The horse (Equus caballus or Equus ferus caballus) is a sizeable ungulate mammal, one of ten modern species of the genus Equus. Horses have long been one of the most economically important domesticated animals, and have played an important role in the transport of people and cargo for thousands of years. Most notably, horses can be ridden by a person perched on a saddle attached to the animal, and are also widely harnessed to pull objects like wheeled vehicles or plows. In some human cultures, horses are also widely used as a source of food. Though isolated domestication may have occurred as early as 4500 BC, clear evidence of widespread use by humans dates to no earlier than 2000 BC, as evidenced by the Sintashta chariot burials, thus firmly establishing the domestication of the horse.
Presently, there are many types of horses. Some are very well known for certain particular qualities or abilities: Thoroughbreds are known for being race horses, while miniature horses serve as mounts for very small children, if they are ridden at all.
Until the middle of the 20th century, armies used horses extensively in warfare; soldiers still refer to the groups of machines that have replaced horses on the battlefield as "cavalry" units, and sometimes preserve traditional horse-oriented names for military units (Lord Strathcona's Horse).
All equids are part of the family Equidae, which dates back approximately 54 million years to the Eocene period. Horses and other equids are odd-toed ungulates of the order Perissodactyla, a relatively ancient group of browsing and grazing animals that first arose less than 10 million years after the dinosaurs became extinct. Perissodactyls were the dominant group of large terrestrial browsing animals until the Miocene (about 20 million years ago), when even-toed ungulates, with stomachs better adapted to grass digestion, began to outcompete them. At one time there were twelve families of odd-toed ungulates, though today only three survive; tapirs and rhinoceroses are the closest living relatives of the modern horse. Horses are believed by scientists to have first evolved in what is now North America.
One of the first true horse species was the tiny Hyracotherium, also known as eohippus, "the dawn horse". In the course of about 5 million years, this early equid evolved into the Orohippus. The vestiges of the 1st and 2nd toes vanished, but the addition of a new "grinding" tooth was significant in that it signalled a transition to improved browsing of tougher plant material. This would allowing grazing not just leafy plants but plains grasses. Their primary food source could transition from leaf-eating forest-dwellers to grass-eating inhabitants of the Great Plains.
Horse evolution was characterized by a reduction in the number of toes, from 5 per foot, to 3 per foot, to only 1 toe per foot (late Myocine 5.3 million years ago). The genus Equus, to which all living equids belong, evolved a few million years ago. Examples of extinct horse genera include: Propalaeotherium, Mesohippus, Miohippus, Orohippus, Pliohippus, Anchitherium, Merychippus, Parahippus, Hipparion and Hippidion.
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