<P> The evolution of the horse, a mammal of the family Equidae, occurred over a geologic time scale 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 . Much of this evolution took place in North America, where horses originated but became extinct about 10,000 years ago . </P> <P> 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 Cretaceous--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 . Other species of Equus are adapted to a variety of intermediate conditions . </P> <P> 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 equids' diets shifted from foliage to grasses, leading to larger and more durable teeth . At the same time, as the steppes began to appear, the horse'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 . </P> <P> Wild horses were known since prehistory from central Asia to Europe, with domestic horses and other equids being distributed more widely in the Old World, but no horses or equids of any type were found in the New World when European explorers reached the Americas . When the Spanish colonists brought domestic horses from Europe, beginning in 1493, escaped horses quickly established large feral herds . In the 1760s, the early naturalist Buffon suggested this was an indication of inferiority of the New World fauna, but later reconsidered this idea . William Clark's 1807 expedition to Big Bone Lick found "leg and foot bones of the Horses", which were included with other fossils sent to Thomas Jefferson and evaluated by the anatomist Caspar Wistar, but neither commented on the significance of this find . </P>

What changes occurred in the size of the horse from hyracotherium to equus