NewsThe horse’s evolutionary pedigree is one of the best-documented in paleontology. The horse family, Equidae, first appeared during the Eocene Epoch, which lasted from around 56 million to 33.9 million years ago. The earliest ancestral horse arose during the early Eocene, a hoofed, browsing animal technically named Hyracotherium but more often known as Eohippus, the “dawn horse.” Eohippus fossils discovered in North America and Europe depict an animal that stood 4.2 to 5 hands (approximately 42.7 to 50.8 cm, or 16.8 to 20 inches) tall, small in contrast to the contemporary horse, with an arched back and elevated hindquarters. The legs terminated in padded feet with four functioning hooves on each forefeet and three on each rear foot, in contrast to current equines’ unpadded, single-hoofed foot. The skull lacked the current horse’s broad, flexible snout, and the size and form of the cranium suggest that the brain was much smaller and less sophisticated than that of today’s horse. The teeth, which were tailored to a fairly broad browser’s diet, also varied greatly from those of current equines. Eohippus was so unhorselike that its evolutionary link to contemporary equines was first unknown. The relationship to Eohippus was not discovered until paleontologists discovered bones of later extinct horses.
The evolutionary trends leading from Eohippus to the modern horse are as follows: increase in size, decrease in the number of hooves, loss of footpads, lengthening of the legs, fusion of the independent bones of the lower legs, elongation of the muzzle, increase in brain size and complexity, and development of crested, high-crowned teeth suited to grazing. This is not to say that there was a smooth, progressive growth of these qualities from Eohippus to those of the contemporary horse. Some of these characteristics, such as grazing dentition, arise suddenly in the fossil record rather than as the result of a series of progressive adaptations. Eohippus, moreover, gave rise to many now-extinct branches of the horse family, some of which differed substantially from the line leading to the modern equines.
While Eohippus remains may be found in both the Old and New Worlds, the horse’s subsequent development occurred mostly in North America. The primary evolutionary changes in dentition occurred over the rest of the Eocene. Orohippus, a middle Eocene genus, and Epihippus, a late Eocene genus, were similar in size and limb shape to Eohippus. Nevertheless, the shape of the cheek teeth—four premolars and three molars located in each side of both jaws—had altered somewhat. Eohippus had separate premolars and molars, with the molars being bigger. In Orohippus, the fourth premolar had evolved to resemble molars, but in Epihippus, both the third and fourth premolars had evolved to resemble molars. In addition, the individual cusps that characterized the cheek teeth of Eohippus had given way in Epihippus to a system of continuous crests or ridges running the length of the molars and molariform premolars. These changes, which represented adaptations to a more-specialized browsing diet, were retained by all subsequent ancestors of the modern horse.
Mesohippus fossils, the next major progenitor of the modern horse, have been discovered in the early and middle Oligocene of North America (the Oligocene Epoch lasted from about 33.9 million to 23 million years ago). Mesohippus was significantly more horselike than its Eocene forefathers: it was taller (averaging about 6 hands [approximately 61 cm, or 24 inches]); the head was more muzzlelike; and the legs were longer and slimmer. Mesohippus had a bigger brain as well. The fourth toe on the forefoot had been reduced to a vestige, so that both the forefeet and hind feet carried three functional toes and a footpad. The teeth remained adapted to browsing.
Mesohippus had developed into a somewhat bigger type known as Miohippus by the late Oligocene. Throughout the early Miocene, Miohippus’ descendants divided into many evolutionary branches (the Miocene Epoch lasted from about 23 million to 5.3 million years ago). One of these branches, known as the anchitheres, comprised numerous species of three-toed browsing horses. Anchitheres were successful, and some genera spread from North America across the Bering land bridge into Eurasia.
Yet, a separate branch went from Miohippus to the contemporary horse. Parahippus was the first representative of this line to arise in the early Miocene. Parahippus and its ancestors were notable for having teeth specialized to chewing grass. Grasses were increasingly common over the North American plains at the period, providing Parahippus with an abundant food source. Grass is a coarser meal than succulent leaves and necessitates a different tooth configuration. The crests of the cheek teeth became bigger and stronger as they adapted to the side-to-side action of the lower jaw required to ground grass blades. Each tooth also possessed an extraordinarily lengthy crown, the majority of which was buried behind the gumline in the juvenile animal. Some of the hidden crown sprouted out when grinding wore away the visible surface. Its high-crowned tooth shape ensured that the animal had an appropriate grinding surface for the duration of its natural life. Adaptations in the digestive tract must have occurred as well, but the organs of digestion are not preserved in the fossil record.
Merychippus, which developed from Parahippus throughout the middle and late Miocene, largely completed the transition from browsing to grazing dentition. Merychippus must have resembled a contemporary pony. It stood around 10 hands (101.6 cm, or 40 inches) tall, and its cranium was identical to that of a contemporary horse. The lower leg’s long bones had fused; this structure, which has been conserved in all current equines, is an adaptation for fast sprinting. The feet remained three-toed, but the footpad was removed in several species, and the two side toes grew quite tiny. The animal’s weight was supported by the huge middle toe in these versions. Powerful ligaments connected the hoofed middle toe to the ankle and lower leg bones, creating a spring mechanism that drove the flexed hoof forward after collision with the ground. Throughout the late Miocene, Merychippus gave birth to multiple evolutionary lineages. Most of them, notably Hipparion, Neohipparion, and Nannippus, preserved their progenitors’ three-toed foot. One line, however, led to the one-toed Pliohippus, the direct predecessor of Equus. Pliohippus fossils occur in the early to middle Pliocene beds of North America (the Pliocene Epoch lasted from about 5.3 million to 2.6 million years ago).
Equus, the genus that includes all contemporary equines such as horses, asses, and zebras, originated from Pliohippus around 4 million to 4.5 million years ago during the Pliocene. Equus shows even greater development of the spring mechanism in the foot and exhibits straighter and longer cheek teeth. By the early Pleistocene, this new variety had spread from the plains of North America to South America and across the Old World (the Pleistocene Epoch lasted from about 2,600,000 to 11,700 years ago). Equus thrived in its North American habitat during the Pleistocene, but subsequently vanished from North and South America about 10,000 to 8,000 years ago. Researchers have proposed a variety of causes for its demise, such as the introduction of deadly illnesses or the arrival of human populations (which presumably hunted the horse for food). Notwithstanding these ideas, the causes for Equus’ demise in the New World remain unknown. The submergence of the Bering land bridge precluded any return migration of horses from Asia, and Equus was not reintroduced into its home continent until the early 16th century, when Spanish explorers imported horses.
Equus evolved in the Old World during the Pleistocene epoch, giving birth to all extant species of the genus. Equus caballus, the modern horse, moved from Central Asia to much of Europe. Local types of horses, all breeds of this single species, undoubtedly evolved, and three of these—horse Przewalski’s (E. ferus przewalskii or E. caballus przewalskii) from Central Asia, the tarpan from Eastern Europe and the Ukrainian steppes, and the forest horse of Northern Europe—are widely regarded as the domestic horse’s ancestral stock. (Przewalski’s horse may be the last surviving distinct breed of wild horse when compared genetically with domesticated horses.) According to this line of thinking, Przewalski’s horse and the tarpan formed the basic breeding stock from which the southerly “warm-blooded” horses developed, while the forest horse gave rise to the heavy, “cold-blooded” breeds.
Related Questions
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What was the original size of a horse?
The earliest ancestral horse arose during the early Eocene, a hoofed, browsing animal technically named Hyracotherium but more often known as Eohippus, the “dawn horse.” Fossils of Eohippus, which have been found in both North America and Europe, show an animal that stood 4.2 to 5 hands (about 42.7 to 50.8 …
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Why did horses become bigger?
The forest was replaced by grassland with shrubs, akin to steppes or grasslands. The horses of the time altered as they adapted and responded to their changing surroundings. They expanded in size (Mesohippus was roughly the size of a goat) and lengthened their legs, allowing them to run quicker.
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Did horses used to be the size of cats?
According to US experts, the Planet was hotter than it is now more than 50 million years ago, when horses the size of household cats roamed the woods of North America.
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How did horses originally look?
Hyracotherium, a little woodland mammal from the early Eocene, was the first equid. It looked nothing like a horse (10 – 20″ tall). It had an arched back, a short neck, a short nose, short legs, and a long tail, much like a dog.
