of Chad, Central Africa, Michel Brunet, et al., Nature, 11 July 2002. DOI: 10.1038/nature00879.
Virtual cranial reconstruction of Sahelanthropus tchadensis, Christoph P.E. Zollikofer et al., Nature, 7 April 2005. DOI: 10.1038/nature03397