In the history of the Earth, there have been five mass extinctions—periods when a large number of species die off within a short period of time. The most recent event, theCretaceous-Tertiary mass extinction, wiped out the dinosaurs 66 million years ago.

Researchers have been trying to determine for years whether or not a sixth mass extinction is upon us already. Critics say scientists may be overestimating the rate at which current species are dying out, and underestimating the historic rate of extinction. So Ceballos and his colleagues wanted to conduct an analysis that used only very conservative estimations.

Extinction is a natural part of evolution. New species emerge, other species die out; it’s always happened, and it will continue to happen as long as there’s life on Earth. But the natural rate of extinction—called background extinction—is typically pretty low. Most studies on extinction rates peg it between 0.1 and 1 extinction per 10,000 species per 100 years (a measure called E/MSY). In other words: for every 10,000 species on Earth, up to one of them will disappear every 100 years.

Sounds perfectly natural. No need for alarm.

What's the current rate for the last 100 years?

Current rate of extinction? Likely worse the last 100 years than the next 100.