Ancient human ancestor walked on two legs 7 million years ago, scientists say: ScienceAlert

The switch to walking on two legs, instead of four, is a major moment in the evolution of our species, which is why scientists are keen to determine exactly when it happened – and a new study indicates that the Adaptation happened about 7 million years ago. .

It is based on a detailed analysis of the thigh (femur) and forearm (ulna) fossils of Sahelanthropus tchadensis, the oldest representative species of mankind. These fossils were first discovered at Toros-Menalla in Chad in 2001.

At the same time, it is likely that these early hominins did quite a bit of tree climbing using all four limbs as well – as one would expect if the species gradually changed from four legs to two legs.

“Here we present post-cranial evidence of the locomotor behavior of S.tchadensiswith new insights into bipedalism early in the evolutionary history of hominins,” the researchers write in their published article.

Bones of Sahelanthropus tchadensis
3D models of the Sahelanthropus tchadensis fossils. (Franck Guy/PALEVOPRIM/CNRS – University of Poitiers)

By comparing the thigh and forearm fossils with the equivalent bones of humans, chimpanzees and gorillas, the researchers were able to understand the mechanics of their use and the way this species moved (its “locomotor mode”). ).

A total of 20 different characteristics of the fossilized bones were used to determine whether S.tchadensis walked on two or four legs, including the outer shape of the remains and the internal structures, assessed via microtomographic imaging.

They concluded that “usual bipedalism” with some tree climbing was the most likely scenario.

The team also suggests that there is a difference between how the species climbed trees compared to today’s gorillas and chimpanzees: with firm grips, rather than leaning on the bones of fingers and toes.

“The curvature and cross-sectional geometric properties of the ulna…are indicative of habitual arboreal behaviors, including climbing and/or ‘cautious climbing’, rather than terrestrial quadrupedalism”, write the researchers.

The research is based on a previous study of a skull fossil unearthed at the same site and thought to also belong to S.tchadensis. Skull analysis suggested that these ape-like creatures were bipedal, but there is now more complete evidence.

The fossils date back to the time (between 6 and 8 million years ago) when humans genetically split from chimpanzees and bonobos, who are our closest living relatives, so this is a crucial step – and who has already attracted a lot of scientific debate.

These early hominids would likely have lived in an environment that mixed forests, palm groves and grasslands, with walking on two legs and climbing trees being options for them as they searched for food and water.

“The most parsimonious hypothesis remains that the postcranial morphology of sahelanthrope is indicative of bipedalism and that any other hypothesis would have less explanatory power for the set of characteristics exhibited by the material from Chad”, write the researchers.

The research has been published in Nature.

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