The coccyx, or tailbone, "is an attachment point of a number of muscles at the pelvis. We need it for upright locomotion. It would be catastrophic if it went away," says Kenneth Saladin, an anatomist and physiologist at Georgia College and State University. The appendix, which helped our distant ancestors digest grass, has slowly evolved to take on a new purpose. Research led by William Parker and R. Randall Bollinger of Duke University has shown that the appendix now serves as a kind of "safe house" for the microbes that aid in digestion. "Each of us has 900 to 1,600 species of bacteria in our gut to make sure we have a healthy immune system," says Stephen Stearns, an evolutionary-biology professor at Yale University. "If one takes over, or they all get flushed out by a disease, then the appendix works like a holding tank for the good bacteria." Even the pinky toe helps keep our balance and diffuses impact throughout the foot when we run.