The world got hot in a hurry 56 million years ago. In just 10,000 years—an eyeblink of geologic time—temperatures shot up by as much as eight degrees Celsius, according to paleoclimate data. In one swampy lowland that is now part of Wyoming, the average annual temperature reached about 26 degrees C, similar to the humid tropical coasts of today's Indonesia. One of the locals, loping along in the heat, was a small, brownish animal, with pointy ears and long legs made for running. At roughly four kilograms, the creature was the size of a small dog, perhaps a diminutive terrier. It was, in fact, a horse.