When postdoctoral researcher Nicholas Holowka first met Leo and Hercules, he didn’t realize the two chimpanzees would help him demonstrate that, while walking on two limbs, human feet are actually more mobile than those of chimpanzees.
The study, entitled “Chimpanzee and human midfoot motion during bipedal walking and the evolution of the longitudinal arch of the foot” and published in the March 2017 issue of the Journal of Human Evolution, now prompts scientists to reevaluate how our ancestors’ feet operated.
“Chimps’ [feet] are more rigid, humans’ are more rigid, or they’re equal. Those three possibilities, people have discussed in papers in the past,” Kristian J. Carlson, associate professor of Clinical Cell and Neurobiology at the University of Southern California, said. “But until this study, nobody’s really, really been able to show that humans are a lot more mobile than we think.”
Chimpanzees, our closest living relatives, and all other primates lack something crucial that humans possess – the longitudinal arch. This arch in human feet spans multiple joints in the midfoot area, which is thought to help humans walk more efficiently, according to the study. Scientists have observed that this arch has made human feet stiffer than primate feet, at least when the heel is first lifted off the ground in a walking step. Meanwhile, primates have mobile foot joints that allow them to easily grasp branches when climbing trees.
However, Holowka, principal investigator of the study, wanted to find out how mobile chimpanzees are when they’re walking on two limbs, rather than climbing or knuckle-walking – “to understand what individual joints are doing and really quantify their motion in a precise way,” Holowka said.
To find out, Holowka and colleagues Matthew C. O’Neill, Nathan E. Thompson and Brigitte Demes used high-speed motion capture in individual recording sessions with the two chimpanzees and five male humans at Stony Brook University from 2013 through 2015. Researchers painted small, circular markers on each foot and leg. Every marker corresponded with a bony structure deep in the skin. Each chimpanzee and human then walked in front of multiple cameras at what Holowka called a normal walking pace.
“The advanced motion capture entails several cameras on the wall mounted on one side of the room, and then there’s a runway on the other side of the room where the chimps move, following the animal trainer,” Brigette Demes, Ph.D. and co-researcher, said.
As each subject moved in a steady line, the cameras captured the movement of those markers. This allowed the researchers to reconstruct three-dimensional positions of the moving legs, feet and underlying bony structures – something they could not have calculated with the naked eye. From those 3-D positions, they could calculate 3-D angles or the range of motion for each of the foot’s planes. One range of motion that particularly caught their eye was flexion-extension.
“What I mean by flexion-extension here is motion in a plane that splits the foot approximately in 2 (going through the heel and the 3rd toe, roughly). The axis about which this motion takes place is perpendicular to the plane, going through roughly the midfoot area,” Holowka said in an email.
They had noticed something extraordinary. Chimpanzees had about 11 degrees of total motion. Humans had 18.
Statistically speaking, the seven-degree difference is quite a lot, Holowka said. It means that humans use about 65 percent more motion than chimpanzees. That number also demonstrated something critical: a human’s midfoot region flexes more dramatically at the end of a walking step than researchers had previously believed. Human feet are actually more mobile than chimpanzee feet while walking on two limbs.
“Improving the understanding of that form-function relationship is essential to interpreting the fossil record and reconstructing how extinct species of hominims moved around in their environments,” Scott A. Williams, Ph.D. and assistant professor of anthropology at New York University, said in an email.
This finding may not only lead other researchers to reexamine their current stance on how our ancestors’ fossil feet work, but also help us learn more about how humans adapted and how feet have evolved, with the help of our closest living relatives.
“The project was about chimpanzees, but it’s actually also about human feet. You can’t really understand about what our feet do unless you’re able to look at it in comparison to something else,” Holowka said. “Chimpanzees tell us a lot about ourselves.”
Correction: An earlier version of this story misquoted Brigette Demes saying, “… and then there’s a one-way on the other side of the room where the chimps move, following the animal trainer.” Demes actually said, “… and then there’s a runway on the other side of the room where the chimps move, following the animal trainer.”