It's often said that extinction is the rule, rather than the exception—after all, 99.9 percent of all species that have ever existed on Earth have gone extinct. In a sense, that adage is true. Life on this planet has toughed it out through five mass extinction events, in which huge numbers of species disappeared during relatively short periods of time. After each, life eventually rebounded.

The keyword in that last sentence is eventually, though. Many scientists say we’re in the midst of a sixth mass extinction, with species dying off 100 times faster than they have in the past. And, according to a new study, it’ll take several million years for mammals to bounce back from the extinctions that