STUART PIMM:

Well, there's two parts to this. One of them is what's happening now or at least what's happened for the last 50 to 100 years. We can follow species from when we first learned about them to the present and see what fraction of them are dying off. And they're dying off at a pretty hefty rate. Working out what happened before humans came along is a little more difficult. But the fossil record can tell us a lot about that. We also learn a lot from their DNA, from the molecular evidence of how species are born and how fast they die off. And it's comparing those two things – what's happening to species now with what happened in the geological past that allowed us to come up with this statement of how fast species are going extinct.