Book excerpt: A revolutionary examination of why we age, what it means for our health, and how we just might be able to fight it. In Cracking the Aging Code, theoretical biologist Josh Mitteldorf and award-winning writer and ecological philosopher Dorion Sagan reveal that evolution and aging are even more complex and breathtaking than we originally thought. Using meticulous multidisciplinary science, as well as reviewing the history of our understanding about evolution, this book makes the case that aging is not something that “just happens,” nor is it the result of wear and tear or a genetic inevitability. Rather, aging has a fascinating evolutionary purpose: to stabilize populations and ecosystems, which are ever-threatened by cyclic swings that can lead to extinction. When a population grows too fast it can put itself at risk of a wholesale wipeout. Aging has evolved to help us adjust our growth in a sustainable fashion as well as prevent an ecological crisis from starvation, predation, pollution, or infection. This dynamic new understanding of aging is provocative, entertaining, and pioneering, and will challenge the way we understand aging, death, and just what makes us human.