Humans are the only mammals to walk on two, rather than four legs-a locomotion known as bipedalism. We strive to be upstanding citizens, honor those who stand tall and proud, and take a stand against injustices. We follow in each other's footsteps and celebrate a child's beginning to walk. But why, and how, exactly, did we take our first steps? And at what cost? Bipedalism has its drawbacks: giving birth is more difficult and dangerous; our running...