60-45 million years agoThe oldest known horse, alternately referred to as Eohippus or Hyracotherium, is about the size of a dog.
c. 4 million B.C.E.Evidence suggests genus Equus (which includes modern horses, zebras and burros) originates in North America.
c. 3-2 million B.C.E. Ancient horses cross over to Eurasia, likely by the Bering land bridge. Several migrations back and forth (and individual species extinctions) likely follow. Though it is widely held that our wild horses descend from stock brought over by Spanish explorers, some scientists trace their lineage back to ancient horses of North America.
c. 1.7 million B.C.E. Estimated origin of Equus caballus, the species we know today.
c. 8000 B.C.E. The last ancient horses left in North America die out toward the end of the Pleistocene. Horses now exist exclusively in Asia, Europe and Africa.
1493-1800 The horse returns to North America with the second voyage of Spanish explorer Christopher Columbus and with subsequent voyages made by the Conquistadors. Over time Native American populations become skilled horsemen, while ranchers and farmers rely heavily on the horse for the hard task of "taming" the West. The wild horse population begins to re-emerge as some of these horses get loose.
1800sBy the 19th century, some accounts estimate the wild horse population in North America at as many as two million or more.
1860-1861The Pony Express famously employs riders on horseback to carry mail across the continent in record time-a mere 10 days.
Early 20th CenturyThe tractor begins to replace the horse on American farms. And the automobile replaces the horse and buggy as a means of transportation. Horses become less valuable. Slaughterhouses begin buying them for meat and other commercial purposes.
1959 Congress passes the Wild Horse Annie Act prohibiting the use of airplanes to hunt the animals on federal land. Velma B. Johnston, later nicknamed Wild Horse Annie by her opponents, was instrumental in building support for the legislation after she followed a mustanger and saw how the animals were rounded up and slaughtered.
1971Congress passes Wild Free-Roaming Horses and Burros Act recognizing the animals as "living symbols of the historic and pioneer spirit of the West." The Bureau of Land Management assumes responsibility over the animals and gradually begins to reduce their population through a controversial adoption program.
2007The last horse-slaughtering plant in the U.S. is forced to close under Illinois law. The DeKalb plant, run by Belgian company Cavel International, had been slaughtering 40,000 to 60,000 horses a year mainly for meat in overseas markets. Still, the Humane Society maintains that many wild horses sold off by the BLM end up in Mexican or Canadian slaughterhouses. The BLM disputes this.
Today Roughly 45,000 wild horses under BLM protection live in some 200 herd management areas in 10 western states (shown in the map). Half of the wild horses living on government land are located in Nevada.Sources:
Journal of Heredity
Natural History Magazine
Rockford Register Star
Bureau of Land Management, U.S. Department of the Interior