We humans are odd creatures: tailless bipeds with sinuous spines, long limbs, arched feet, agile hands, and enormous brains. Our bodies are a mosaic of features shaped by natural selection over vast periods of time—both exquisitely capable and deeply flawed. We can stand, walk, and run with grace and endurance, but we suffer aching feet and knee injuries; we can twist and torque our spines, and yet most of us are plagued by back trouble at some point in our lives; we can give birth to babies with big brains, but only through great pain and risk. Scientists have long sought to answer the question of how our bodies came to be the way they are. Now, using new methods from a variety of disciplines, they are discovering that many of the flaws in our "design" have a common theme: They arise primarily from evolutionary compromises that came about when our ancestors stood upright—the ﬁrst step in the long path to becoming human.