Ambition, genius, thought, imagination, love, hate, greed and, above all, consciousness ourselves as alive and as part of our world--all this is somehow enabled by the brain. The brain is the person, and if it goes wrong, a person is ruined. This book is about part of what the brain does -- a role of which many of us are hardly aware, but one that has ensured, the survival of mankind. Despite famine, drought, wars, cold, infections and hostile environments, we survive as a species -- though not always as individuals. All this time, our brains have been coping with what fate throws at us -- a process that some call adaptation. How does the brain do it? How does it know what's needed? How does it enable us to provide that need? How much do we depend on our own brains, or on those of others? This book is different from other books on the brain. It deals with the brain's role in survival, rather than "higher" cognitive functions (such as language or thought). It describes the special part of the brain that keeps you alive: that makes you feel hungry when you need energy, makes you feel thirsty when you need water, drives you to reproduce so that your species survives, makes you fearful of things or individuals that might harm you, and defends you against adversity.