The common ancestor of Perissodactyls (horses, rhinos, tapirs) appeared in North American during the Paleocene, about 60 million years ago (MYA). The first known member of the horse lineage was Hyracotherium, formerly known as Eohippus, a small deer-like herbivore of subtropical forests that fed on tender vegetation and fruit. Appearing in the early Eocene, some 55 MYA, this dog-sized mammal evolved through a series of early horse ancestors to Oligocene forms such as Mesohippus (37 MYA) and Epihippus (30 MYA) that began to emerge from the forest to take advantage of newly evolved and nutritious grasslands; in concert, their teeth took on traits vital to grazing and their dominant third toe would evolve toward the modern hoof.
Diversification in the early Miocene led to Parahippus, a pony-sized ancestor, and Meryhippus (17 MYA), the first true grazer and fleet, horse-sized member of the lineage. Meryhippus diverged into at least three groups, one of which, Hipparion, was the first ancestral horse to cross into Eurasia. Equus, the genus of all modern horses, first appeared in the Pliocene, about 5 MYA; some members of that genus spread into South America by 1.5 MYA while others crossed the Bering land bridge into Eurasia throughout the Pleistocene. By the end of the Wisconsin Glacial Period, 10,000 years ago, the horse had disappeared from the Americas, likely a result of changing flora and hunting pressure from humans; they would be domesticated by human tribes in Central Asia about 6000 years ago and brought back to the Americas by Spanish explorers in the 15th and 16th Centurys.
The above summary leaves out numerous intermediate and side-branch species in what is one of the most complete and well documented evolutionary trees for any mammal. Perhaps most enlightening is the gradual, physical adaptation of ancestral horses to their changing environment, progressing from small, forest dwelling creatures to large, fleet and hardy residents of our vast grasslands. What better creature, so vitally important to the advance of human culture, to fully understand and appreciate?
