The evolution of the horse involves the gradual development of the modern horse from the fox-sized, forest-dwelling Hyracotherium. Paleozoologists have been able to piece together a more complete picture of the modern horse's evolutionary lineage than that of any other animal.
The horse belongs to an order known as the Perissodactyls, or "odd-toed ungulates", which all share hoofed feet and an odd number of toes on each foot, as well as mobile upper lips and a similar tooth structure. This means that they share a common ancestry with tapirs and rhinoceros. The Perissodactyls originally arose in the late Paleocene, less than 10 million years after the extinction of the dinosaurs. This group of animals appears to have been originally specialized for life in tropical forests, but whereas tapirs and, to some extent, rhinoceroses, retained their jungle specializations, horses instead adapted to life on dryer land in the much-harsher climatic conditions of the steppes.
The early ancestors of the modern horse walked on several spread-out toes, an accommodation to life spent walking on the soft, moist grounds of primeval forests. As grass species began to appear and flourish, the equids' diets shifted from foliage to grasses, leading to larger and more durable teeth. At the same time, as the steppes began to appear, the horse's predecessors needed to be capable of greater speeds in order to outrun predators. This was attained through the lengthening of limbs and the lifting of some toes from the ground in such a way that the weight of the body was gradually placed on one of the longest toes, the third.
During the Beagle survey expedition the young naturalist Charles Darwin had remarkable success with fossil hunting in Patagonia. On 10 October 1833 at Santa Fe, Argentina, he was "filled with astonishment" when he found a horse's tooth in the same stratum as fossil giant armadillos, and wondered if it might have been washed down from a later layer, but concluded that this was "not very probable". After the expedition returned in 1836 the anatomist Richard Owen confirmed that the tooth was from an extinct species which he subsequently named Equus curvidens, and remarked that "This evidence of the former existence of a genus, which, as regards South America, had become extinct, and has a second time been introduced into that Continent, is not one of the least interesting fruits of Mr. Darwin's palæontological discoveries.
In 1848 a study On the fossil horses of America by Joseph Leidy systematically examined Pleistocene horse fossils from various collections including that of the Academy of Natural Sciences and concluded that there had been at least two ancient horse species in North America, Equus curvidens and another which he named Equus americanus, but a decade later he found that name had already been taken and renamed it Equus complicatus. In the same year he visited Europe and was introduced by Owen to Darwin.
The original sequence of species believed to have evolved into the horse were based on fossils discovered in North America in the 1870s by paleontologist Othniel Charles Marsh. The sequence, from Hyracotherium (popularly called Eohippus) to the modern horse (Equus), was popularized by Thomas Huxley and became one of the most widely-known examples of a clear evolutionary progression. The horse's evolutionary lineage became a common feature of biology textbooks, and the sequence of transitional fossils was assembled by the American Museum of Natural History into an exhibit which emphasized the gradual, "straight-line" evolution of the horse.
Since then, as the number of equid fossils has increased, the actual evolutionary progression from Hyracotherium to Equus has been discovered to be much more complex and multi-branched than was initially supposed: the straight, direct progression from the former to the latter has been replaced by a more elaborate model with numerous branches in different directions, of which the modern horse is only one of many. It was first recognized by George Gaylord Simpson in 1951 that the modern horse is not the "goal" of the entire lineage of equids (the notion of a goal would contradict modern evolutionary theory); it is simply the only genus of the many horse lineages that has happened to survive.
Detailed fossil information on the rate and distribution of new equid species has also revealed that the progression between species was not as smooth and consistent as was once believed: although some transitions, such as that of Dinohippus to Equus, were indeed gradual progressions, a number of others, such as that of Epihippus to Mesohippus, were relatively abrupt and sudden in geologic time, taking place over only a few million years. Both anagenesis (gradual change in an entire population's gene frequency) and cladogenesis (a population "splitting" into two distinct evolutionary branches) occurred, and many species coexisted with "ancestor" species at various times. The change in equids' traits was also not always a "straight line" from Hyracotherium to Equus: some traits reversed themselves at various points in the evolution of new equid species, such as size and the presence of facial fossae, and it is only in retrospect that certain evolutionary trends can be recognized.
The earliest animal to bear recognizably horse-like anatomy was the Hyracotherium ("hyrax-like beast"). Its scientific name is derived from initial confusion over early partial fossils' relationship with living species: Richard Owen likened early Hyracotherium fossils "to a hare in one passage and to something between a hog and a hyrax in another". A later name for the Hyracotherium, "eohippus" ("dawn horse"), is also popular, though the earlier name takes precedence due to scientific naming conventions.
Hyracotherium lived in the Ypresian (early Eocene), about 52 mya (million years ago). It was an animal approximately the size of a fox (250–450 mm in height), with a relatively short head and neck and a springy, arched back. It had 44 low-crowned teeth, in the typical arrangement of an omnivorous, browsing mammal: 3 incisors, 1 canine, 4 premolars, and 3 molars on each side of the jaw. Its molars were uneven, dull, and bumpy, and used primarily for grinding foliage. The cusps of the molars were slightly connected in low crests. The Hyracotherium browsed on soft foliage and fruit, probably scampering between thickets in the mode of a modern muntjac: the Hyracotherium had a small brain, and possessed especially small frontal lobes.
Its limbs were decently long relative to its body, already showing the beginnings of adaptations for running. However, all of the major leg bones were unfused, leaving the legs flexible and rotatable. Its wrist and hock joints were low to the ground. The forelimbs had developed five toes, out of which only four were equipped with a small proto-hoof; the large fifth "toe-thumb" was off the ground. The hind limbs had three out of the five toes equipped with small hooves, while the vestigial first and fifth toes did not touch the ground. Its feet were padded, much like a dog's, but with the small hooves on each toe in place of claws.
For a span of about 20 million years, the Hyracotherium thrived, with few significant evolutionary changes occurring. The most significant change was in the teeth, which began to adapt to the changing diet of Hyracotheria as these early equids shifted from a mixed diet of fruits and foliage to one focused increasingly on browsing foods. During the Eocene, a Hyracotherium species (most likely Hyracotherium vassacciense) branched out into various new types of equids. Thousands of complete, fossilized skeletons of these animals have been found in the Eocene layers of North American strata, mainly in the Wind River basin in Wyoming. Similar fossils of horses have also been discovered in Europe, such as Propalaeotherium (which is not considered ancestral to the modern horse).
The most dramatic change between Hyracotherium and Orohippus was in their teeth: the first of the premolar teeth were dwarfed, the last premolar shifted in shape and function into a molar, and the crests on the teeth became more pronounced. Both of these factors gave the teeth of Orohippus greater grinding ability, suggesting that Orohippus was subsisting on tougher plant material.
In the mid-Eocene, about 47 million years ago, Epihippus, a species which continued the evolutionary trend of increasingly efficient grinding teeth, evolved from Orohippus. Epihippus had five grinding, low-crowned cheek teeth with well-formed crests. A late form of Epihippus, sometimes called Duchesnehippus, had teeth similar to Oligocene equids, although slightly less developed. Whether Duchesnehippus was a subgenus of Epihippus or a distinct genus is disputed.
In response to the changing environment, equids, too, began to change. In the late Eocene, they began developing tougher teeth and becoming slightly larger and leggier, allowing for faster running speeds in open areas, and thus for evading predators in non-wooded areas. About 40 million years ago, the Mesohippus ("middle horse") suddenly developed in response to strong new selective pressures to adapt, beginning with the species Mesohippus celer and soon followed by Mesohippus westoni.
In the early Oligocene, Mesohippus was one of the more widespread mammals in North America. It walked on three toes on each of its front and hind feet (the first and fifth toes remained, but were small and not used in walking). The third toe was stronger than the outer ones, and thus more weighted; the fourth front toe was diminished to a vestigial nub. Judging by its longer and slimmer limbs, Mesohippus was an agile animal.
Mesohippus was slightly larger than Epihippus, about 610 mm (24") at the shoulder. Its back was less arched, and its face, snout, and neck were somewhat longer. It had significantly larger cerebral hemispheres, and had a small, shallow depression on its skull called a fossa, which in later horses became quite detailed, and serves as a useful marker for identifying an equine fossil's species. Mesohippus had six grinding "cheek teeth", with a single premolar in front—a trait all later equid species would retain. Mesohippus also had the sharp tooth crests of Epihippus, improving its ability to grind down tough vegetation
The Miohippus was significantly larger than its predecessors, and its ankle joints had subtly changed. Its facial fossa was larger and deeper, and it also began to show a variable extra crest in its upper cheek teeth, a trait that became a characteristic feature of later equid teeth.
The Miohippus ushered in a major new period of equid diversification. While Mesohippus died out in the mid-Oligocene, Miohippus continued to thrive, and in the early Miocene (24–5.3 million years ago), it began to rapidly diversify and speciate. It branched out into two major groups, one of which adjusted to the life in forests once again, while the other remained suited to life on the prairies.
In the middle of the Miocene epoch, an animal called Merychippus was alive. Merychippus had wider molars than its predecessors, which are believed to have been used for crunching the hard grasses of the steppes. The hind legs, which were relatively short, had side toes equipped with small hooves, but they probably only touched the ground when running.
In North America, Hipparion and its relatives (Cormohipparion, Nannippus, Neohipparion, and Pseudhipparion), proliferated into many kinds of equids, at least one of which managed to migrate to Asia and Europe during the Miocene epoch. (European Hipparion differs from American Hipparion in its smaller body size – the best-known discovery of these fossils was near Athens.)
Until recently, Pliohippus was believed to be the ancestor of present-day horses because of its many anatomical similarities. However, though Pliohippus was clearly a close relative of Equus, its skull had deep facial fossae, whereas Equus had no fossae at all. Additionally, its teeth were strongly curved, unlike the very straight teeth of modern horses. Consequently, it is unlikely to be the ancestor of the modern horse; instead, it is a likely candidate for the ancestor of Astrohippus.
Plesippus is often considered an intermediary stage between Dinohippus and the present day horse, Equus.
The famous fossils found near Hagerman, Idaho were originally thought to be a part of the genus Plesippus Hagerman Fossil Beds (Idaho), a Pliocene site, dating about 3.5 Ma ago. The fossilized remains were originally called Plesippus shoshonensis but further study by paleontologists determined that fossils represented the oldest remains of the genus Equus. Their estimated average weight was 425 kg, roughly the size of an Arabian horse. About 2.5 Ma ago Plesippus reached Eurasia over the Bering Strait from North America.
At the end of the Pliocene, the climate in North America began to cool down significantly and the animals were forced to move south. One group of the Plesippus species escaped to South America, and the other moved across the land bridge around the Bering Strait into Asia and Europe. A portion also remained in the southern part of North America.
In South America a form named Hippidion developed from Plesippus. Hippidion was relatively short-legged with a deeply recessed nasal notch, very thin and delicate nasals, and long ectoflexids in the lower premolars. It continued to live on the South American pampas for a long time, but eventually died out.
Equus stenonis crossed into North America, where similar forms known as Equus scotti are common; some types (Equus scotti var. giganteus) exceeded the modern horse in size. However, all the horses in North America ultimately became extinct approximately 11,000 years ago. The causes of this extinction (simultaneous with a variety of other American megafauna) are still a matter of debate, particularly given the suddenness of the event and the fact that these mammals had clearly been surviving for millions of years previous. Often-mentioned possibilities include climate change, pandemic, or hunting by the possibly simultaneous arrival of humans.
Recent studies by a team of geneticists headed by C. Vila indicate that the horse line split from the zebra/donkey line between 4 and 2 million years ago. Equus ferus, ancestor species to Equus caballus, appeared 630,000 to 320,000 years ago. Equus caballus was formed from several subspecies of Equus ferus by selective breeding widely over Eurasia for an extended time. The details of this process are currently a target of research by archaeologists and geneticists.
There is a theory that four basic "proto" horses developed in Europe through natural selection with adaptations to their environment prior to domestication of the horse. There are competing theories, some arguing that the prototypes were separate species, others suggesting that the prototypes were physically different manifestations of Equus ferus or Equus caballus. Either way, the most common theories of historical wild species from which other types are thought to have developed suggests the following base prototypes:
The Spanish began to import Iberian horses to breeding ranches on Cuba, Haiti, and other large Caribbean islands offshore of the Americas beginning with Columbus' second voyage in 1493. The first horses to return to the main continent were 16 specifically identified horses brought by Hernan Cortes. Subsequent explorers, such as Coronado and DeSoto brought ever-larger numbers, some from Spain and other from breeding establishments set up by the Spanish in the Caribbean. Later, as Spanish missions were founded on the mainland, horses would eventually be lost or stolen, and proliferated into large feral herds that became known as the Mustang.