Built for the long run.

If walking upright first set our human ancestors apart from their ape cousins, it may have been their ability to run long distances with a springing step over the African savanna that marked the transition to today's human body form. By studying fossil records, Dennis M. Bramble, a biologist at the University of Utah, and Daniel. E. Lieberman, a Harvard paleontologist, found evidence that humans began to be able to run long distances about 2 million years ago. Traces of tendon-attachment points on bones of early human species revealed an extensive network of springy tendons along the backs of the legs and feet. These tendons, while ideal, for running, were not needed for ordinary walking. According to Bramble, "Running made us human, at least in the anatomical, sense." Endurance running, unique to humans among primates, probably let our ancestors hunt and scavenge over great distances. This ability, the two scientists say, "may have made possible a diet rich in fats and proteins thought to account for the unique human combination of Large bodies, small guts, big brains, and small, teeth."