Extinction is a recurring theme over the history of life on Earth. Ninety-nine percent of species that have ever lived on Earth have gone extinct,¹ and more than one third of the plant and animal species alive today are threatened with extinction.² Usually, extinction operates at a fairly constant rate, culling some species while speciation generates new ones. However, at a few points in life’s history — and likely today — the humdrum of regular extinction has been interrupted by mass extinctions. During these catastrophic periods, extinction levels rose far above background rates and huge swathes of life’s diversity disappeared forever.