Infants and children are the often-ignored heroes when it comes to understanding human evolution. Evolutionary pressures acted upon the young of our ancestors more powerfully than on adults. Changes over the course of development in our ancestors were primarily responsible for the species and the people we have become. This book takes an evolutionary developmental perspective, emphasizing that developmental plasticity—the ability to change our physical and psychological selves early in life—is the creative force in evolution, with natural selection serving primarily as the Grim Reaper, or a filter, eliminating novel developmental outcomes that did not benefit the survival of those individuals who possessed them, while letting the more successful outcomes through. Over generations as embryos, infants, and children continued to change and to produce slightly different adults, a new species was born—Homo sapiens. This book is about becoming—of becoming human and of becoming mature adults. One theme of this book is about how an understanding of our species’ evolution can help us better understand current development and how to better rear successful and emotionally healthy children. The second theme turns the relation between evolution and development on its head: How can an understanding of human development help us better understand human evolution? The short answer to this second question is that children invented humanity, and that human evolution can be seen as children setting the stage and leading the way to species innovation.