Evolution
Living things change through time in a continual process called evolution. Since life on Earth began, millions of new species have appeared and disappeared. Organisms alive today represent only a tiny proportion of all the living things that have ever existed. Evolution occurs largely as a result of natural selection. This process favors the survival, and thus the ability to reproduce, of individuals that are better adapted to their environment than other members of the same species.
Over the past 50 millions years, horses has evolved from forest dwellers to grass grazers.Hyracotherium, the earliest known horses live in North American forest. The leaf eating Mesohippus appeared 30 million years ago. The grass eating Merychippus followed five million years later. Modern horses appeared about 2 million years ago.