But horse evolution was not smooth and gradual. Different traits evolved at different rates, didn't always evolve together, and occasionally reversed "direction." Also, horse species did not always come into being by gradual transformation (anagenesis) of their ancestors; instead, sometimes new species split off from ancestors (cladogenesis) and then coexisted with those ancestors for some time. Some species arose gradually, others suddenly. Overall, the horse family demonstrates the diversity of evolutionary mechanisms. The most modern equids (descendants of Parahippus) are called equines. Strictly speaking, only the very modern genus Equus contains "horses." Kathleen Hunt has produced an Equus evolutionary tree that graphically shows the development of horses.

Meanwhile, across the sea, horses were becoming a fixture of many ancient civilizations, and establishing their place in human history. In 1,000 B.C. the first horses were domesticated and used for transportation of both humans and cargo. 500 years later, Persian officials began using mounted couriers for message relaying. Horses had become a part of human life.

It wasn't until 1500 A.D. that the horse was reintroduced to North America when the Spanish came to conquer the New World in the 16th Century. Their small, sturdy mounts spread quickly, once again, throughout America.