In human evolution, the transition from walking on all fours to walking upright didn't occur in a single step. Many changes to our ancestors' skeletons and muscles, not to mention their behavior, took place over millennia. Here, compare the skeletons of chimpanzees and modern humans—as well as that of our early bipedal predecessor, Australopithecus afarensis—to see some of the physiological modifications that made walking upright possible. Brian Richmond is an associate professor of anthropology at George Washington University and a Research Associate at the Smithsonian's National Museum of Natural History. His research includes the study of how hominid locomotion evolved.