Horses, those majestic, slender creatures that play football on Budweiser commercials had a humble beginning at the start of the Eocene.
Horses began as a small fawn-looking animal called Hyracotherium. Eating shrubs in the swampy forests of North America and Europe, Hyracotherium had the need for speed. With its four-toed hooves, it could outrun any modern Olympic track star (or any vicious carnivore predator).
By the Oligocene, horses had more-than-doubled their size. They were now almost two feet long. This new breed was called Mesohippus (Mesohippus later evolved into a larger version, Miohippus). But instead of having 4 toes, Mesohippus had 3, with a big toe in the middle. It could now run with greater ease.
Now looking much more like the modern horse, the pony-sized Merychippus ran on its extra-large toe. The other toes, becoming more and more useless, began to shrink away. Merychippus also adapted for plains life in the Miocene: it got tougher teeth for tougher grass.
Horses developed more during the Pliocene, when the one-toed Pliohippus made an entrance. And just an epoch later, in the Pleistocene, the modern horse, Equus, arrived. Equus lived in large herds, and they were the ancestors of Przewalski' s horse, a type of wild , and three types of zebras. All of these have not changed much since Equus, and these wild species can give us a pretty good concept for studying their ancestors.