Dogs vary more in size than any other mammal on earth, from teacup-sized Chihuahuas to towering Great Danes. Most of this extreme variation has arisen over the past two centuries with the rise of modern dog breeding. But Victorian dog fanciers didn’t create these breeds out of whole cloth, a new study finds. Instead, they hijacked two tiny genetic changes that have been present in the ancestors of dogs since at least the last Ice Age. The study, which I cover this week in Science, doesn’t just help explain why dogs are among the most malleable creatures on the planet. It may also elucidate how and why the sizes of wolves and their relatives have changed over evolutionary history.