Evolution of the Horse
The first members of the horse family, Equidae, appeared in North America about 55 million years ago. These small, leaf-eating horses thrived in the warm, wet forests that covered much of the continent.

Then, about 35 mya, global temperatures dropped and dry grasslands replaced much of the forest. Horses with traits that enable them to adapt to this new habitat survived. For example, some early horses had 3 or 4 toes, each ending in a small hoof. But those with bigger, strong-er middle toes were better adapted to run and stand on the harder soil of the prairie. This eventually led to the near-disappearance of the side toes. The remaining middle toe became the larger, broadened hoof that sported the horse's weight.

Horses of the grasslands also had longer teeth than horses of the forests. Short, broad teeth may have been fine for chewing soft leaves. But they would wear down quickly grazing on tough grasses. All these adaptations helped the grass-eating horses of the prairie to survive, as their
leaf-eating relatives of the forest slowly disappeared.