The evolution of the horse occurred over a period of 50 million years, transforming the small, dog-sized, forest-dwelling Eohippus into the modern horse. Paleozoologists have been able to piece together a more complete outline of the evolutionary lineage of the modern horse than of any other animal.
The horse belongs to the order Perissodactyla (odd-toed ungulates), the members of which all share hooved feet and an odd number of toes on each foot, as well as mobile upper lips and a similar tooth structure. This means that horses share a common ancestry with tapirs and rhinoceroses. The perissodactyls arose in the late Paleocene, less than 10 million years after the Cretaceousâ€“Paleogene extinction event. This group of animals appears to have been originally specialized for life in tropical forests, but whereas tapirs and, to some extent, rhinoceroses, retained their jungle specializations, modern horses are adapted to life on drier land, in the much harsher climatic conditions of the steppes.
The early ancestors of the modern horse walked on several spread-out toes, an accommodation to life spent walking on the soft, moist grounds of primeval forests. As grass species began to appear and flourish, the equidsâ€™ diets shifted from foliage to grasses, leading to larger and more durable teeth. At the same time, as the steppes began to appear, the horseâ€™s predecessors needed to be capable of greater speeds to outrun predators. This was attained through the lengthening of limbs and the lifting of some toes from the ground in such a way that the weight of the body was gradually placed on one of the longest toes, the third.
Indigenous modern horses died out in the New World at the end of the Pleistocene, about 12,000 years ago, and thus were absent until the Spanish brought domestic horses from Europe, beginning in 1493. Escaped horses quickly established large wild herds. William Clarkâ€™s 1807 expedition to Big Bone Lick found â€œleg and foot bones of the Horsesâ€, which were included with other fossils sent to Thomas Jefferson.