A new study has found never-before-seen ridges on the foot bones of a 7-million-year-old fossil ape, indicating that they walked upright on two legs while on the ground.
This lump, called the femoral tubercle, is only found in members of the human lineage. This species, Sahelanthropus tchadensis, is the oldest known hominid, according to a study published January 2 in the journal Science Advances. (Hominidae is a group of species including humans that existed after the split from chimpanzees and bonobos. Upright bipedalism is a characteristic of hominids.)
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Reanalysis of the S. tchadensis femur also identified two additional human-like anatomical features. First, like modern humans, the bones were twisted inward, placing the knees closer than the hips. Second, there is a distinct lump on the side of the fossil’s largest hip muscle, which is absent in modern non-human apes.
The curved arm bones of S. tchadesis suggest that the species was adapted to tree climbing, similar to modern chimpanzees and bonobos. However, its hips and knees function similarly to those of hominids, suggesting that the ape frequently walked bipedally during its time on land.
“I think they must have been on the ground for quite some time to evolve bipedalism,” Williams said.
Fossils are hotly debated
Found in modern-day Chad, S. tchadensis was first described in 2002 and remains highly controversial. The authors of that study argued that fossil apes were the earliest known hominins based on the location of an opening in the skull to which the spinal cord attaches, called the foramen magnum. The opening was in the center of the skull, suggesting that the ape stood upright like humans, but some have argued that the position does not prove that S. tchadensis walked on two legs.
Twenty years later, fragments of two forearm bones, an ulna and a femur, belonging to S. tchadensis were uncovered. The authors claimed that the femur belonged to a bipedal ape. However, other scientists disagreed with this assessment, saying that the shape of the femur was not indicative of frequent bipedalism.
Williams said he was unsure whether S. tchadensis was bipedal and therefore a hominin because it is “very ancient.” The ape lived about 6 million to 7 million years ago, when scientists believe the last common ancestor of humans and chimpanzees lived. Rather than being a hominid, S. tchadensis may have been an ancient ape more closely related to chimpanzees and bonobos than humans, he explained.
Curious about the answer anyway, Williams and his team examined 3D scans of limb bones. They examined various distinctive features of the femur and compared these bones to the femurs of all living and extinct apes in which they exist.
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This analysis revealed that the size and shape of the ulna and femur of S. tchadensis are similar to those of modern chimpanzees and bonobos. “We were receiving very great ape signals,” Williams said.
But there was an important difference that led the researchers to believe that the great ape was bipedal. Their analysis confirmed the presence of a medially twisted femoral shaft and a maximal gluteus muscle attachment, both of which are associated with the hominid way of moving.
But crucially, they discovered something no one had noticed before: a small bump on the upper front of the femur. “This is a very subtle little bump that you couldn’t see at first just by looking at the fossil, but you could identify it by rubbing your thumb along the fossil and bumping into it,” Williams said. The team then confirmed that the original S. tchadensis fossil also had this lump.
“It’s beyond convincing,” Jeremy DeSilva, a biological anthropologist at Dartmouth College who was not involved in the study, told Live Science. “I pulled this out right away. [the femur 3D scan] I said, “Wait, why didn’t I see this?” And certainly some of the important anatomical structures that they point out in this paper can be seen in this fossil,” he said. I wish I had seen these things. ”
The study makes the question of what the last common ancestor of humans and chimpanzees looked like “even more puzzling and interesting,” DeSilva said.
If S. tchadensis was a hominin, as Williams believes, it could suggest that its ancestor was more chimpanzee-like than human. However, DeSilva said S. tchadensis could potentially be a bipedal ape not of the human lineage.
“So the question we have now as an area to work on is can non-hominin bipedalism be possible? Is that possible?” he said.
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