“Our oldest known representatives (on the ground and in trees) practiced bipedalism,” said Frank Guy, a research associate at the Université de Poitiers in France. Remains of ancient species show that bipedalism emerged after chimpanzees and human ancestors diverged on their evolutionary tracks, he added.
There are many more of these fossils. According to the study, their characteristics show that Sahelanthropus tchadensis has also maintained the ability to climb trees efficiently.
These ancestors are species more closely related to humans than to hominins or chimpanzees, and represent an early stage of our evolutionary divergence, said Daniel Lieberman, a paleoanthropologist and professor of human evolutionary biology at Harvard University. Lieberman was not involved in the study.
Bipedalism in these ancestors is not surprising. The arm and leg bones analyzed in this study were discovered in Chad in 2001 along with a nearly complete skull, the study said. However, it is not clear whether they came from the same person, said study author Guillaume Daver, assistant professor of paleontology at the Université de Poitiers.
The skull showed a downward-pointing point where the head and spinal cord meet — which would make walking on all fours more difficult, Lieberman said.
New analysis of the limbs from that discovery provides further evidence that hominins traveled on two legs when they roamed the Earth about 7 million years ago, he said.
“It’s a glimpse into what set human descent on a separate evolutionary path from our ape relatives,” Lieberman said. While the latest findings support what earlier studies had already suggested, fossils from this period are rare, so each discovery is an important piece of evidence, he added.
The new study “makes it unlikely that the common ancestor we share with chimpanzees looked like a chimpanzee,” Guy said.
The double bridge caught fire
Bipolarity is important to our evolution, but it didn’t make much sense to our ancestors, Lieberman said.
Walking on two legs makes an animal slower, more unstable and at risk for back pain, neither of which is conducive to survival, he said.
“There must have been some big advantages,” Lieberman said. Scientists have a hypothesis about what it might have been.
Our common ancestor with apes was like a chimpanzee, and we know that they used a lot of energy to walk — twice as much as humans when you adjust for body size, Lieberman said.
When the evolutionary paths of humans and chimpanzees diverged, Earth’s climate was changing and rainforests were breaking up in Africa, so our ancestors had to travel farther to get food, he said. The hypothesis is that walking on two legs gave them more energy to travel.
“What really put us on this different evolutionary path was that we were bipedal, or we walked on two legs,” Lieberman said. “It helps us really understand the origins of humanity.”
He said many things like language, tools, fire have defined us as humans. In the 1870s, Charles Darwin — without any evidence we have now — speculated that walking on two legs was the spark that started it all, Lieberman said.
“Now we can see that bipedalism was a big differentiator from apes and helped free our hands to make tools,” Lieberman said.
“We proved Darwin right,” he said. “That’s kind of cool.”
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