Apart from a couple of bothersome side branches, horse evolution presents a neat, orderly picture of natural selection in action. The basic story line goes like this: as the woodlands of North America gave way to grassy plains, the tiny proto-horses of the Eocene epoch (about 50 million years ago) gradually evolved single, large toes on their feet, more sophisticated teeth, larger sizes and the ability to run at a clip, culminating in the modern horse genus Equus. (See a gallery of prehistoric horse pictures.)
This story has the virtue of being essentially true, with a couple of important "ands" and "buts." But before we embark on this journey, it's important to dial back a bit and place horses in their proper position on the evolutionary tree of life. Technically, horses are "perissodactyls," that is, ungulates (hoofed mammals) with odd numbers of toes. The other main branch of hoofed mammals, the even-toed "artiodactyls," are represented today by pigs, deer, sheep, goats and cattle, whereas the only other significant perissodactyls beside horses are tapirs and rhinoceroses.
What this means is that perissodactyls and artiodactyls (which counted among the mammalian megafauna of prehistoric times) both evolved from a common ancestor, which lived only a few million years after the demise of the dinosaurs at the end of the Cretaceous period, 65 million years ago. In fact, the earliest perissodactyls (like Hyracotherium, the ancestor of all horses) looked more like small deer than majestic equines!
The Earliest Horses - Hyracotherium and Mesohippus
Until an even earlier candidate is found, paleontologists agree that the ultimate ancestor of all modern horses was Hyracotherium, a tiny (no more than 50 pounds), deer-like herbivore with four toes on its front feet and three toes on its back feet. (If the name Hyracotherium sounds unfamiliar, that's because this horse ancestor was once known by the more popular Eohippus, or "dawn horse.") The giveaway to Hyracotherium's status is its posture: this perissodactyl put most of its weight on a single toe of each foot, anticipating later equine developments. Hyracotherium was closely related to another early ungulate, Palaeotherium, which occupied a distant side branch of the horse evolutionary tree.
Five to ten million years after Hyracotherium came Orohippus ("mountain horse"), Mesohippus ("middle horse"), and Miohippus ("Miocene horse," even though it went extinct long before the Miocene epoch). These perissodactyls were about the size of large dogs, and sported slightly longer limbs with enhanced middle toes on each foot. They probably spent most of their time in dense woodlands, but may have ventured out onto the grassy plains for short jaunts.
Toward True Horses - Epihippus, Parahippus and Merychippus
During the Miocene epoch, North America saw the evolution of "intermediate" horses, bigger than Hyracotherium and its ilk but smaller than the equines that followed. One of the most important of these was Epihippus ("marginal horse"), which was slightly heavier (possibly weighing a few hundred pounds) and equipped with more robust grinding teeth than its ancestors. As you might have guessed, Epihippus also continued the trend toward enlarged middle toes, and it seems to have been the first prehistoric horse to spend more time feeding in meadows than in forests.
Following Epihippus were two more "hippi," Parahippus and Merychippus. Parahippus ("almost horse") can be considered a next-model Miohippus, slightly bigger than its ancestor and (like Epihippus) sporting long legs, robust teeth, and enlarged middle toes. Merychippus ("ruminant horse") was the largest of all these intermediate equines, about the size of a modern horse (1,000 pounds) and blessed with an especially fast gait.
(At this point, it's worth asking the question: what drove the evolution of horses in the fleet, single-toed, long-legged direction? During the Miocene epoch, waves of tasty grass covered the North American plains, a rich source of food for any animal well-adapted enough to graze at leisure and run quickly from predators if necessary. Basically, prehistoric horses evolved to fill this evolutionary niche.)
Next Step, Equus - Hipparion and Hippidion
Following the success of intermediate horses like Parahippus and Merychippus, the stage was set for the emergence of bigger, more robust, more "horsey" horses. Chief among these were the similarly named Hipparion ("like a horse") and Hippidion ("like a pony"). Hipparion was the most successful horse of its day, radiating out from its North American habitat (by way of the Siberian land bridge) to Africa and Eurasia. Hipparion was about the size of a modern horse; only a trained eye would have noticed the two vestigial toes surrounding its single hooves.
Lesser known than Hipparion, but perhaps more interesting, was Hippidion, one of the few prehistoric horses to have colonized South America (where it persisted until historical times). The donkey-sized Hippidion was distinguished by its prominent nasal bones, a clue that it had a highly developed sense of smell. Hippidion may well turn out to have been a species of Equus, making it more closely related to modern horses than Hipparion was.
Speaking of Equus, this genus--which includes modern horses, zebras and donkeys--evolved in North America during the Pliocene epoch, about four million years ago, and then, like Hipparion, migrated across the land bridge to Eurasia. The last Ice Age saw the extinction of both North and South American horses, which disappeared from both continents by about 10,000 B.C. Ironically, though, Equus continued to flourish on the plains of Eurasia, and was reintroduced to the Americas by the European colonizing expeditions of the 15th and 16th centuries A.D.
Here's a list of the most notable prehistoric horses; just click on the links for more information.
American Zebra Also known as the Hagerman horse.
Anchitherium A long-lived "side branch" on the equine tree of life.
Dinohippus This prehistoric horse wasn't quite as fearsome as its name.
Epihippus This tiny, prehistoric horse lived about 30 million years ago.
Hipparion One of the most successful horses of the Miocene epoch.
Hippidion This donkey-sized horse had a prominent snout.
Hypohippus This Miocene horse had unusually short legs.
Hyracotherium The horse formerly known as Eohippus.
Merychippus An important intermediate step in equine evolution.
Mesohippus This "middle horse" was about the size of a deer.
Miohippus This "Miocene horse" actually lived much earlier.
Orohippus This prehistoric horse was a close relative of Hyracotherium.
Palaeotherium This tapir-like beast was remotely related to modern horses.
Parahippus This "almost horse" had noticeably enlarged middle toes.
Pliohippus This prehistoric horse was built for speed.
Quagga This South African zebra went extinct in 1883.
Tarpan The immediate predecessor of the modern horse.