The Science of Discworld III: Darwin's Watch

Roundworld is in trouble again, and this time it looks fatal. Having created it in the first place, the wizards of Unseen University feel vaguely responsible for its safety. They know the creatures who lived there escaped the impending Big Freeze by inventing the space elevator - they even intervened to rid the planet of a plague of elves, who attempted to divert humanity onto a different time track. But now it's all gone wrong. Unless something drastic is done, there won't be time for anyone to invent spaceflight and the human race will be turned into ice-pops.