“Our earliest known representatives practiced bipedalism (on the ground and in trees),” said study author Franck Guy, a researcher at the University of Poitiers in France. Fossils suggest that bipedalism arose soon after the evolutionary separation of chimpanzees and human ancestors.
More will be found in these fossils. According to the study, their characteristics show that Sahelanthropus tchadensis also retains the ability to skillfully climb trees.
These ancestors were more closely related to humans than to hominins or chimpanzees, and they mark an early stage of our evolutionary divergence, says Daniel Lieberman, a professor of human evolutionary biology at Harvard University and a paleoanthropologist. Lieberman did not participate in the study.
Bipedalism in these ancestors is not at all surprising. The arm and leg bones analyzed in the study were found in Chad in 2001, along with a nearly complete skull. It is unclear whether they came from the same individual, said study author Guillaume Daver, associate professor of paleontology at the University of Poitiers.
Lieberman said the skull showed a downward-pointing point where the head and spinal cord meet — a feature that would have made walking on all fours very difficult.
A new analysis of the limbs from this find provides more evidence that hominins traveled on two legs as they roamed the Earth around 7 million years ago.
“It’s a look at what set the human race on a separate evolutionary path from our ape cousins,” Lieberman said. Although recent findings support what early research suggested, fossils from this time are rare, so each discovery is important evidence.
And the new study “makes it very unlikely that the common ancestor we share with chimpanzees is chimpanzee-like,” Guy said.
Bipedalism fueled the fire
Lieberman said bipedalism was extremely important to our evolution, but it didn’t mean much to our ancestors.
He added that walking on two legs makes the animal slower, more unstable and more at risk of back pain, neither of which is conducive to survival.
“There was a really big advantage,” Lieberman said. Scientists have speculations about what it could be.
Our common ancestor with apes was very similar to a chimpanzee, Lieberman says, and we know that they must use a lot of energy to walk — twice as much as humans when you adjust for body size.
When humans and chimpanzees diverged, Earth’s climate was changing and African rainforests were fragmenting, so our ancestors had to travel farther to get food, he said. The hypothesis is that walking on two legs gives them more energy to travel.
“What put us on this different evolutionary path is that we were bipedal, or we walked on two legs,” Lieberman said. “It helps us understand the origins of humanity.”
There are many things that define us as humans, such as language, tools and fire, he said. And in the 1870s, Charles Darwin speculated — without any evidence we have now — that walking on two legs was the spark that started it all, Lieberman said.
Lieberman said we can now see that bipedalism was a big differentiator from apes and helped free up our hands to make tools.
“We proved Darwin right,” I said. “That’s pretty cool.”
