We all assume we know what life is, but the more scientists learn about the living world - from protocells to brains, from zygotes to pandemic viruses - the harder they find it to define exactly what it is and what it isn't.

What is life? In this riveting and thought-provoking book, Carl Zimmer explores the question by journeying to the edges of life in every direction, from viruses to computer intelligence, from its origins on earth to the search for extra-terrestrial life and the strange experiments that have attempted to recreate life from scratch in the lab. The question is not only a scientific issue; it hangs over some of society's most charged conflicts - whether a fertilized egg is a living person, for example, and when we ought to declare a person legally dead.

Whether he is handling pythons or searching for hibernating bats, Zimmer investigates life in its most unfamiliar forms. He tries his own hand at evolving life in a test tube with unnerving results, explores our cultural obsession with Dr. Frankestein's monster and how Coleridge came to believe the whole universe was alive. The result is an entirely gripping exploration of one of the most crucial questions of all: the meaning of life.