Horses and humans have an ancient relationship. Asian nomads discovered horses about 4,000 years ago. They still hold a place of honor in many cultures. Most horses are domestic, but some still remain wild. There is only one species of domestic horse, but around 400 different breeds that specialize in everything from pulling wagons to racing. The close relationship between horses and humans has changed us both. People have remade horses, creating dozens of breeds in our efforts to make horses faster, stronger, bigger, or smaller. Horses have also changed us. The ways we travel, work, and fight wars have all been profoundly shaped by our use of horses. By 55 million years ago, the first members of the horse family, the dog-sized Hyracotherium, were scampering through the forests that covered North America. For more than half their history, most horses remained small, forest browsers. But changing climate conditions allowed grasslands to expand, and about 20 million years ago, many new species rapidly evolved.