One significant way we differ from our ape ancestors is in our arched feet. As humans evolved, the foot developed from something that could grasp branches easily to one more suited for upright movement. To propel yourself forward, you need an arch that keeps the foot stiff when you push off with the toes, but is also flexible enough to act as a shock absorber when your heel hits the ground. The discovery several years ago of a three-million-year-old foot bone with a full arch suggests that our early human relatives (A. afarensis) were upright walkers.
Nevertheless, plenty of highly evolved people—as many as one in five—have flat feet, technically called pes planus and once referred to as “fallen arches.” One or both feet may be affected.
There are two general types. “Rigid” flat feet are flat all the time, whether they are bearing weight or not. More common are “flexible” flat feet, in which the arch collapses only when weight is put on it, though flexible flat feet may become more rigid over time.
Flat feet tend to run in families and increase with age. They may result from congenital bone deformities or injuries to bones or soft tissue; most frequently, they occur in adults due to overstretching and degeneration of the posterior tibial tendon on the inner side of the ankle. People who have occupations that involve lots of weight-bearing activity, are very active, are very overweight or obese, or have tight Achilles tendons are at higher risk—as are those with diabetes and inflammatory disorders like rheumatoid arthritis.
How do you know if you have flat feet? Wet your feet and stand on a dry surface that will leave a footprint. A foot with a normal arch leaves a print that looks like a wide band from the heel to the ball of the foot, with a large indent on the inner side. If there is little or no indent, your feet are flat.
Other telltale signs are toes that point outward, ankles that turn in, difficulty standing on tiptoe—and a tendency to get bunions, calluses and hammertoes.