The first equid was Hyracotherium, of early Eocene age (approximately 55 million years ago). A small forest animal 10-20" at the shoulder, it bore no resemblance to the modern horse. It had a "doggish" look with an arched back, short neck, short snout, short legs, and long tail. It browsed on fruit and fairly soft foliage, and was probably a slow-moving animal.

For the next 35 million years, the first adaptations toward the modern horse were a decrease in the number of weight-bearing toes and the premolars becoming shaped more like molars, increasing the grinding efficiency of the teeth.

As this third line of Miocene (18 million years ago) horses began to specialize in eating grasses, several changes occurred. This was one of the most interesting times in horse evolution, with two main changes that were carried forth to today's horse:

• The teeth changed to be better suited for chewing harsh, abrasive grass.
  • Small crests on the teeth enlarged and connected in a series of ridges for grinding.
  • There was a gradual increase in the height of the tooth crowns, so that the teeth could grow out of the gum continuously as the tops were worn down ("hypsodont" teeth).
  • And, in addition, the tooth crowns became harder due to the development of a cement layer on the teeth.
• These horses started to become specialized runners, an increase in body size, leg length, and length of the face.
  • The bones of the legs began to fuse, and the leg bones and musculature became specialized for efficient forward-and-back strides, with flexible leg rotation being eliminated.
  • Most significantly, the horses began to stand permanently on tiptoe (another speed adaptation); instead of walking on doglike pads, their weight was supported by springy ligaments that ran under the fetlock to the big central toe. All these changes occurred rapidly, and we are lucky to have a fairly good fossil record during this time.

A third one-toed horse called Dinohippus (recently discovered) arose about 12 MYA. They resemble Equus in terms of foot morphology, teeth, and skull. Dinohippus was the most common horse in North America in the late Pliocene, and almost certainly gave rise to Equus.

Equus arose approximately 4 million years ago and is the genus of all modern equines. 13.2 hands tall (pony size), with a classic "horsey" body. Characteristics of a rigid spine, long neck, and long legs, with fused leg bones that prevent rotation, a long nose, a flexible muzzle, a deep jaw, a slightly larger brain, one-toed, featuring side ligaments that prevent the hoof from twisting, high-crowned, and straight grazing teeth with strong crests lined with cement.

They still had some primitive traits from Dinohippus, including a slight facial fossa. They had zebra-like bodies (relatively stocky with straight shoulders and thick necks) and short, narrow, donkey-like skulls. They probably had stiff, upright manes, ropy tails, medium-sized ears, striped legs, and at least some striping on the back. All these traits are shared bythe various species of modern equines.

They quickly diversified into at least 12 new species in 4 different groups. All these Equus species coexisted with other one-toed horses (such as Astrohippus) and with various successful hipparions and protohippines, which had been merrily evolving on their own paths but later disappeared.

During the first major glaciations of the late Pliocene (2.6 Million Years Ago), certain Equus species migrated to the Old World.

• Some entered Africa and diversified into the modern zebras.
• Others spread across Asia, the Middle East, & North Africa as desert-adapted onagers and asses.
• Still others spread across Asia, the Middle East, and Europe as the true horse, E. caballus.
• Other Equus species spread into South America.
• Equus then disappeared from the New World, probably an inability to adapt to the changing climates and environments, and had to wait for the Europeans to reestablish them in the Americas.

The Equus genus was perhaps the most successful perissodactyl genus that ever lived -- even before domestication by humans.