On most computers there is a button called ‘reset’ that clears the screen and starts all over again, wiping out whatever is in the memory at the time. You can start afresh, unburdened by the clutter of the recent past. (Desks should have reset buttons.)

The planet earth has a reset button, called mass extinction. It has been pressed five times so far. The most drastic occasion was at the end of the Permian period, 225 million years ago, when 95 per cent of all marine animal species became extinct. It has been pressed twice since, most recently 65 million years ago, when the finger that punched it was an errant comet that struck Mexico with the force of a billion Hiroshimas. The result was a lot of dead dinosaurs. But mass extinctions are opportunities as well as ends. The Permian wipe-out gave the molluscs their lucky break to take over from their rivals, the brachiopods. The death of the dinosaurs was the opportunity for which our ancestors, the mammals, had been waiting tens of millions of years.

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