Evolution of the horse

The evolution of the horse, a mammal of the family Equidae, occurred over a geologic time scale of 50 million years, transforming the small, dog-sized, forest-dwelling '' Eohippus'' into the modern horse. Paleozoologists have been able to piece together a more complete outline of the evolutionary lineage of the modern horse than of any other animal. Much of this evolution took place in North America, where horses originated but became extinct about 10,000 years ago, before being reintroduced in the 15th century.

The horse belongs to the order Perissodactyla ( odd-toed ungulates), the members of which one will share hooved feet and an odd number of toes on each foot, as well as mobile upper lips and a similar tooth structure. This means that horses share a common ancestry with tapirs and rhinoceroses. The perissodactyls arose in the late Paleocene, less than 10 million years after the Cretaceous–Paleogene extinction event. 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, modern horses are adapted to life in the climatic conditions of the steppes, which are drier and much harsher than forests or jungles. Other species of ''Equus'' are adapted to a variety of intermediate conditions.

The early ancestors of the modern horse walked on several spread-out toes, an accommodation to life spent walking on the soft, moist ground of primeval forests. As grass species began to appear and flourish, the equids' diets shifted from foliage to silicate-rich grasses; the increased wear on teeth selected for increases in the size and durability of teeth. At the same time, as the steppes began to appear, selection favored increase in speed to outrun predators. This ability was attained by 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.

History of research

Wild horses have been known since prehistory from central Asia to Europe, with domestic horses and other equids being distributed more widely in the Old World, but no horses or equids of any type were found in the New World when European explorers reached the Americas. When the Spanish colonists brought domestic horses from Europe, beginning in 1493, escaped horses quickly established large feral herds. In the 1760s, the early naturalist Buffon suggested this was an indication of inferiority of the New World fauna, but later reconsidered this idea. William Clark's 1807 expedition to Big Bone Lick found "leg and foot bones of the Horses", which were included with other fossils sent to Thomas Jefferson and evaluated by the anatomist Caspar Wistar, but neither commented on the significance of this find.

The first Old World equid fossil was found in the gypsum quarries in Montmartre, Paris, in the 1820s. The tooth was sent to the Paris Conservatory, where it was identified by Georges Cuvier, who identified it as a browsing equine related to the tapir. His sketch of the entire animal matched later skeletons found at the site.

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 this was "not very probable". After the expedition returned in 1836, the anatomist Richard Owen confirmed the tooth was from an extinct species, which he subsequently named '' Equus curvidens'', and remarked, "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 at least two ancient horse species had existed in North America: ''Equus curvidens'' and another, which he named ''Equus americanus''. A decade later, however, he found the latter 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 was based on fossils discovered in North America in 1879 by paleontologist Othniel Charles Marsh. The sequence, from ''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 that emphasized the gradual, "straight-line" evolution of the horse.

Since then, as the number of equid fossils has increased, the actual evolutionary progression from ''Eohippus'' to ''Equus'' has been discovered to be much more complex and multibranched 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. George Gaylord Simpson in 1951 first recognized that the modern horse was not the "goal" of the entire lineage of equids, but is simply the only genus of the many horse lineages to survive.

Detailed fossil information on the distribution and rate of change 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 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 ''Eohippus'' 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 only in retrospect can certain evolutionary trends be recognized.

Before odd-toed ungulates

Phenacodontidae

Phenacodontidae is the most recent family in the order Condylarthra believed to be ancestral to the odd-toed ungulates. It contains the genera '' Almogaver'', '' Copecion'', '' Ectocion'', '' Eodesmatodon'', '' Meniscotherium'', '' Ordathspidotherium'', '' Phenacodus'' and '' Pleuraspidotherium''. The family lived from the Early Paleocene to the Middle Eocene in Europe and were about the size of a sheep, with tails making slightly less than half of the length of their bodies and, unlike their ancestors, good running skills.

Eocene and Oligocene: early equids

''Eohippus''

''Eohippus'' appeared 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: three incisors, one canine, four premolars, and three 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. ''Eohippus'' browsed on soft foliage and fruit, probably scampering between thickets in the mode of a modern muntjac. It had a small brain, and possessed especially small frontal lobes.

Its limbs were 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, of which four were equipped with small proto-hooves; the large fifth "toe-thumb" was off the ground. The hind limbs had small hooves on three out of the five toes, whereas 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 in place of claws.

For a span of about 20 million years, ''Eohippus'' thrived with few significant evolutionary changes. The most significant change was in the teeth, which began to adapt to its changing diet, as these early Equidae shifted from a mixed diet of fruits and foliage to one focused increasingly on browsing foods. During the Eocene, an ''Eohippus'' species (most likely ''Eohippus angustidens'') branched out into various new types of Equidae. 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 have also been discovered in Europe, such as '' Propalaeotherium'' (which is not considered ancestral to the modern horse).

''Orohippus''

Approximately 50 million years ago, in the early-to-middle Eocene, ''Eohippus'' smoothly transitioned into ''Orohippus'' through a gradual series of changes. Although its name means "mountain horse", ''Orohippus'' was not a true horse and did not live in the mountains. It resembled ''Eohippus'' in size, but had a slimmer body, an elongated head, slimmer forelimbs, and longer hind legs, all of which are characteristics of a good jumper. Although ''Orohippus'' was still pad-footed, the vestigial outer toes of ''Eohippus'' were not present in ''Orohippus''; there were four toes on each fore leg, and three on each hind leg.

The most dramatic change between ''Eohippus'' and ''Orohippus'' was in the teeth: the first of the premolar teeth was 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 increased the grinding ability of the teeth of ''Orohippus''; the change suggest selection imposed by increased toughness of ''Orohippus'' plant diet.

''Epihippus''

In the mid-Eocene, about 47 million years ago, ''Epihippus'', a genus 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 species of ''Epihippus'', sometimes referred to as '' Duchesnehippus intermedius'', had teeth similar to Oligocene equids, although slightly less developed. Whether ''Duchesnehippus'' was a subgenus of ''Epihippus'' or a distinct genus is disputed. ''Epihippus'' was only 2 feet tall.

''Mesohippus''

In the late Eocene and the early stages of the Oligocene epoch (32–24 mya), the climate of North America became drier, and the earliest grasses began to evolve. The forests were yielding to flatlands, home to grasses and various kinds of brush. In a few areas, these plains were covered in sand, creating the type of environment resembling the present-day prairies.
In response to the changing environment, the then-living species of Equidae also 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 nonwooded areas. About 40 mya, '' 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 in) 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 modern horses is quite detailed. The fossa 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 descendant Equidae would retain. ''Mesohippus'' also had the sharp tooth crests of ''Epihippus'', improving its ability to grind down tough vegetation.

''Miohippus''

Around 36 million years ago, soon after the development of ''Mesohippus'', '' Miohippus'' ("lesser horse") emerged, the earliest species being ''Miohippus assiniboiensis''. As with ''Mesohippus'', the appearance of ''Miohippus'' was relatively abrupt, though a few transitional fossils linking the two genera have been found. ''Mesohippus'' was once believed to have anagenetically evolved into ''Miohippus'' by a gradual series of progressions, but new evidence has shown its evolution was cladogenetic: a ''Miohippus'' population split off from the main genus ''Mesohippus'', coexisted with ''Mesohippus'' for around four million years, and then over time came to replace ''Mesohippus''.

''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 equine teeth.

''Miohippus'' ushered in a major new period of diversification in Equidae.''Fossil Horses In Cyberspace''
Florida Museum of Natural History and the National Science Foundation.

Miocene and Pliocene: true equines

''Kalobatippus''

The forest-suited form was ''Kalobatippus'' (or ''Miohippus intermedius'', depending on whether it was a new genus or species), whose second and fourth front toes were long, well-suited to travel on the soft forest floors. ''Kalobatippus'' probably gave rise to '' Anchitherium'', which travelled to Asia via the Bering Strait land bridge, and from there to Europe. In both North America and Eurasia, larger-bodied genera evolved from ''Anchitherium'': '' Sinohippus'' in Eurasia and '' Hypohippus'' and '' Megahippus'' in North America. ''Hypohippus'' became extinct by the late Miocene.

''Parahippus''

The ''Miohippus'' population that remained on the steppes is believed to be ancestral to '' Parahippus'', a North American animal about the size of a small pony, with a prolonged skull and a facial structure resembling the horses of today. Its third toe was stronger and larger, and carried the main weight of the body. Its four premolars resembled the molar teeth; the first were small and almost nonexistent. The incisor teeth, like those of its predecessors, had a crown (like human incisors); however, the top incisors had a trace of a shallow crease marking the beginning of the core/cup.

''Merychippus''

In the middle of the Miocene epoch, the grazer '' Merychippus'' flourished. It 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. ''Merychippus'' radiated into at least 19 additional grassland species.

''Hipparion''

Three lineages within Equidae are believed to be descended from the numerous varieties of ''Merychippus'': '' Hipparion'', '' Protohippus'' and '' Pliohippus''. The most different from ''Merychippus'' was ''Hipparion'', mainly in the structure of tooth enamel: in comparison with other Equidae, the inside, or tongue side, had a completely isolated parapet. A complete and well-preserved skeleton of the North American ''Hipparion'' shows an animal the size of a small pony. They were very slim, rather like antelopes, and were adapted to life on dry prairies. On its slim legs, ''Hipparion'' had three toes equipped with small hooves, but the side toes did not touch the ground.

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.)

''Pliohippus''

'' Pliohippus'' arose from ''Callippus'' in the middle Miocene, around 12 mya. It was very similar in appearance to '' Equus'', though it had two long extra toes on both sides of the hoof, externally barely visible as callused stubs. The long and slim limbs of ''Pliohippus'' reveal a quick-footed steppe animal.

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''.

''Dinohippus''

'' Dinohippus'' was the most common species of Equidae in North America during the late Pliocene

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