The ancestry of the horse family is obscure; small, multiple-toed, primitive representatives, known as Hyracotherium (or Eohippus) appeared simultaneously in Europe and America near the beginning of the Cenozoic era about 70 million years ago. From this beginning, an animal anatomically similar to the modern horse developed in western North America during the early Ice Age (Pleistocene) but became extinct in the "New World" before European explorers arrived. Natural selection of horses for survival, evolved an animal of tenfold greater stature than the ancestor; only the central digit of the feet was retained as a hoof and the teeth and jaws modified to consume harsher food.
The Eocene horse, EOHIPPUS, attained the size of a modern fox, carried itself on the four digits of the hands or forefeet and on the three of the hind feet. Each foot had the middle digit slightly enlarged, as in all primitive perissodactyls, and was adapted to woodland travel. The teeth were almost omnivore-like, suited to macerating leaves, bushes, buds, and fruits, but not grass. The teeth underwent gradual modification as Eocene time progressed, but the skeleton had few changes.
In the Oligocene horse, MESOHIPPUS, the central digit enlarged to bear about half of the animal's weight and the teeth had a higher crown of enamel. These anatomical changes coincided with elevation of such moisture barriers as the ancestral Alps in Europe and the Cascade-Sierra Nevada ranges in America; moisture reduction eradicated most trees on the leeward side of the mountains, allowing grass to become the plains vegetation, and made firm footing for animals. The harsh sand-coated grass wore down the teeth of grazing creatures and gave variants with higher crown teeth the benefit of long service. The firm ground of the steppes removed the need for a broad foot; variants with long feet and carrying the weight on the central digit had a definite advantage of speed in travel for either food or escape. The long-lasting teeth and fleetness to flight from predatory carnivores gave the specialized strain improved chances for survival.
The greater size, increase in crown height (hypsodonty), and elimination of the side toe progressed in MERYCHIPPUS of the Miocene Age and PLESIPPUS of the Pliocene. In Plesippus the upper bone of the middle digit is almost as long as the lower limb bone; Plesippus was truly "on his toes". This horse was a fast runner, and with warning could out distance any predator.
Many other changes were associated with those in teeth and feet. The hypsodont teeth required more jaw space and this evolved progressively. Lengthening of the toes raised the animal from the ground and grazing was easier for the long necked forms. Another trend, apparently independent, was gradual increase in complexity of the brain.
The principal evolution of the horse occurred on the plains east of America's Sierras. At various stages in the evolution, herds migrated to Eurasia and Africa; there some of the latest arrivals survived Pleistocene Age extinction in the homeland. Not a single horse remained in the "New World" when the Europeans arrived. Escape of a few individuals from the Spanish caravans saw the horse repopulate many parts of the southwest. Their successful return to a region from which they had been exterminated, so few years previously, poses one of the great unsolved problems in cause for extinction or influence over survival.