(ORDO NEWS) — The transition to walking on two legs instead of four is an important moment in the evolution of our species, so scientists are racing to pinpoint exactly when this happened – and in the new study, this transition occurred about 7 million years ago.
This is based on a detailed analysis of the femur (femur) and forearm (ulna) fossils of Sahelanthropus tchadensis, the oldest member of the human race. These fossils were first discovered at Toros Menalla in Chad in 2001.
At the same time, it is likely that these early hominins climbed trees using all four limbs—as would be expected if the species gradually transitioned from four legs to two.
“Here we present postcranial evidence for locomotor behavior in S. tchadensis, providing new insight into bipedalism early in hominin evolutionary history,” the researchers write in their published paper.
By comparing hip and forearm fossils with those of humans, chimpanzees and gorillas, the researchers were able to figure out the mechanics of their use and the species’ mode of locomotion (its “locomotor mode”).
To determine whether S. tchadensis walked on two or four legs, 20 different characteristics of the fossilized bones were used, including the external shape of the remains and internal structures assessed using microtomography.
They concluded that “habitual bipedalism” with some tree climbing is the most likely scenario.
The team also suggests that there is a difference between how members of this species climbed trees compared to modern gorillas and chimpanzees: using strong hand grips rather than resting on the bones of their fingers and toes.
“The curvature and geometric cross-sectional properties of the ulna … are indicative of habitual dendrological behavior, including climbing and/or ‘careful climbing’, rather than a terrestrial tetrapod,” the researchers wrote.
The study is based on an earlier study of a fossil skull excavated at the same site and believed to be S. tchadensis. Analysis of the skull suggested that these ape-like creatures were bipedal, but now there is more complete evidence.
Fossils date back to around the time (between 6-8 million years ago) when humans genetically diverged from chimpanzees and bonobos, which are our closest living relatives, so this is a pivotal milestone that has already sparked a lot of scientific debate.
These early hominins likely lived in an environment where forests, palm groves, and pastures intermingled, and walking on two legs and climbing trees were options for them to find food and water.
“The simplest hypothesis remains that the postcranial morphology of Sahelanthropus is bipedal, and that any other hypothesis has less explanatory power for the set of characters represented by material from Chad,” the researchers write.
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