By directing the evolution of a worm, scientists have confirmed answers to the age-old question: “What is the point of having sex with someone else?” For most people, that would hardly be a tricky query but it’s no reflection on the lives of evolutionary scientists that sex has been one of biology’s oldest puzzles.

The problem is this: many creatures can reproduce by fertilising themselves instead of getting someone else to do it, and at first glance they should do much better individuals that cross-fertilise. For a start, they’d ensure that all of their genes reach the next generation, while mating with another individual reproduction halves their genetic legacy. And without having to find males, self-fertilising females should be able to