So complete was the disaster that many of today’s big American mammals, such as caribou, moose, and brown bear, are immigrants from Asia. The die-off happened amazingly fast—the statistics, according to the study, suggest that it was instantaneous. And when it was complete, naturalist Alfred Russell Wallace wrote, the Americas had become “a zoologically impoverished world, from which all of the hugest, and fiercest, and strangest forms [had] recently disappeared.”
