Evolution of the horse
 | This article needs additional citations for verification. |
The evolution of the horse involves the gradual development of the modern horse from fox-sized, forest-dwelling mammals. Zoologists have been able to piece together a more complete picture of the modern horse’s evolutionary line than that of any other animal.
Overview
Horses belong to a group 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 similar tooth structure. This means that they share a common ancestor with tapirs and rhinoceri. 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 first 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 the mainland was drying out, the steppes began to appear, and with it came large numbers of dry land predators. This in turn required the horse’s predecessors to possess more speed in order to survive. The ability to run faster was accomplished by 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. On solid ground, pushing-off with a single toe and equipped at the last evolutionary link with a hoof, the horse was able to reach fast speeds. The modern horse is a single-toed, hoofed mammal, while its predecessors were multi-odd-toed animals.
Hyracotherium
The earliest animals to bear recognisably horse-like anatomy are found in the lower Eocene and are called Hyracotherium, an animal approximately the size of a fox with a relatively short head, 44 teeth with uneven, dull and bumpy molars, a short neck, a springy, arched back, and wrist and hock joints that are still low to the ground. Its limbs were relatively long, already showing the beginnings of adaptations for running. The forelimbs had developed five toes out of which only four were equipped with a small hoof; the fifth large “toe–thumb” was off the ground. The hind limbs had three out of the five toes equipped with small hooves, while the first and fifth toes did not touch the ground.
During the Eocene, the Hyracotheria branched out into various types resembling the fox in size (from 250 mm to 450 mm in height). Thousands of complete, fossilized skeletons of these animals have been found in the Eocene layers of North America, mainly in the Wind River basin of Wyoming. Similar fossils of horses have also been discovered in Europe.
Orohippus
Orohippus (also called Protorohippus) lived in the middle of the Eocene. It resembled Hyracotherium is size, but had a slimmer body, an elongated head, slimmer forelimbs and longer hind legs, all of which are characteristics of a good jumper. The outer toes of Hyracotherium are not present in the Orohippus; hence on each forelimb there were four toes and on three on each hind leg. The first of the premolar teeth were dwarfed.
Mesohippus
In the early stages of the Oligocene epoch, the North American environment was changing. During the still-warm and dry weather conditions, the forests were yielding to flatlands, which were home to grasses and various kinds of brush. In some places these plains were covered in sand, creating the type of environment resembling the present-day prairies.
During the younger 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. Judging by its slim limbs, Mesohippus, which was about 500 mm tall, was a fast animal.
Miohippus and its descendants
At the end of Oligocene epoch and the beginning of the Miocene epoch, Miohippus emerged, which was somewhat bigger than its predecessors. It branched out into two forms, one of which adjusted to the life in forests, while the other remained suited to life on the prairies.
The forest form was Kalobatippus (or Miohippus intermedius), whose second and fourth front toes were long, to suit 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 form known as Hyohippus, which became extinct near the beginning of the Pliocene.
From the Miohippus that remained on the steppes are 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
The form of Meryhippus that lead to present day horses was the Pliocene Pliohippus. It had long extra toes on both sides of the hoof, but they were externally barely visible as callused stubs. The long and slim limbs of Pliohippus reveal a quick-footed steppe animal.
Pleshippus
Plesippus is an intermediary stage between Pliohippus and the present day horse.
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 Pleshuippus. Hippidium was relatively short-legged with an especially long nose that also formed the lower part of the skull. It continued to live on the South American pampas for a long time, but eventually died out.
Equus, the 'true horse'
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 American, where it is known as Equus scotti; it did not differ in any way from the European form; however, some types (Equus scotti var. giganteus) exceeded the modern horse in size. However, all the horses in North America ultimately became extinct, perhaps due to some mass contagion, but it is also theorized that humans hunted them 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.
Divide between horses and zebras
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.
Horses return to America
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.
Details
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, not only due to plentiful food, but also to the increase in variety.