One of the greatest questions concerning human evolution is when and why our ape ancestors developed upright trunk posture. The vertebrae in the lower backs of humans and other members of the ape family exhibit differences from those of their closest living relatives—monkeys, which use a horizontal posture to walk on four legs.
It may seem a natural choice to compare human and other ape spines to monkey spines in order to determine some form of evolutionary shift.
However, this can be challenging because apes display a multitude of different behaviors involving upright postures, including arm swinging and climbing, that make it difficult to pinpoint which factors caused which evolutionary shift. Therefore, scientists have had trouble attributing the evolution of the differences in the lower backs of apes and monkeys to specific behaviors.
A study by Gabrielle Russo, an assistant professor in the Department of Anthropology at Stony Brook University, and her collaborator Scott Williams, an assistant professor in the Department of Anthropology at New York University, has helped shed some light on this issue.
In an attempt to isolate certain behavioral factors from others, Russo and Williams chose to compare the lower backs of different species of bears.
“This study is a good example of what researchers call the comparative method,” Russo said, “which is to look at distantly related taxa that do similar behaviors and see if they might converge morphologically in certain ways.”
Bears may not seem like an intuitive choice, but one species of bear in particular tends to spend some time sitting on its bottom with its back upright.
“Most bears use horizontal trunk postures and walk on four legs, but giant pandas are different because they spend a considerable amount of their time using upright trunk postures,” Russo said. “When they are feeding, particularly on bamboo, they spend a lot of time sitting upright.”
Russo and Williams found that giant pandas have fewer lower back vertebrae than other bears and the giant panda spines had features that were different from those of other bears’ spines. These differences mirror the differences between the lower backs of apes and monkeys.
Unlike apes, giant pandas do not exhibit complex motions that involve upright posture, such as arm swinging, but they still display the same types of differences in lower-back anatomy from their closest living relatives that apes do. The convergence of giant panda and ape lower-back adaptations may explain that the differences in lower backs between apes and monkeys are simply due to upright posture.
“These anatomies could have evolved simply in the context of upright posture,” Russo said. “We don’t know what behaviors those postures would have been pat of, so that’s still an open door. But what we can say is that it could be upright posture in general and that all of these specialized behaviors could have come afterwards.”