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Horses are emigrants from and immigrants to North America
The modern horses you observe today grazing in fields and pastures in the countryside belong to a family of animals that have taken a long, adventurous journey around the world.
Primitive ancestors of the horse first evolved in the Western U.S. during the Age of Mammals, about 55 million years ago. The earliest horselike animal was a little forest dweller, Hyracotherium. This creature resembled a dog more than a horse, with an arched back, short legs and a long tail. Rather than hooves, it had four toes on each of its front feet and three on each of its back feet. Its teeth were suited for the mixed diet of an omnivore, unlike the grazing and grinding teeth that later evolved in grassland horses.
During the next 50 million years, an amazing family tree radiated out from Hyracotherium, with many diverse branches. These early horselike creatures filled a variety of ecological niches. At least 27 genera and hundreds of species evolved, but nearly all of them ultimately became extinct. This is not to suggest they werent successful; many of the genera survived for millions of years.
By 5 million years ago, the genus Equus had arrived, and its features were very close to the horses we see today. Horses in this branch of the family tree had developed fewer toes, and finally, hooves. Their teeth had evolved for grinding grasses of the Great Plains. And their legs had grown longer, although the animals hadnt reached the height of modern horses. Equus flourished in North America, and eventually migrated to Eurasia by crossing the Bering Land Bridge (a stretch of land that connected Alaska to Siberia as a result of a drop in sea levels). After the land bridge re-submerged, North American horse populations were isolated from horses in the rest of the world.
Some time between 13,000 and 11,000 years ago as the last Ice Age ended retreating glaciers and rising temperatures dramatically changed the environment. Large North American animals (megafauna) were under stress as their native habitats changed or vanished. Humans had arrived on the continent, and evidence shows that horses were among the megafauna hunted for meat. Under such pressures, much of the North American megafauna, including horses, went extinct.
Fortunately for Equus, horses had already spread to every continent in the world, with the exception of Australia and Antarctica. They would finally return to the continent, brought back by the same species that had helped eliminate them here. On his second voyage in 1493, Columbus brought horses to the Virgin Islands. By 1519, horses were also brought to Mexico. Escaped horses soon spread across the Great Plains.
From the July 27 – Aug. 2, 2005, issue