How did unicellular organisms develop into the animals and human beings of today? In this "refreshingly lucid treatment of profound questions" (Booklist), neuroscientist Joseph LeDoux digs into the natural history of life on earth to provide a new perspective on the similarities between us and our ancestors in deep time. Indeed, LeDoux shows how even the earliest single-cell organisms had to solve the same problems that we and our cells have to solve each day. This page-turning survey of the whole of terrestrial evolution sheds new light on how nervous systems evolved in animals, how the brain developed, and what it means to be human.