Scientific Background Why are humans hairless? Why is childbirth so painful, and dangerous? Why is the human intellect so highly developed? Because we run. The Perfect Runner cracks open the modern-day sport of endurance running by exploring our evolutionary past as a species formed and defined by long-distance running adaptations. New archaeological research into the physiology of early hominids and a deeper understanding of climate conditions at critical points in our evolution are now revealing that two critical adaptations occurred simultaneously to produce modern Homo sapiens . The first was the transformation between 3 and 2 millions years ago of the tree-climbing body of our Australopithecus predecessors into the bipedal body we possess today. The second was the development of the large Homo brain, and the reasoning and language abilities it enabled. The order and cause of these changes have spurred debate for decades, but now some scientists are arguing neither would have been possible without a critical third adaptation, namely a supreme advantage over other animals bestowed by our ability to run long distances. The emergence of Homo sapiens The famous Australopithecene skeleton we call Lucy, discovered in the Ethiopian portion of Africa's Great Rift Valley, reveals that 3.2 millions years ago, our ancestors moved in a vertical world. With short legs, a wide waist, long arms and grasping toes, Lucy would have waddled as she walked, recruiting her arms alongside her legs for faster movement. Her elongate feet and side-pointing big toes would have made running almost impossible.