Although our human ancestors climbed down from the trees millions of years ago, our feet have retained the type of flexibility seen in today’s tree-dwelling primates, new research shows.
For about 80 years at least, scientists have been under the impression that human feet are special. Studies comparing human and chimpanzee footprints in the 1930s suggested that human feet were much stiffer than those of other apes. Humans have arches in the mid-foot region, as well as rigid outer edges, leading scientists to conclude that our mid-foot regions weren’t capable of touching the ground, unlike those of our ape relatives.
More recent evidence has suggested the contrary. Studies that measured foot mechanics with machines have hinted that some people’s arches often make contact with the ground as they walk. However, these machines were able to measure only a few footsteps at a time.
Thanks to a pressure-sensitive treadmill, researchers at the University of Liverpool were able to examine 25,000 footsteps made by 45 healthy human volunteers. About two-thirds of those volunteers took some steps in which their mid-foot touched the ground, they found.
The team also used a pressure-sensitive platform to measure the footsteps of 11 bonobos and eight orangutans, two species of tree-dwelling great apes. The range of their mid-foot motion had some overlap with those of humans, the researchers found.
“Humans are supposed to have unique feet, but it looks like we don’t,” said Robin Crompton, a musculoskeletal biologist at the University of Liverpool, who worked on the study.
Why human feet have remained somewhat suited to life in the trees is still unclear. Crompton thinks that adaptations to tree-dwelling may also be useful for moving over irregular terrain.
But some scientists remain unconvinced that the study will change our current understanding of how walking evolved in humans.
Though the walking patterns of humans and other primates showed some overlap, they were still “fundamentally different,” said Carol Ward, an evolutionary biologist at the University of Missouri who wasn’t involved in the study. Most apes placed their weight almost entirely on their mid-foot region, while the majority of human subjects concentrated only about half the pressure on their feet in the mid-foot.
“This paper doesn’t change the basic story,” Ward said.
The human subjects wore shoes on a regular basis, which might have affected the flexibility of their feet, added Stony Brook University anthropologist William Jungers, who wasn’t involved in the study either. The researchers might have come to a different conclusion if they had tested people in developing countries who didn’t grow up wearing shoes, he said.
The findings could help doctors who care for patients with arthritis or diabetes. A flatfooted gait has been widely considered a symptom of both of these diseases. Now, “we should perhaps be less concerned about individuals whose arch support is less than perfect,” Jungers said.