In this week's PNAS, scientists have compared how much energy is used in human bipedal walking compared to the four-legged gait of chimpanzees (and even bipedal walking in chimpanzees). In general, they found that energy is saved with longer steps and less active muscle mass, and in fact that humans use about 75% less energy getting around than apes do. The difference is huge. It is said that natural selection generally proceeds by very small advantages but if the first human to start walking more upright was able to conserve even a percentage of this, it must have given them a great opportunity to spend that energy elsewhere (Feeding a large brain? Using language?). USA Today, NatGeo.