In the dark early chapters of human history, there is an unknown ancestor who made the profound transition from walking on all fours to standing upright – an act that has come to define us.
The chances of stumbling upon fossilized evidence of such an evolutionary prize are slim, but in a new study, scientists say an ape-like animal that lived in Africa 7 million years ago is the best candidate to date.
After a fresh analysis of bones belonging to a species called Sahelanthropus chadensisThe researchers concluded that although it resembled an ape, its bones were adapted for walking upright rather than walking on all fours. It is considered the oldest known hominin, or member of the human species, since its evolutionary split with chimpanzees.
“Based on the features we found, it could have been a bipedal ape, most similar to a chimpanzee or bonobo,” said Dr. Scott Williams, assistant professor at New York University and lead author of the study. Although chimpanzees and bonobos can walk upright for short distances, they primarily move on their knuckles.
The work is the last one in the debate that raged since 2001, when several Sahelanthropus the fossils were discovered in the Jourab Desert in Chad. When detection was made publicsuggested the team's lead researcher, Professor Michel Brunet from the University of Poitiers in France. Sahelanthropus walked upright because he held his head up. He declared this species “the ancestor of all mankind.”
Other scientists were less convinced that Sahelanthropus belonged to the human race. And without more bones, especially the lower body, it was difficult to understand how this happened. Partial femur and forearm bones from Sahelanthropus appeared later, but were unable to resolve the debate: researchers have still not reached a consensus on whether or not it belonged to an upright walker.
In this latest study, Williams and his colleagues set out to reexamine the femur and forearm bones using new techniques, comparing their size, proportions and three-dimensional contours to the bones of known hominins and apes. One feature especially caught their attention: a protrusion on Sahelanthropus the thigh bone, called the femoral tubercle.
“This is the attachment point for the largest and most powerful ligament in our body,” Williams said. “When we sit, this ligament is relaxed, and when we stand, it tightens. It prevents the torso from falling backward or side to side when walking, so it's a really important adaptation for bipedal walking. To my knowledge, this has only been found in bipedal hominins.”
The analysis revealed additional hallmarks of upright walking that other teams had noticed, such as the natural curve of the femur, which helps point the leg forward, and the muscles of the buttocks, which hold the hips in a stable position and help stand, walk and run. Details published in Science Advances.
According to Williams, the evidence points to an ape-like animal that lived around the time of the evolutionary split between humans and chimpanzees and walked the earth upright, if not all the time. “We think that the earliest hominids adapted to terrestrial walking,” he said, “but still relied on trees for food and for safety.”
But the case is far from closed. Dr Marine Cazenave, from the Max Planck Institute for Evolutionary Anthropology in Germany, said most of the findings pointed to similarities with African apes or extinct apes, and called the evidence for bipedalism “weak”. She also found the femoral tubercle inconclusive, adding that it was not directly related to upright posture and was “very weak” in a “severely damaged” area of the femur.
Dr Rhianna Drummond-Clark from the same institute found some of the evidence compelling, but she still had questions. “More work is needed to determine whether bipedal walking was used for walking in trees or for locomotion on land, the latter of which is a defining feature of human origins,” she said. The results may equally suggest Sahelanthropus She said it was an early chimpanzee that became less upright and began walking on its knuckles.
Dr. Guillaume Daver and Dr. Frank Guy from the University of Poitiers, who argued for a long time What Sahelanthropus was bipedal, welcomed the new evidence but said the debate would not be resolved without new fossils, which they hope to find when the Chadian-French team returns to the site this year. Everyone seems to agree on this. “I think it's a matter of too few fossils and too many researchers,” Williams said.
