The switch to walking on two legs, instead of four, is an important moment in the evolution of our species, so scientists are keen to determine exactly when it happened, and a new study suggests that the adaptation happened about 7 million years ago. .
This is based on a detailed analysis of fossils of the thigh (femur) and forearm (ulna) of Sahelanthropus tchadensis, the oldest representative species of mankind. These fossils were first discovered in Toros-Menalla in Chad in 2001.
At the same time, it is likely that these early hominids did some tree climbing on all four limbs as well, as would be expected if the species made the gradual switch from four legs to two legs.
“Here we present postcranial evidence of locomotor behavior in S. tchadensis, with new insights into bipedalism early in hominid evolutionary history,” the researchers write in their published paper.
3D models of Sahelanthropus tchadensis fossils. (Franck Guy/PALEVOPRIM/CNRS – University of Poitiers)
By comparing the thigh and forearm fossils to the equivalent bones of humans, chimpanzees and gorillas, the researchers were able to figure out the mechanics of how they were used and how this species moved (its ‘locomotor mode’ ).
A total of 20 different features of the fossilized bones were used to establish whether S. tchadensis walked on two or four legs, including the external shape of the remains and internal structures, assessed by microtomography.
They concluded that “habitual bipedalism” with some tree climbing was the most likely scenario.
The team also suggests there is a difference in the way the species climbed trees compared to today’s gorillas and chimpanzees: using firm handholds, rather than leaning on bones fingers and toes.
“Curvature and cross-sectional geometric properties of the ulna … are indicative of common arboreal behaviors, including climbing and/or ‘cautious climbing,’ rather than terrestrial quadrupedalism,” the researchers write.
The research builds on an earlier study of a fossil skull excavated at the same site, which is also believed to belong to S. tchadensis. Analysis of the skull suggested that these ape-like creatures were bipedal, but now there is more complete evidence.
The fossils date to the time (6-8 million years ago) when humans split genetically from chimpanzees and bonobos, our closest living relatives, making it a crucial stage , and which has already attracted a lot of scientists. debate
These early hominins would likely have lived in an environment that mixed forests, palm trees, and grasslands, with both 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 Sahelanthropus is indicative of bipedality and that any other hypothesis would have less explanatory power for the set of features presented by the Chad material,” the researchers write.
The research has been published in Nature.