Until about 13,000 years ago, North America was home to a menagerie of massive mammals. Mammoths, camels, and lions walked the ground that has become Wilshire Boulevard in Los Angeles and foraged on the marsh land now buried beneath Chicago's streets. Then, just as the first humans reached the Americas, these Ice Age giants vanished forever.

In Once and Future Giants, science writer Sharon Levy digs through the evidence surrounding Pleistocene large animal ("megafauna") extinction events worldwide, showing that understanding this history--and our part in it--is crucial for...