Working the “borderland between archeology and paleontology,” a paleomammalogist examines a suite of causes for the extinction of large animals during the late Pleistocene. In 1814, when John James Audubon observed a flock of passenger pigeons over the Ohio River, the population numbered in the untold billions. A century later, the last passenger pigeon died in a zoo. That, notes MacPhee (Race to the End: Amundsen, Scott, and the Attainment of the South Pole, 2010, etc.), who has worked at the American Museum of Natural History for three decades, is the “fastest decline on record for a vertebrate with an initially large population size”—and, he adds cautiously, the “most supported” cause is overhunting. Why the caution, when the historical record is full of accounts of passenger pigeons being blasted out of the sky? Because the author is wrestling with the thorny question of what brought on the deaths of so many large animals, including mastodons, marsupial lions, dodos, and baboon lemurs. In a noir film, the smoking gun would be in the hands of the humans who just happened to show around the time of extinction. In science, greater care must be taken in establishing causality, and even if MacPhee bridles at the line of argument established by Paul Martin and his followers blaming the megafaunal extinction on the human “blitzkrieg,” he at least includes human killing among the causes. Others factors include climate change, the collapse of food webs, and the introduction of invasive species. Concluding that there is no compelling common cause in the extinctions, MacPhee closes by considering the possibilities of genetically reviving lost species, or “de-extinction,” noting, finally, “it is to be hoped that any brave new creations can be integrated into existing ecosystems without destroying them—something we have been unable to do with ourselves, for at least the last 50,000 years." So why did all those critters go extinct? MacPhee suggests rather than asserts, but his book, featuring beautiful illustrations from Schouten, adds thoughtful fuel to a scholarly debate that shows no signs of ending.