Are we at the beginning of what would be just the sixth mass extinction in the past 540 million years? A group of scientists think so, and they lay out their argument in a new edition of Nature. They argue that, if the rate of animal extinctions continues unabated, we would reach “mass extinction” levels in the next 300 to 2,200 years. So far, Earth has lost only a few percentage of all species, not the 75 percent wipeout that was seen in previous mass-extinction events. Still, the extinction rate is accelerating.