This is based on a detailed analysis of the femur (femur) and forearm (ulna) fossils from 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’s likely that these early hominins did a fair amount of tree climbing using all four limbs—as would be expected if the species made the gradual shift 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 the fossil Sahelanthropus tchadensis. (Franck Guy/PALEVOPRIM/CNRS – University of Poitiers) By comparing the femur and forearm fossils to the equivalent bones from humans, chimpanzees and gorillas, the researchers were able to understand the mechanics of how they were used and how this species moved (its ‘motor mode’). A total of 20 different features of the fossil bones were used to determine whether S. tchadensis walked on two or four legs, including the external shape of the remains and internal structures, assessed via microtomographic imaging. They concluded that “ordinary 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 hand grips, rather than resting back on the bones of the fingers and toes. “The curvature and cross-sectional geometric properties of the ulna … are indicative of common arboreal behaviors, including climbing and/or ‘careful climbing,’ rather than a terrestrial tetrapod,” the researchers write. The research builds on a previous study of a fossil skull discovered at the same site, also believed to belong to S. tchadensis. Analysis of the skull has shown that these ape-like creatures were bipedal, but now there is more complete evidence. The fossils date back to the time (6-8 million years ago) when humans split genetically from chimpanzees and bonobos, our closest living relatives, so it’s a critical stage – and one that has already attracted many scientific public debate. These early hominins would probably have lived in an environment that mixed forests, palm trees 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 Sahelanthropus is indicative of bipedalism, and that any other hypothesis would have less explanatory power for the set of features exhibited by the material from Chad,” the researchers write. The research was published in Nature.
