Section 1 : The Original North American Horses

In the Nevada desert, a herd of wild horses gallops across the land, leaving behind a cloud of dust and a trail of hoof prints in the soft earth. It's a scene from a Western, but there are no barbed wire fences, no cattle, and no cowboys anywhere in sight. This is years--thousands of years--before the Spanish landed in North America, bringing with them the ancestors of today’s herds. And wild horses are running free.

North America is the birthplace of the modern horse. The evolution of the horse involves multiple branches and many extinctions (see Appendix A), but the simplified version of the evolution of the modern horse starts about 50 million years ago, during the Eocene Epoch, the "dawn horse" Eohippus roamed the land. It was a small, fox-sized creature with four toes that lived in the hot, swampy bog lands and grazed on vegetation. As the climate and the land changed, so did Eohippus. Forest land became thinner and grasslands began to emerge, and the horse grew larger, developed a more powerful jaw and teeth for digesting grass, and longer legs to outrun predators in the open. This horse, called Mesohippus, developed a larger central toe that bore most of its weight across the firmer terrain. About 17 million years ago, Merychippus emerged. It stood about 35 inches high and its diet consisted mainly of hard plains grasses, so the grinding teeth became more prominent. The central toe elongated and hardened for travel over hard ground, and the smaller toes became less prominent. About 12 millions years ago, the horse had evolved into Pliohippus, the first monodactyl animal. Pliohippus roamed the great plains of the US; fossils have been found all over Colorado, Nebraska, the Dakotas, and Canada. During this time, some horses migrated to Asia, Europe, and Africa, and others spread into South America. The modern horse, Equus, emerged about two million years ago during the Pleistocene epoch, a time when multiple glaciations occurred separated by warm periods.

2: The Extinction of the Horse >