The discovery of a never-before-seen bump on the leg bone of a 7 million-year-old fossil ape shows it walked upright on two legs while it was on the ground, a new study finds.
Only members of the human lineage have this lump, called the femoral tubercle. That makes the species, Sahelanthropus tchadensis, the earliest known hominin, according to the study, published Jan. 2 in the journal Science Advances. (Hominins are the group of species, including humans, that existed after the split from chimpanzees and bonobos. Walking upright on two legs is a defining characteristic of hominins.)
"That [bump] really sold it to us," study lead author Scott A. Williams, an associate professor of paleoanthropology at New York University, told Live Science. "That really convinced me that, OK, we think it's a biped, and therefore, we think it's a member of the hominin lineage."
The reanalysis of S. tchadensis' femur also confirmed two more human-like anatomical features. First, the bone twisted inward, placing the knees closer together than the hips, as in modern humans. Second, there was a distinct lump on the side of the fossil where the largest glute muscle attaches, which isn't found in living nonhuman apes.
S. tchadesis' curved arm bones suggest that, like modern-day chimps and bonobos, the species was adapted to climbing trees. But its hips and knees functioned like those of hominins which suggests the ape frequently walked bipedally while on the ground.
"I think it must have been on the ground a significant amount of the time in order to evolve bipedalism," Williams said.
A hotly debated fossil
Discovered in modern-day Chad, S. tchadensis was first described in 2002 and remains highly controversial. The authors of that study claimed the fossil ape was the earliest known hominin based on the position of the opening in the skull where the spinal cord attaches, called the foramen magnum. The opening was in the middle of the skull, which suggests the ape stood upright like humans do, but others argued that positioning didn't prove S. tchadensis walked on two feet.
Twenty years later, two forearm bones, or ulnae, and a femur fragment belonging to S. tchadensis were revealed. The authors argued that the femur belonged to an ape that walked on two legs. But other scientists disagreed with this assessment, stating that the thigh bone shape did not indicate frequent bipedality.
Williams said he was on the fence about S. tchadensis being bipedal — and, therefore, a hominin — because it is "really old." The ape lived around the time scientists believe the last common ancestor of humans and chimpanzees lived, approximately 6 million to 7 million years ago. Rather than being a hominin, S. tchadensis may have been an ancient ape more closely related to chimps and bonobos than to humans, he explained.
Because he was interested in the answer either way, Williams and his team inspected the 3D scans of limb bones. They looked at various hallmark features on the femur and compared them to the thigh bones of all living and extinct ape species for which these bones exist.
This analysis revealed that the size and shape of S. tchadensis' ulna and femur resembled those of modern-day chimps and bonobos. "We were getting a very great-ape signal," Williams said.
But there were key differences that convinced the team that this ape was bipedal. Their analyses confirmed the presence of an inwardly twisted femur shaft and the attachment for the largest glute muscle, both linked to a hominin way of moving.
Critically, however, they spotted something no one had previously noticed: a tiny bump on the top front of the femur. "It's a very subtle little bump that I actually didn't identify initially by looking at the fossil but by rubbing my thumb along it and bumping into it," Williams said. The team then verified 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 research, told Live Science. "I immediately pulled this [the femur 3D scan] out and said, 'Wait, how did I not see this?' And sure as day, some of the key anatomies that they point out in this paper, I can see in this fossil," he said. "I'm kind of kicking myself. I wish I had seen these things."
This research makes the question of what the last common ancestor between humans and chimpanzees looked like "even more puzzling and fascinating," DeSilva said.
If S. tchadensis was a hominin, it could suggest, as Williams believes, that this ancestor was more chimpanzee-like than human-like. However, DeSilva said S. tchadensis could potentially be a bipedal ape not on the human lineage.
"So the question we now have as a field that we have to contend with is," he said, "can you be bipedal and not be a hominin? Is that possible?"
