The horse as it is known today adapted by evolution to survive in areas of wide-open terrain with sparse vegetation, surviving in an ecosystem where other large grazing animals, especially ruminants, could not.[98] Horses and other equids are odd-toed ungulates of the order Perissodactyla, a group of mammals that was dominant during the Tertiary period. In the past, this order contained 14 families and many species, but only three families—Equidae (the horse and related species), the tapir, and the rhinoceros—containing 18 known species have survived to the present day.[99] The earliest known member of the Equidae family was the Hyracotherium, which lived between 45 and 55 million years ago, during the Eocene period and had 4 toes on each front foot, and 3 toes on each back foot.[100] The extra toe on the front feet soon disappeared with the Mesohippus, which lived 32 to 37 million years ago,[101] and by about 5 million years ago, the modern Equus had developed.[102] The extra side toes shrank in size until they vanished. All that remains of them in modern horses is a set of small vestigial bones on the leg above the hoof,[103] known informally as ergots, chestnuts, or splint bones.[104] Their legs also lengthened as their toes disappeared and until they were a hoofed animal capable of running at great speed.[103]
Over millions of years, equid teeth also evolved from browsing on soft, tropical plants to adapt to browsing of drier plant material, and grazing of tougher plains grasses. Thus the proto-horses changed from leaf-eating forest-dwellers to grass-eating inhabitants of semi-arid regions worldwide, including the steppes of Eurasia and the Great Plains of North America.
About 15,000 years ago Equus ferus was a widespread, holarctic species. Horse bones from this time period, the late Pleistocene, are found in Europe, Eurasia, Beringia, and North America.[105] Yet by 10,000 years ago, the horse became extinct in North America and rare elsewhere.[106][107] The reasons for this extinction are not fully known, but one theory notes that extinction was contemporary with human arrival.[108] Another theory points to climate change, noting that approximately 12,500 years ago, the grasses characteristic of a steppe ecosystem gave way to shrub tundra, which was covered with unpalatable plants.[109]
Wild species surviving into modern times
A truly wild horse is a species or subspecies which has no ancestors that were ever domesticated. Therefore, most "wild" horses today are actually feral horses, animals that escaped or were turned loose from domestic herds and the descendants of those animals.[110]
Only two types of truly wild horses survived into recorded history. One, the Tarpan (Equus ferus ferus) survived into the historical era, but became extinct in 1909, when the last captive died in a Russian zoo.[111] Its pure genetic line was lost, but three attempts have been made to recreate the Tarpan. None of the breeding programs were completely successful, although all three resulted in horses with many similarities to the Tarpan. The Heck horse was created by the German zoologist brothers Heinz Heck and Lutz Heck at the Tierpark Hellabrunn (Munich Zoo) in Germany in an attempt to breed back the Tarpan.[112] In 1936, Polish university professor Tadeusz Vetulani began a program using Konik horses,[111] and in the 1960s Harry Hegardt started a program in the United States using feral mustangs and local working ranch horses that has resulted in the Hegardt or Stroebel's Horse.[113]
There is only one true wild horse species alive today, the Przewalski's Horse (Equus ferus przewalskii). It is a rare Asian animal, also known as the Mongolian Wild Horse; Mongolian people know it as the taki, and the Kyrgyz people call it a kirtag. Small wild breeding populations of this animal, named after the Russian explorer Nikolai Przhevalsky, exist in Mongolia.[114][115] There are also small populations maintained at zoos throughout the world. The species was considered extinct in the wild between 1969 and 1992, but a small breeding population was reestablished in the wild due to the conservation efforts of numerous zoos.[116]
In 1995, a population of horses was discovered in the Riwoche Valley of Tibet, unknown to the rest of the world. It was speculated that these small Riwoche horses were a relic population of wild horses,[115] but testing did not reveal genetic differences with domesticated horses,[117] which is in line with news reports indicating that they are used as pack and riding animals by the local villagers.[118] These horses are hypothesized to be a possible missing link between wild and domesticated horses, in part due to their resemblance to the images known as "horse no 2" seen in prehistoric cave paintings alongside of images of the Przewalski's horse.[117]
Other modern equids
Besides the horse, there are seven other species of genus equus in the equidae family. These are the ass or donkey, Equus asinus; the Mountain Zebra, Equus zebra; Plains Zebra, Equus burchelli; Grévy's Zebra, Equus grevyi; the Kiang, Equus kiang; and the Kulan, Equus hemionus, including its subspecies, the Onager, Equus hemionus onager.[119]
Horses can crossbreed with other members of the equus genus. The most common hybrid is the mule, a cross between a "jack" (male donkey) and a mare. A related hybrid, a hinny, is a cross between a stallion and a jenny (female donkey).[120] Other hybrids include the zorse, a cross between a zebra and a horse that is bred in Africa and used for trekking on Mount Kenya.[121] With rare exceptions, most hybrids are sterile and cannot reproduce.
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