The evolutionary stages of the horse (family Equidae) serve as an excellent illustration of the evolutionary process because it allows us to observe a step-by-step change in the shape of the bodies, the build of the limbs, and the structure of the teeth. In accordance with the changes in the environment, development continued from a five-toed mammal the size of a fox, to the single-hoofed, modern-sized horse we see today.
At the beginning of this evolutionary line, it is very difficult to distinguish the ancestors of horses from the ancestors of tapirs and rhinoceroses. Both of these animals have obviously similar origins and similarities in the structure of their teeth, odd-toed limbs, obvious mobility of the upper lip, and other aspects by which they join the evolutionary line of odd-toed, hoofed mammals, the Perissodactyls. The tapirs and rhinoceroses remained adapted to their original style of life, conserving forms suitable for life in tropical forests, but the evolutionary line of the horse 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 his predecessors were multi-odd-toed animals.
The evolutionary development of the odd-toed predecessors to the present single-toed Family Equidae can be observed in the Tertiary period of North America, where layers of many well-preserved fossil skeletons of horse predecessors have been discovered. Paleontologists have been able to piece together a more complete picture of the modern horse’s evolutionary line than of any other mammal species.
The evolutionary line of horses began in the lower Eocene epoch in a form called Eohippus. This “horse” was approximately the size of a fox and had various characteristics reminding us of its older predecessors: 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. The limbs are relatively long, obviously showing the beginnings of adaptation to gaining more speed. 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 Eohippus branched out into various types resembling the fox, mainly 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 limbs of four-toed horse ancestors have also been discovered in the Eocene layers of Europe. This mammal was called Hyracotherium .
Orohippus, the successor of Eohippus, appeared in the middle of the Eocene. It has also been called Protorohippus. It still very much resembled the Eohippus; same size, slimmer body, elongated head, slimmer forelimbs and longer hind legs, all of which are characteristics of a good jumper. The outer toes of Eohippus are no longer present in the Orohippus; hence on each forelimb there were four fingers (toes) and on each hind leg three toes, and the first of his premolar teeth were dwarfed.
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; hence the type of environment resembling our present prairies.
The younger Pliocene epoch revealed the Mesohippus, which was one of the more widespread types of mammals in North America at the time. Mesohippus walked on three toes (fingers) on both front and hind feet. The third toe was stronger than the outer ones and thus more weighted. Although the first and fifth toe remained, they were very small. Judging by its slim limbs, Mesohippus, which was about 500 mm tall, was a fast animal.
At the end of Oligocene epoch and the beginning of the Miocene epoch, the Mesohippus evolved into a form known as Miohippus. Miohippus had some parts bigger than its predecessor. It had branched out into two branches from which one adjusted to the life in the primeval forests, while the other remained suited to life on the prairies. The forest form led to the birth of Kalobatippus (Miohippus intermedius), whose second and fourth finger again elongated for travel on the softer primeval forest grounds. The Kalobatippus managed to relocate to Asia via the Bering Strait land bridge, and from there moved into Europe, where its fossils were formerly described under the name Anchitherium. Kalobatippus is then believed to have evolved into a form known as Hyohippus , which became extinct near the beginning of Pliocene.
From the Miohippus that remained on the steppes evolved the North American breed of Parahippus. This little “horse” was the size of a small pony, with a prolonged skull and the facial structure resembling horses of today. The third toe became stronger and larger, hence carrying the main weight of the body. His four premolars resembled the molar teeth and the first almost disappeared. The incisive teeth of the Parahippus, like its predecessors, had a crown as humans do; however, the top incisors had a trace of shallow crease marking the beginning of the core/cup.
In the middle of the Miocene epoch, Merychippus appeared, following after the Parahippus. Meryhippus’ “crunching” on hard steppe, grassy plants caused it to grow relatively longer. It also had wider molars than its predecessors. The hind legs, which were relatively short, still had the side toes equipped with small hooves, though they probably only touched the ground when running. From the numerous varieties of Meryhippus evolved three new kinds of equids: Hipparion , Protohippus and Pliohippus .
According to shapes, the most diverted of the three was the Hipparion. The main difference here was in the structure of tooth enamel. In comparison with other horses, the inside, or tongue side had a completely isolated parapet. On his slim legs, the Hipparion had three toes equipped with small hooves, but the side toes did not touch the ground. The American Hipparion, also known as Neohipparion, proliferated in several 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 most known discovery of these fossils was near Athens.)
The complete and well-preserved skeleton of the North American Hipparion shows an animal the size of a small pony. They were very slim, similar to antelopes, and equally timid. They easily adjusted to life on dry prairies. According to newer research, they’re being excluded from the direct ancestry of horses, as it’s believed Hipparion more likely led to the evolution of zebras and asses. Outside the diversities of head features, today’s horses, asses, and zebras differ only slightly in their teeth and bones. The branch of Protohippus that is believed to be the line leading to the modern horse died off in the Pliocene. The form of Meryhippus that lead to present horses was the Pliocene Pliohippus. It still had long extra toes on both sides of the hoof, but they were externally barely visible, callused stubs. The long and slim limbs of Pliohippus reveal a quick-footed steppe animal. The Pliohippus became the Plesippus, and further evolution into the form of genuine Equus occurred during the upper Pliocene.
It still remains uncertain how these horses came from their “home land” of North America to Europe.
At the end of the Pliocene, the climate in North America began to cool down significantly. The animals were forced to move south. One part 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 section of North America. The Ice Age spread five times over Europe and North America and five times again receded (the interglacial periods). This of course lasted many millennia and it is estimated that approximately one million years elapsed from the Ice Age (the Quaternary period) to our era.
The oldest species of true horse, Equus stenonis, was discovered in Italy, and is believed to have evolved from the Plesippus at the end of the Tertiary and beginning of the Quaternary periods. Equus stenonis suddenly proliferated into two branches, one lighter in body mass and one heavier. The North American genuine form of the diluvium horse was named Equus scotti and did not differ in any way from the European form; however, some types exceeded the modern horse (Equus scotti var. giganteus) in size.
In South America the Plesippus was evolving into a form named Hippidium . The 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 off. All the horses in North America became extinct as well, but more likely due to some mass contagion.
Horses in Historical Times
At the end of the 15th century, when the first Europeans came to America, there were no horses; the cultural tribes of Indians (in today’s Mexico and Peru) did not even have a name for the animal. The Spanish imported predecessors of all the 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.
Emergence of the Genuine Equus Species
The species of genuine horse came to walk only on the end of the third toe and from both side toes, as did their predecessors. Skeletal remnants of 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. It is often believed that the splint bones on the modern horse are a useless attachment, but they indeed 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 in a certain embryonic stage.
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 were forced to find food by grazing on the steppes. Finally, the size of the body was grew as well, not only due to plentiful food, but also to the increase in variety.
