The horse family preceded humans on earth by some 50,000,000 years. Fossil remains of members of this family are found in both the Eastern and the Western Hemisphere. The fossils found in North America provide one of the most complete records of animal evolution.

The earliest known ancestor of the horse was Eohippus, or Hyracotherium, present 55 to 55 million years ago, in the early Eocene epoch. There were several species of Eohippus, the smallest about 10 inches (25 cm) high. Eohippus had 44 teeth, so constructed that the animal could have eaten only soft leaves and fruit. Through millions of years, changes took place in the descendants of Eohippus. Their legs and feet became more and more adapted to speed, and their teeth to chewing coarse vegetation. The most successful adaptations took place among the North American species. Branches of the horse family in other parts of the world died out completely.

Merychippus, a primitive North American horse of the Miocene epoch, was perhaps the first grazing member of the family. By the end of the Pliocene epoch, Equus, the modern type of horse, appeared in what is now the United States. Equus spread to all continents except Australia over land bridges then in existence. After the end of the Pleistocene epoch (Ice Age), horses disappeared from the Western Hemisphere.

Early humans hunted horses for food. In early historic times humans tamed the horse and mounted it, and for many centuries the horse was important in military conquest and in the building of the great civilizations. The first domestic horses appeared in China and in the Tigris-Euphrates Valley in southwestern Asia, between 3000 and 2000 B.C. By 1000 B.C. domestic horses were found all over Asia, Europe, and North Africa.

As a symbol of conquest, the horse soon became identified with the ruling classes. Kings and noblemen rode horseback and in chariots for pleasure, and watched horse and chariot races.

Horses could not be utilized fully for pulling until the collar harness was invented. The Chinese are believed to have invented this type of harness in the fifth century A.D.; it reached Europe in about the 10th century.

Horses were introduced into America by 16th-century Spanish explorers, and were later brought in by European settlers. American Indians soon learned the value of horses for war and hunting, and obtained them from the Europeans through trading or theft. Many horses escaped, and at one time vast herds of wild horses roamed the western Plains. The horse-drawn stagecoach provided transportation and communication before the days of railways and automobiles. In some areas the stagecoach served until the early years of the 20th century.

Although horses were used for farm work in England in the early Middle Ages, it was on the vast farms and ranches of America that horses proved essential in pulling plows and heavy wagons, and in herding cattle. In cities, horses provided power for hauling wagons and streetcars. Horses were also used for pulling canal boats.

The widespread use of motorized vehicles and farm implements in the 20th century marked the decline of the horse as a work animal. However, the work horse has retained its importance in nonindustrialized countries. In the United States, the stock horse is still used for ranch work, and the use of horses for recreation and sport has increased steadily since the 1950's. Horseback riding, horse shows, hunting on horseback, and horse racing are enjoyed in many parts of the world.