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Paleoanthropologists have examined three fossilized limb bones of Sahelanthropus tchadensis, one of the oldest known species in the human family tree.
Representation of the modes of locomotion practiced by Sahelanthropus tchadensis. Image credit: Sabine Riffaut, Guillaume Daver, Franck Guy / PALEVOPRIM / CNRS / University of Poitiers.
Sahelanthropus tchadensis is an extinct hominin that lived 7-6 million years ago (Miocene epoch) in west-central Africa.
The species’ first fossils were discovered in 2001 at three localities in the Lake Chad Basin, northern Chad.
The finds included three jaw pieces, several isolated teeth and a small but relatively complete skull nicknamed Toumaï.
“This skull, and 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 Sahelanthropus tchadensis was capable of bipedalism,” said University of Poitiers paleoanthropologist Franck Guy and his colleagues.
One of the localities in the Lake Chad Basin also yielded two ulnae (forearm bone) and a femur (thigh bone).
These bones were tentatively attributed to Sahelanthropus tchadensis because no other large primate was found at this locality.
In the new study, Dr. Guy and co-authors compared the fossils to extant and extinct hominoid specimens, including extant apes (humans, common and bonobo chimpanzees, gorillas and orangutans), Miocene apes and fossil hominins representing Orrorin tugenensis, australopiths and early Homo.
They also used high-resolution micro-CT images taken from the original femur and ulnae to assess the inner morphology of the bones.
“The structure of the femur indicates that Sahelanthropus tchadensis was usually bipedal on the ground, but probably also in trees,” the authors said.
“This bipedalism co-existed 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.”
“All these data reinforce the concept of a very early bipedal locomotion in human history, even if at this stage other modes of locomotion were also practiced,” they concluded.
Their paper was published this week in the journal Nature.
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G. Daver et al. Postcranial evidence of Late Miocene hominin bipedalism in Chad. Nature, published online August 24, 2022; doi: 10.1038/s41586-022-04901-z
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