1 in 13 humans has chimp-like feet

When our ancient ancestors left their arboreal homes, they ditched their flexible feet for rigid tootsies best suited for walking on the ground. But according to a new study, 1 out of every 13 people may have bendy, tree-ready feet, without even knowing it.

Though chimp feet may look pretty different from human feet, they actually retain similar bone structures. But there is a big difference in the foot ligaments of humans and chimps, and this difference guides how flexible the foot is.

Like chimps, humans have joints in the middle of our feet. But our ligaments are stiff, keeping our feet rigid. Our rigid midfoot likely acts as an efficient lever to propel us forward as we walk. Chimps have soft ligaments, making their feet more flexible for grasping objects and branches.

In the new study, Boston University anthropologist Jeremy DeSilva and occupational therapist Simone Gill tested the midtarsal flexibility of 398 people, as they walked around barefoot at the Boston Museum of Science. They filmed the participants' feet up close and discovered that 32 of them (about 8 percent) had midfoot flexibility characteristic of a midtarsal break. These people also exhibited elevated pressures on the sides of their midfoot as they walked, and had significantly flatter feet than the people without the midtarsal break.

The owners of the chimp-like feet didn't know that there was anything different about their feet and didn't actually appear to walk differently. But the differences were obvious to the researchers when they looked at the close-ups of the feet as they unrolled while walking.

In a yet-to-be-published analysis, Robin Huw Crompton of the University of Liverpool found that people with midtarsal breaks may be even more common than DeSilva and Gill's work suggests, according to New Scientist.

Comptom thinks that flexible feet may have always been a part of our species, but DeSilva believes otherwise. New Scientist explains:

He thinks flexible feet make walking less efficient – something he intends to test. This would have been a disadvantage once our ancestors left the trees. If so, he says, it is more likely the trait reappeared recently. "My guess is that we are getting more variation than ever before, perhaps because shoes have impacted foot anatomy."

Read more at New Scientist, or check out the abstract of the study in the American Journal of Physical Anthropology.