New research suggests that one of the earliest horses started out small—then got even smaller. As temperatures rose 55 million years ago during the ancient Eocene epoch, a North American horse species shrank from the size of a small dog to that of a house cat.
It would take millions of years and dozens of different genera until nature produced something approaching the modern horse (Equus ferus). The leggy show-jumper and powerful rodeo steed are of course products of modern breeding on top of eons of evolution. But the road to the arena and racetrack had humble beginnings.