At seven million years old, Sahelanthropus tchadensis is a species pursued by controversy. Considered by the scientific community as the oldest representative of human beings, studies such as the one published in 2020 in the journal Journal of Human Evolution questioned its human status, based on the analysis of a partial femur.
When the Franco-Chadian Paleoanthropological Mission, led by the paleontologist Michel Brunet, found several of its remains in 2001 in the Djurab desert (Chad), in the Central Africa region, it was also questioned whether this species would have the ability to walk on its two lower limbs. The shape of the Toumaï skull, as this fossil was dubbed, suggested a bipedal position (particularly the orientation and anterior position of the foramen magnum where the vertebral column inserts), but not all researchers agreed with this view.
Skull cast of ‘Sahelanthropus tchadensis’ TM 266-01-060-1, nicknamed Toumaï,. /Wikipedia
“The evidence from the earliest found remains of S. tchadensis was, above all, a skull, which presented a morphology clearly compatible with a habitual upright posture. So we had good reason to think that this species was bipedal. However, some colleagues did not agree with this interpretation,” tells SINC Jean-Renaud Boisserie, paleontologist and research director at the French National Center for Scientific Research (CNRS), who is now leading a study on the femur and two ulnas of the forearm of S. tchadensis.
We had good reason to think that this species was bipedal. However, some colleagues disagreed with this interpretation.
This new research, published in the journal Nature, reopens the debate on the modalities and date of the appearance of bipedalism, since the number of such ancient human fossils is very small.
“Limb bones are a more direct source on the locomotion of a fossil species and now provide confirmation of the evidence found in the skull, as well as more detailed information on the complete locomotion of S. tchadensis,” explains the French scientist.
Collection work session between Franck GUY (left) and Guillaume DAVER (right), at the PALEVOPRIM laboratory, Poitiers (CNRS/University of Poitiers)./ © Franck Guy / Palevoprim / CNRS – University of Poitiers
Able to climb in bipedal or quadrupedal mode
The acquisition of bipedalism is considered a decisive step in human evolution. The description of these three limb bones from the most distant ancestor of man would confirm habitual, but not exclusive, bipedalism. This reinforces the idea that bipedalism was acquired very early in our history, but at a time still associated with the ability to move with four limbs through trees.
Limb bones are a more direct source on the locomotion of a fossil species, and now provide confirmation of evidence found in the skull.
“Our work, based mainly on evidence from the two ulnas, indicates that this species was probably quite skilled in climbing, both in bipedal mode (assisted bipedalism) and in quadrupedal mode. This latter tree climbing was possible thanks to the firm grip of the hands, clearly differentiating it from that of gorillas and chimpanzees, which lean on the back of their phalanges,” highlights the expert.
These bones were analyzed both in their external morphology and in their internal structures by means of microtomography: biometric measurements, geometric morphometrics and biomechanical indicators. 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.
3D models of postcranial material from ‘Sahelanthropus tchadensis’. / © Franck Guy / PALEVOPRIM / CNRS – University of Poitiers
The fossils are undoubtedly of S. tchadensis, because no other great apes were found. However, it is impossible to know if they belong to the same individual as Toumaï’s skull.
According to Boisserie, “the cranial and dental remains found at the TM 266 site indicate that there are at least three different fossilized individuals from this locality. One hundred different documented vertebrates have been found, but the only large primate species found in Toros-Ménalla was S. tchadensis. The evidence from the different specimens can, with very little doubt, be generalized to the entire species. They show clear adaptations to bipedality as a result of evolution, which go beyond simple variations between individuals.”
The evidence from the different specimens can, with very little doubt, be generalized to the entire species. They show clear adaptations to bipedality as a result of evolution, which go beyond simple variations between individuals.
The Djurab Desert, where the fossil deposits that gave rise to the post-cranial remains of ‘Sahelanthropus tchadensis’ are found. / © MPFT / Palevoprim / CNRS – University of Poitiers
Too many unknowns to solve
Analysis of this material began in 2004, as soon as it was identified. The femur in particular was presented by Michel Brunet to several international experts for advice and comparison. This research then slowed down considerably for several reasons, including the priority given to field investigation of other postcranial remains and other work, as well as the difficulty of analyzing this piecemeal material that requires additional knowledge.
“The study intensified in 2017; however, it took five years to complete all the necessary analyses, acquire suitable comparison material, and go through the various stages of a particularly rigorous review process in the journal Nature, which was made very complicated by the global pandemic,” emphasizes the researcher.
Our results are fully compatible with the appearance of later Australopithecus, within the group of the first known hominids of more than 4.2 million years.
For now, the state of conservation of the bones does not allow us to define the effectiveness of S. tchadensis‘s gait on the ground, especially in terms of speed. Paleontologists cannot infer whether it was capable of running or trotting. “As this type of behavior emerged more recently in our history, it is more prudent to say that it was not present in this species,” he says.
Humanity split off from the chimpanzee group during the late Miocene, probably between 10 and 7 million years before the present. / © Franck Guy / PALEVOPRIM / CNRS – University of Poitiers
The relationships among early hominids known (S. tchadensis, Orrorin, and Ardipithecus) are difficult to establish given the currently known evidence. “However, our results are fully consistent with the appearance of later Australopithecus, within the group of the first known hominids of more than 4.2 million years,” emphasizes Boisserie.
For us, the cranial and dental characteristics are enough to attribute this species to our lineage.
What seems clearest in this study is that bipedal locomotion occurred very early in evolutionary history, even though it was combined with other modes of movement. “This study does not address the question of whether S. tchadensis is a hominid or not. It provides information about its locomotor behavior, regardless of its affinities. For us, the cranial and dental characteristics are enough to attribute this species to our lineage. Many colleagues may think that bipedalism is strong evidence to attribute it to the human group. However, we must be careful, because one day we may discover earlier human species that do not show bipedalism.”
Reference:
G. Daver et al. “Postcranial evidence of late Miocene hominid bipedalism in Chad”. Nature.