Evolution of the horse
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Evolution of the horse, showing reconstruction of the fossil species obtained from successive rock strata. The foots diagrams are all front views of the left forefoot. The third metacarpal is shaped throughout. The teeth are shown in longitudinal section. It should be noted that the animals shown are only selected examples—the evolution of horses was by no means a linear process. |
The
evolution of the horse involves the gradual development of the modern
horse from the fox-sized, forest-dwelling
Hyracotherium.
Zoologists have been able to piece together a more complete picture of the modern horse's evolutionary lineage than that of any other animal.
Horses belong 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
rhinoceri. 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, rhinoceri, 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.
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 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 is recognized that the horse is not the "goal" of the entire lineage of equids (a notion that would contradict
modern evolutionary theory); it is simply the only equid genus 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
fossae, and it is only in retrospect that certain evolutionary trends can be recognized.
[Hunt, Kathleen (1995). Horse Evolution. TalkOrigins Archive.]Hyracotherium
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".
[Gould, Stephen Jay (1991). "The Case of the Creeping Fox Terrier Clone." Bully for Brontosaurus: Reflections in Natural History (pp. 155–167). New York: W.W. Norton & Co.] A later name for the
Hyracotherium, "eohippus" ("dawn horse"), is also popular, though the earlier name takes precedence due to scientific naming conventions.
[Fossil Horses In Cyberspace. Florida Museum of Natural History and the National Science Foundation.]Hyracotherium evolved in the early
Eocene (54–34 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 law. 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 modern of a modern
muntjac, albeit with less intelligence, speed and agility: 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 that were still 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).
[MacFadden, B. J. (1976). "Cladistic analysis of primitive equids with notes on other perissodactyls." Syst. Zool. 25(1):1-14.]Orohippus
Approximately 50 million years ago, in the early-to-middle Eocene,
Hyracotherium smoothly transitioned into
Orohippus over a gradual series of changes.
Although its name means "mountain horse",
Orohippus did not live in the mountains. It resembled
Hyracotherium is size, but had a slimmer body, an elongated head, and 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
Hyracotherium were not present in the
Orohippus; there were four toes on each forelimb, and three on each hind leg.
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.
Epihippus
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 subspecies of
Epihippus or a single species is disputed.
Mesohippus
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Mesohippus, an equid more adapted to the changing climate of the early Oligocene. |
In the late Eocene and the early stages of the
Oligocene epoch (32–24 million years ago), 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, 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 horces 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.
[Däggdjurens tid: Hästar ]Miohippus
Around 36 million years ago, Soon after the development of the
Mesohippus, the
Miohippus ("lesser horse") emerged, the earliest species being
Miohippus assiniboiensis. Like
Mesohippus,
Miohippus's evolution was relatively abrupt, though a few transitional fossils linking the two genera have been found. It was once believed that the
Mesohippus had
anagenetically evolved into the
Miohippus by a gradual series of progressions, but new evidence has shown that
Miohippus's evolution was
cladogenetic: a
Miohippus population split off from the main
Mesohippus genus, coexisted with
Mesohippus for around 4 million years, and then over time came to replace
Mesohippus.
[Prothero, D.R. and Shubin, N. (1989). "The evolution of Oligocene horses." The Evolution of Perissodactyls (pp. 142–175). New York: Clarendon Press.]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.
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 travel on the soft forest floors.
Kalobatippus travelled to Asia via the
Bering Strait land bridge, and from there to Europe, where its fossils were formerly described under the name
Anchitherium.
Kalobatippus is believed to be ancestral to another European species known as
Hyohippus, which became extinct near the beginning of the Pliocene.
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 and the first were small and almost nonexistent. The incisive teeth of
Parahippus, like those of its predecessors, had a crown as humans do; 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, 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.
Hipparion
Three new equids are believed to be descended from the numerous varieties of
Merychippus:
Hipparion,
Protohippus and
Pliohippus. The most different from
Merychippus was
Hipparion. The main difference was in the structure of
tooth enamel: in comparison with other equids, 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.
An American form of
Hipparion, also known as
Neohipparion, proliferated into many kinds of equids several of which managed to migrate to Asia and Europe during the Pliocene epoch. (The European
Hipperia differs from the American
Hipparion in the smaller body size – the best-known discovery of these fossils was near
Athens.)
Recent research suggests that
Hipparion is an ancestor of the
zebra and the
donkey, rather than the horse.
Pliohippus
Pliohippus arose from
Calippus in the middle Miocene, around 15 million years ago. 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 was 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.
[MacFadden, B. J. (1984). "Astrohippus and Dinohippus". J. Vert. Paleon. 4(2):273-283.]Dinohippus
Plesippus
Plesippus is often considered an intermediary stage between
Dinohippus and the present day horse,
Equus.
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. The
Ice Age spread five times over Europe and North America and five times again receded; it is estimated that approximately one million years elapsed from the Ice Age (the Quaternary period) to our era.
In South America a form named
Hippidium developed from
Plesippus.
Hippidium 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
The oldest species of "true" horse,
Equus stenonis, was discovered in
Italy, and is believed to have evolved from
Plesippus-like animals at the end of the Tertiary or beginning of the Quaternary periods.
Equus stenonis proliferated into two branches, one lighter in body mass and one heavier.
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, perhaps due to
climate change or some mass
contagion. It has also been suggested that humans hunted horses to extinction, as the appearance of humans in the Americas occurred at about the same time as the extinction of most large mammals in the Americas. However, there are no known kill sites of Pleistocene horses in North America, and so this scenario remains unsupported.
[Vilà, Carles; Leonard, Jennifer A. Leonard; Götherström, Anders; Marklund, Stefan; Sandberg, Kaj; Lidén, Kerstin; Wayne, Robert K.; Ellegren, Hans (2001). "Widespread Origins of Domestic Horse Lineages". Nature Vol. 291 (pp. 474–477).]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 bp.
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.
At the end of the
15th century, when the first Europeans came to America, there were no horses in the Americas; the
natives of modern-day
Mexico and
Peru did not even have a name for the animal. The
Spanish imported horses back to America. Runaway horses and
cattle went wild on the pampas and proliferated into large herds, only to be caught again later and domesticated.
Toes
The ancestors of the horse came to walk only on the end of the third toe and both side toes. Skeletal remnants show obvious wear on the back of both sides of
metacarpal and
metatarsal bones, commonly called the "splint bones". They are the remnants of the second and the fourth toe. Modern horses retain the splint bones; it is often believed that they are a useless attachment, but they in fact play an important role in supporting the carpal joints (front knee) and even the tarsal joints (hock).
It is not unheard of that foals are occasionally born with three toes equipped with hooves. This is called a phylogenetic
atavism caused by arrested development at a certain
embryonic stage.
Teeth
Throughout the phylogenetic development, the teeth of the horse underwent significant changes. The type of the original
omnivorous teeth with short, "bumpy" molars, with which the prime members of the evolutionary line distinguished themselves, gradually changed into the teeth common to
herbivorous mammals. They became long (as much as 100 mm), roughly cubical molars equipped with a flat grinding surface. In conjunction with the teeth, during the horse's evolution the elongation of the facial part of the skull is apparent, and can also be observed in the backward set eyeholes. In addition, the relatively short neck of the equine ancestors became longer with equal elongation of the legs. This is because they adapted to finding food by grazing on the steppes. Finally, the size of the body grew as well.
