A chimp-sized human ancestor walked upright 6 million years ago, far earlier than anyone had been able to show before, researchers report.
Specialized X-ray scans of the top of a fossil thighbone show clear evidence that the creature walked upright, like pre-humans, and not like apes, the researchers said.
Their findings, published in Friday’s issue of the journal Science, take the dawn of human gait back another 3 million years from “Lucy,” the earliest known pre-human to have walked on two legs.
“We have solid evidence of the earliest upright posture and bipedalism securely dated to 6 million years,” said Dr. Robert Eckhardt, a professor in the Laboratory of Comparative Morphology and Mechanics at Pennsylvania State University.
This older species, known scientifically as Orrorin tugenensis, lived in what is now the Kenyan Lukeino Formation.
The international team of researchers studied bones dug up nearly four years ago. One thighbone includes the intact head of the left thighbone — the ball that is inserted into the hip socket joint. The bones are about the same size as a modern chimpanzee’s. But they look quite different.
The researchers ran computed tomography or CT scans on the bones. These computer-enhanced X-rays create a three-dimensional image. They found that the neck connecting the ball to the shaft was thinner on top than it was on the bottom, a sign that the creature walked on two legs.
“In present-day chimps and gorillas, the thicknesses in the upper and lower parts of that bone are approximately equal,” Eckhardt said in a statement. “In modern humans, the bone on top is thinner than on the bottom by a ratio of one to four or more. The ratio in this fossil is one to three.”
Genetic evidence suggests that chimps and human diverged from a common ancestor 7 million years ago.