Earth has seen five major extinction episodes in its time — the Ordovician, Devonian, Permian, Triassic, and Cretaceous, in that order — and many smaller ones. The Ordovician (440 million years ago) and Devonian (365 million) each wiped out about 80 to 85 percent of species. The Triassic (210 million years ago) and Cretaceous (65 million years) each wiped out 70 to 75 percent of species. But the real whopper was the Permian extinction of about 245 million years ago, which raised the curtain on the long age of the dinosaurs. In the Permian, at least 95 percent of animals known from the fossil record check out, never to return. Even about a third of insect species went — the only occasion on which they were lost en masse. It is as close as we have ever come to total obliteration.

It was, truly, a mass extinction, a carnage of a magnitude that had never troubled the Earth before, says Richard Fortey. The Permian event was particularly devastating to sea creatures. Trilobites vanished altogether. Clams and sea urchins nearly went. Virtually all other marine organisms were staggered. Altogether, on land and in the water, it is thought that Earth lost 52 percent of its families — that's the level above genus and below order on the grand scale of life (the subject of the next chapter) — and perhaps as many as 96 percent of all its species. It would be a long time — as much as eighty million years by one reckoning — before species totals recovered.