Our great-ape cousins such as chimpanzees have feet that are very flexible in their middle region due to something called the midtarsal break that allows their feet to bend in the middle, enabling them to grasp at branches for easier climbing through trees. So when a chimp lifts his foot off the ground, it just flops about - there's nothing to hold the bones together. Most humans, on the other hand (or should I say foot?), have the same joint but have ligaments that stretch across it making the foot more rigid and stable for upright walking. Australopithecus sediba, a human ancestor that lived 2 million years ago, has a foot structure that is more ape-like than human, so somewhere along the line our feet evolved probably to accommodate our bipedalism.
The study was done by Jeremy DeSilva, a functional morphologist from Boston University, whose main interest is the evolution of the human foot and ankle. In this recent study, museum visitors were requested to walk barefoot across a mechanized carpet while DeSilva's team observed their gaits and the structure of their feet as they walked.
The surprising results showed that 8 percent of the nearly 400 participants possessed a flexible midtarsal break in their foot, and displayed a pressure signature in their footprint that looked like that found in the footprints of non-human primates. Perhaps more surprising is the fact that those subjects who had the unusual foot-joint structure weren't even aware of it until DeSilva revealed it to them.
The study was published in the American Journal of Physical Anthropology.