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 line leading from Eohippus to the modern horse exhibits the following evolutionary trends: increase in size, reduction in the number of hooves, loss of the footpads, lengthening of the legs, fusion of the independent bones of the lower legs, elongation of the muzzle, increase in the size and complexity of the brain, 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. Moreover, Eohippus gave birth to multiple now-extinct branches of the horse family, some of which varied significantly from the line that led to contemporary 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 adaptations to a more specialized browsing diet were kept by all succeeding progenitors 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 far more horselike than its Eocene ancestors: it was larger (averaging about 6 hands [about 61 cm, or 24 inches] high); the snout was more muzzlelike; and the legs were longer and more slender. Mesohippus had a bigger brain as well. The forefoot’s fourth toe had been reduced to a remnant, leaving both the forefeet and rear feet with three functioning toes and a footpad. The teeth retained their browsing adaptation.
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, included a variety of three-toed browsing horses comprising several genera. 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 had an extremely long crown, most of which, in the young animal, was buried beneath the gumline. Some of the hidden crown sprouted out when grinding wore away the visible surface. This high-crowned tooth structure assured the animal of having an adequate grinding surface throughout its normal life span. The digestive system must have evolved as well, although the digestive organs 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. Strong ligaments attached this hoofed central toe to the bones of the ankles and lower leg, providing a spring mechanism that pushed the flexed hoof forward after the impact of hitting 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. Yet, one branch led to the one-toed Pliohippus, the direct ancestor of Equus. Pliohippus fossils may be found in North American Pliocene layers from the early to middle Pliocene (the Pliocene Epoch lasted from about 5.3 million to 2.6 million years ago).
Equus—the genus to which all modern equines, including horses, asses, and zebras, belong—evolved from Pliohippus some 4 million to 4.5 million years ago during the Pliocene. Equus has a more developed spring mechanism in the foot, as well as 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 prevented any return migration of horses from Asia, and Equus was not reintroduced into its native continent until the Spanish explorers brought horses in the early 16th century.
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 developed, and three of these—Przewalski’s horse (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 generally credited as being the ancestral stock of the domestic horse. (As compared to farmed horses, Przewalski’s horse may be the last remaining separate breed of wild horse.) According to this theory, Przewalski’s horse and the tarpan were the foundational breeding stock from which the southern “warm-blooded” horses sprang, while the forest horse gave origin to the heavier, “cold-blooded” varieties.
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.
