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 is to locate life's edge.

Carl Zimmer investigates one of the biggest questions of all: What is life? The answer seems obvious until you try to seriously answer it. Is the apple sitting on your kitchen counter alive, or is only the apple tree it came from deserving of the word? If we can't answer that question here on earth, how will we know when and if we discover alien life on other worlds? The question 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.Charting the obsession with Dr. Frankenstein's monster and how Coleridge came to believe the whole universe was alive, Zimmer leads us all the way into the labs and minds of researchers working on engineering life from the ground up.