“Our oldest known representatives were bipedal (on the ground and in trees),” said study author Frank Jay, a research fellow at the University of Poitiers in France. He added that the ancient remains show that bipedalism appeared shortly after chimpanzees and human ancestors diverged in their evolutionary paths.
There is more to be found in these fossils. Their characteristics show that Sahelanthropus tchadensis also preserved the ability to climb trees efficiently, according to the study.
These ancestors were hominins, or species more closely related to humans than chimpanzees, and they represent an early stage in our evolutionary divergence, said Daniel Lieberman, professor of human evolutionary biology and paleontologist at Harvard University. Lieberman was not involved in the study.
Walking on two legs in these ancestors is not exactly a surprise. The study said that the arm and leg bones analyzed in this study were found in Chad in 2001 along with a nearly complete skull. Study author Guillaume Daver, assistant professor of palaeontology at the University of Poitiers, said it was unclear if they belonged to the same person.
Lieberman said the skull showed a point pointing downward where the head and spinal cord meet — a feature that would make walking on all fours more difficult.
He added that the new analysis of the limbs from this discovery provides more evidence that hominins were traveling on two legs when they roamed the Earth about 7 million years ago.
“It’s a glimpse into what set the human lineage on an evolutionary path separate from our ape cousins,” Lieberman said. He added that while recent findings support what early studies have already suggested, fossils from this time are scarce, so each discovery is an important piece of evidence.
Jay said the new study “makes it unlikely that the common ancestor we share with chimpanzees is anything like chimpanzees”.
The motion on two feet lit the fire
Lieberman said that walking on two legs was very important to our evolution, but it didn’t make sense for our ancestors.
He added that walking on two legs makes the animal slower, more erratic and more at risk of developing back pain, none of which is beneficial for survival.
“There must have been a really big advantage,” Lieberman said. Scientists have a hypothesis about what it could be.
Our common ancestors with apes are very similar to chimpanzees, Lieberman said, and we know they need to use a lot of energy to walk — twice as much as humans do when they are adapted to body size.
As the evolutionary paths of humans and chimpanzees diverged, he said, the Earth’s climate was changing and the rainforests of Africa were fragmenting, so our ancestors had to travel farther to find food. The hypothesis is that walking on two legs gave them more energy to travel.
“What really drove us down this different evolutionary path is that we were walking on two legs, or we walked on two legs,” Lieberman said. “It helps us really understand the origins of humanity.”
He said that there were many things that defined us as human beings, such as language, tools, and fire. Lieberman said that in the 1870s Charles Darwin guessed—without any evidence we have now—that walking on two legs was the spark that started it all.
We can now see that walking on two legs was a huge difference from monkeys and helped free our hands for tool development, Lieberman said.
“We’ve proven Darwin right,” he said. “That’s kinda cool.”
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