When we humans strut our stuff, we do it by coordinating the movements of our hips and upper bodies. As the pelvis rotates forward, the trunk moves in the opposite direction, cancelling out angular momentum and reducing the amount of energy burned while walking. Finally, swinging arms counterbalance the sway of the hips, completing the characteristic human gait.
Chimpanzees, on the other hand, can be trained to walk on two hind legs and will occasionally do it in the wild, but it is not their preferred means of getting around. When they do walk upright, their compact trunks and tall, wide hips cause them to stoop. As they make their way forward, the trunk appears rigid while the swing of the hips and arms seems overly pronounced and somewhat clumsy.
Pairing that observation with studies of chimp bone structure, researchers had long assumed that our closest relatives lack the counter-rotations characteristic of human motion. Following this logic, scientists also concluded that human ancestors prior to Homo erectus—whose morphology shares commonalities with chimps—likely walked that way, too.
Until now, however, no one ever verified that assumption. And as it turns out, it is not correct.
Using kinematic analysis, a team of researchers from Stony Brook University and the University of Arizona College of Medicine found that chimp and human locomotion share more similarities than previously thought. That suggests our chimp-like human ancestors, such as Australopithecus afarensis, might have been some of the first hominins to stand on their own two feet.
Hercules and Leo, two chimps trained to walk upright, helped the researchers arrive at these findings. The scientists attached motion-measuring markers on numerous points on the chimps as well as on human volunteers, and then measured the paths those markers took as their wearers walked forward. This allowed the team to compare how our two related species move, and also to break down each style of walking into its specific parts.
Contrary to common assumptions, they found that the chimps' upper bodies do twist slightly as they walk but their ribs and hips move in the same direction. Humans, meanwhile, move those structures in the opposite direction.
The chimps’ sway does work to conserve some energy, and the degree to which their rib cages move is almost the same as that of humans. The team found only a 0.4-degree difference in axial rotation between the human and chimp trunks.
“These results show that chimpanzees utilize [trunk] rotations to counter pelvic rotations in much the same was as humans,” the authors write.
As they report this week in Nature Communications, these findings disprove the assumption that chimps are completely rigid up top, and they have interesting implications for the evolution of bipedal walking in humans.
Even if early chimp-like hominins had chimp-like pelvises that rotated up to 50 percent more than those of modern humans, they could probably still walk upright and save energy by swinging their trunks in time with their hips.
Two-legged running, however, which requires larger cancellations between movement of the hips and trunk, “may have been somewhat less effective,” the team writes. Future research may probe when human ancestors switched from in-synch to out-of-synch movements of the hips and trunk, and why evolution favored that path for our upright locomotion.