As the frontiers of New Spain moved north, the horse was reintroduced into the very canyons and mesas of the Southwest where Equus had initially evolved four million years earlier. During the 1680-90 Pueblo revolt, hundreds of Spanish horses escaped from the upper Rio Grande valley into their natal environment, where they prospered and multiplied to form the great mustang herds that forever changed the history of the American West. Further South, during the seventeenth century, a new equestrian adventurer emerged, the gaucho.