Biologists often say that the most dangerous thing a cell can do is divide. This is because, during the complex process of replication—the unspooling of DNA, the assembling of two genomes from the halves of one—there is always the chance that the cell will make a mistake. Mutations can cost an organism its life, but they are also essential to evolution. Without them, there would be no novelty and no change; the slow-churning Darwinian search algorithm would stop. In this sense, transposons—wandering snippets of DNA that hide in genomes, copying and pasting themselves at random—are unsung heroes of natural selection. Although the information that they carry is spare, they account for fifty per cent of all mammalian genetic material. Our own DNA is a battlefield between self and other.