Bipedal Sahelanthropus tchadensis
According to research on three limb bones, the oldest known progenitor of modern humans climbed through trees and walked on two feet roughly seven million years ago.
The age of the first known representative species of humans was advanced by a million years when the skull of Sahelanthropus tchadensis or "Toumai," was found in Chad in 2001. However, at the time, the issue sparked intense controversy among scientists, in part because of the rarity and poor quality of the accessible bones. Some even asserted that it was simply an ancient ape and not a human relative.
Because of the way it’s spinal column articulated with the nearly intact cranium was considered to be bipedal, which was another contested claim. A team of scientists thoroughly examined two forearm bones and a thigh bone discovered at the same location as the Toumai cranium. They published their findings today in the journal Nature. The latest findings on the limb bones show that it preferred to be bipedal, but they would also occasionally weave amongst the trees like other great apes.
The leg and arm bones were discovered in 2001 together with tens of thousands of other fossils, but the scientists were unable to establish that they belonged to the same individual as the Toumai skull. In fact, despite being discovered close to the skull, the femur and two forearm bones were actually not first recognized as being a member of the Sahelanthropus fossil.
After years of testing and analyzing the bones, they discovered 23 traits that were then matched to fossils of hominins rather great apes. Their comparative sample included chimpanzees, gorillas and orangutans, fossil Miocene apes, early hominin bipeds like Orrorin tugenensis and Ardipithecus Ramidus, and remains of prehistoric Homo and Homo sapiens. To find out if the bone's features matched those of those known to be well-suited to the demands of force, balance, and other requirements of upright walking, researchers evaluated the bone's outward shapes, curves, internal structures, and thicknesses.
The examination of the femur, in particular, was challenging because the bone lacks the joints on either end, which are important diagnostic traits that could have settled the question of whether the species was bipedal. If the femur was designed to support the entire weight of the body on one leg at a time, it would be visible at its neck, which links to the hip socket. Another indication of habitual bipedalism is the distal knee end, which would be seen if alignment kept body weight below the center of gravity. S. tchadensis' femur had numerous features with other hominin species, however none of these traits were only present in apes.
The study of the ulnae, the bigger of the two forearm bones that extends from the elbow to the little finger, determined that Sahelanthropus' arms were exceedingly apelike, similar to chimpanzees. Furthermore, unlike gorillas and chimpanzees, the Sahelanthropus did not exhibit any indication of leaning on the back of its hands in the forearm bones. They came to the conclusion that these traits are significantly more similar to those of a hominin than any other ape. The species was therefore quite skilled in the trees as well as bipedal!
According to the authors, this might have suited an opportunistic lifestyle that was probably quite beneficial in the varied environment of Toros-Ménalla seven million years ago. At that time, the northern Chad Basin's stretch of wetlands preserved a patchwork of forest cover, palm groves, and less forested, more grass-rich areas in what is now a desert. Therefore, Sahelanthropus tchadensis had access to resources found in arboreal, terrestrial, and aquatic settings.
This data implies that, from the time of Sahelanthropus through A. afarensis, which exhibited these adaptations up until around three million years ago, hominins may have been equipped to travel both habitually bipedally and in the trees for about four million years.
Sahelanthropus has two of the fundamental adaptations that all later hominins share and which aren't present in the chimpanzee genus; these are a reshaped femur for habitual upright walking and, as demonstrated in earlier studies of the skull, reduced canine teeth that altered the mouth. These characteristics are also present in other fossils of the oldest known hominins, such as Ardipithecus ramidus, which lived approximately 4.4 million years ago, and Orrorin tugenensis, which lived around 6 million years ago.
A research paper published in the Journal of Human Evolutiononly two years ago claimed that the identical femur belonged to a hominin who did not walk on two feet regularly. Even long time anthropology blogger and professor, John Hawks, has questioned Toumai’s bipedalism. He says,
“The two teams who have collected data from the femur seem to disagree entirely about what the femur shows. They’re looking at the same piece of bone. I don’t understand how they disagree about this. If either group could just release (surface 3-D and internal CT scan) data so that we can all examine it, there would be no reason for this disagreement.”
Daver, G., Guy, F., Mackaye, H.T. et al. Postcranial evidence of late Miocene hominin bipedalism in Chad. Nature (2022). https://doi.org/10.1038/s41586-022-04901-z
Macchiarelli R, Bergeret-Medina A, Marchi D, Wood B. Nature and relationships of Sahelanthropus tchadensis. J Hum Evol. 2020 Dec;149:102898. doi: 10.1016/j.jhevol.2020.102898. Epub 2020 Nov 1. PMID: 33142154.