Horses have an unusually complete fossil record. While most organisms' lineages have major gaps in their history, the line of horses can be traced almost back to their beginning, roughly 55 million years ago. During this time, they evolved from deer-like browsers to modern horses.
The earliest horse, the dawn horse, wound up with two scientific names due to confusion of its discovery. Richard Owen gave the genus the name Hyracotherium first, though Orhniel Marsh's name Eohippus remains widely used to this day. The dawn horses lived in the early part of the Eocene Epoch, 55.8 to 33.9 million years ago. It was a small animal, less than two feet tall, with a slender build and legs built for running. Unlike modern horses, it had four distinct toes on the front paws, and three on the back. It lived in North America and Europe.
The dawn horse was superseded by its descendants in the later part of the Eocene, and into the Oligocene Epoch, 33.9 million to 23 million years ago. New genera of horses, like Orohippus, Mesohippus and Miohippus, had teeth adapted to browsing, eating a variety of foliage. Their size also increased gradually through this time, with Miohippus standing three to four feet tall. The number of toes also decreased to three on the hind foot. While the dawn horse lived in North America and Europe, the line leading to modern horses evolved in North America.
Parahippus to Pliohippus
In the Miocene (23 million to 5.3 MYA), grasslands developed in North America. Parahippus appeared during this time, with several adaptions for this new environment. Parahippus lacked a foot-pad, and had teeth better adapted to grazing than browsing. It gave rise to Merychippus in the mid Miocene. Merychippus had teeth almost identical to the modern horse, and had one large central toe, with the rest of the toes reduced in size. In turn, Merychippus gave rise to Pliohippus in the early to mid Pliocene (5.3 to 2.6 MYA). Pliohippus had a singled, hoofed toe and is considered the ancestor to modern horses.
First Modern Horses
The first modern horses appeared near the end of the Pliocene Epoch, 2.6 million years ago. They belong to the genus Equus. Their bodies included some of the "finishing touches" of horse adaptation, like longer cheek teeth better adapted for grazing and and improved "spring" mechanism in their legs. They first evolved in North America, but spread across the Bering Straight into the Old World. At 10,000 to 8,000 years ago, they died out in North America. In central Asia, the horse was domesticated first about 6,000 years ago. The horse was reintroduced to North America in the 15th century.
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