HORSE, mammal of the genus Equus, of the family Equidae. The genus contains the domestic horse, E. caballus, and three groups of species living in the wild state. One group comprises the ZEBRA, native to Africa; another consists of the ASS, including the kiang and onager of Asia and the wild ass of Africa. The third group contains Przhevalski’s horse, E. przhevalskii, now native to western Mongolia only, but formerly found over a much wider range. The only extant true wild horse, it crosses with the domestic horse and produces fertile progeny. Przhevalski’s horse, while not a direct ancestor of E. caballus, is believed to be the survivor of a species that contributed to the origin of the domestic horse. Other so-called wild horses in various parts of the world are descendants of domestic horses that have reverted to the wild state.
The evolution of the horse can be traced through fossil remains to Eohippus, a small, leaf-browsing mammal of the EOCENE EPOCH. Eohippus, about the size of a fox, had four toes on its forefeet and three on its hind feet. Several species and related genera appeared in North America and the Old World during the Eocene. Then, apparently, the Old World species died out, but the American species gave rise, in the Oligocene epoch, to the genus Mesohippus.
In the Miocene epoch Mesohippus was succeeded by Hypohippus and Anchitherium, both of which are thought to have colonized the Old World from North America. Other descendants of Mesohippus were Miohippus and Merychippus; the latter genus developed high-crowned teeth, permitting it to feed by grazing on grass rather than by browsing on leaves. Among the descendants of Merychippus in the Pliocene epoch were Hipparion, which apparently spread from North America to the Old World, and Pliohippus, which appears to be the progenitor of the modern genus Equus. In each of these developmental stages the animal showed an increase in general size and a reduction in the size of the supplementary toes present in Eohippus.
During the Pleistocene epoch the genus Equus apparently spread from North America to Eurasia, Africa, and South America. Subsequently the native American horses died out, possibly as the result of disease. Cave dwellings in Europe indicate that horses were plentiful on that continent during the early Stone Age. Dismembered skeletons of horses have been found in and near such dwellings in sufficient numbers to show that horses were frequently killed and eaten. In Neolithic times, when Europe was largely forested, the number of horses evidently declined. Remains of the Bronze Age include bits and other pieces of harness, and they clearly demonstrate that horses had become domestic animals in this period.
