A moth evolves ears that can hear the sonar of bats, and bats adapt by hushing their calls to whispers. A newt evolves powerful poisons that can kill would-be predators, and a snake evolves immunity to those poisons. A gazelle becomes faster to outrun its hunter, and a cheetah becomes faster still. The natural world is full of these evolutionary arms races—endless battles where one party’s adaptations are met by counter-adaptations from its opponent. Both sides move in and out of check, changing all the time but locked in a perpetual stalemate.

The human genome is engaged in a similar evolutionary arms race… against itself.

The opponents are jumping genes called retrotransposons that can hop around the genome. They increase