When I was studying geology at Columbia, the theory of geologic gradualism was being replaced by a new idea of catastrophism, which proposed that long periods of stability were sometimes broken by huge geological events: volcanoes would erupt after long periods of quiet, drowning a landscape in lava; or an asteroid would strike the planet and create an enormous crater.

Now biology, too, has come to accept the notion that the history of life is broken by sudden, sharp events that can cause widespread extinction and rapid evolution, and fracture the otherwise slow development of species. It's called punctuated equilibrium. And there is some question whether this might be cyclical.