Twenty years ago, scientists discovered a 7-million-year-old skull that they concluded belonged to a creature that walked upright and was our earliest known ancestor. Not everyone was convinced. Now the researchers are back with more evidence they say strengthens their case.
Their new study published Wednesday analyzed arm and leg fossils found near the skull in Africa, looking for signs of walking on two feet rather than all fours. When early humans began to walk upright, it marked a key moment in our split from apes
In the article in the journal Nature, researchers again place the creature only on the human side of the evolutionary divide. The fossil species, called Sahelanthropus tchadensis, walked upright while still being able to climb around trees, they reported.
The species has been dated to around 7 million years ago, making it the oldest known human ancestor by a long shot. It is about a million years older than other early known hominins.
But it has been a source of heated debate since the fossils were first uncovered in Chad in 2001.
Researchers – also led by researchers at the University of Poitiers in France – first looked at the fossil creature’s skull, teeth and jaw. They argued that the creature must have walked on two feet and held its head upright, based on the location of the hole in the skull where the spinal cord connects to the brain.
Other experts were not swayed by the early evidence.
The latest work includes a femur that was not initially associated with S. tchadensis and was not studied for years. Other researchers at the French university found the bone in the laboratory’s collection and realized that it probably belonged to the fossil species.
Compared with bones from other species, the femur was a better fit with upright humans than knuckle-walking apes, according to the study.
“It’s not a feature. It’s just a total pattern of features,” co-author Franck Guy said of their analysis at a news conference.
Nevertheless, the debate about the species is likely to continue.
Ashley Hammond, a researcher at the American Museum of Natural History in New York, said more research is needed to determine the creature’s place on the evolutionary tree.
“I’m not completely convinced yet,” Hammond said. “This could still also be a fossil monkey.”
Another researcher at the French university, Roberto Macchiarelli, had previously examined the femur and determined that the species was probably an ape. Looking at the new study, Macchiarelli said he still doesn’t believe the species was a hominin, even though it may have walked on two legs at times.
Rick Potts, director of the Smithsonian’s Human Origins Program, said the femur puts the species on “better footing” as a possible early human ancestor. But the real confirmation comes down to a common saying in the field: “Show me more fossils.”
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