Until a few thousand years ago, creatures that could have been from a sci-fi thriller--including gorilla-sized lemurs, 500-pound birds, and 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? No one event can be pinpointed as a specific...