Until a few thousand years ago, creatures that could have been from a sci-fi thriller--including gorilla-sized lemurs, 800-pound birds, crocodiles that weighed a ton or more--roamed the earth. These great beasts, or megafauna, lived on every habitable continent and on many islands. With a handful of exceptions, all are now gone. What caused the disappearance of these prehistoric behemoths? Paleomammologist Ross D. E. MacPhee explores that question, examining the leading extinction theories, weighing the evidence, and presenting his own conclusions. He shows how theories of human overhunting and catastrophic climate change fail to explain critical features of these extinctions, and how new thinking is needed to elucidate these mysterious losses. He comments on how past extinctions can shed light on future losses, and on the possibility of bringing back extinct species through genetic engineering. Gorgeous four-color illustrations by Peter Schouten bring these megabeasts back to life in vivid detail.