At what point did our ancestors learn to walk on two legs? It has long been assumed that because early human ancestors had a similar pelvis and ribcage set up – known as the “trunk” – to chimpanzees, that their ability to walk on two legs must have been limited. But new research has shown that chimps are actually far more efficient at bipedal walking than was assumed from looking solely at their skeleton, suggesting that early humans were too.
“For a long time we've have had this textbook idea that chimpanzees (and other great apes) have a rigid trunk,” Nathan Thompson, from Stony Brook University, explained to IFLScience via email. This is due to the build of their skeleton, which has a big, wide ribcage and pelvis, and a short lower back thought to limit movement. Humans on the other hand have longer, taller trunks which are much more flexible. Because the early human Australopithecus afarensis has a similar skeletal build to chimps, it’s long been assumed they also had rigid trunks.
Walking bipedally with such a robust trunk is more energetically demanding than for those with a greater range of movement, largely due to increased muscle engagement. This is thought to be the main reason that bipedalism is restricted in those with rigid trunks. But it turns out that despite what skeletons might imply, the trunk of chimpanzees is surprisingly flexible. “What we found though was that the amount of rotation that happens between the pelvis and ribcage during walking was virtually identical between humans and chimpanzees,” said Thompson, who coauthored the paper published in Nature Communications.
The researchers compared the bipedal walking of two chimps, Hercules and Leo, with that of a human. Stony Brook University/YouTube/Nature Video
By placing markers on different parts of both human and chimpanzees and tracking their movements as they walked bipedally, the researchers were able to analyze the way the pelvis and ribcage rotated in relation to each other. When humans walk, their ribcage rotates in the opposite direction to our hips, which help us save energy. When chimps walk upright, the researchers found to their surprise that not only did the trunk rotate in the first place, but that their hips rotated in the same direction as their ribcage.
“The fact that chimpanzees can use their upper bodies to compensate for pelvis motion tells us that our early ancestors probably could have as well, and that there’s no reason to think that the chimp-like aspects of A. afarensis pelvis and ribcage would have hindered their walking ability,” said Thompson. “Depending on how much their pelvis rotated, they may have been able to use the type of opposite rotations humans do, in order to save energy during locomotion.”
The results go to show just how much we still have to learn, even about our closest living relative the chimpanzee. Even though it’s been textbook knowledge that chimps have a rigid trunk, the researchers have been able to prove this is not the case. It also raises questions about the evolution of our own physiology. If our long, slender trunk didn’t evolve for walking as was thought, then what behavior selected for it?