Domestication of the horse
How and when horses became domesticated has been disputed. Although horses appeared in Paleolithic cave art as early as 30,000 BC, these were wild horses and were probably hunted for meat. The clearest evidence of early use of the horse as a means of transport is from chariot burials dated c. 2000 BC.[1][2] However, an increasing amount of evidence began to support the hypothesis that horses were domesticated in the Eurasian Steppes in approximately 3500 BC.[3][4] Discoveries in the context of the Botai culture had suggested that Botai settlements in the Akmola Province of Kazakhstan are the location of the earliest domestication of the horse.[5] Warmouth et al. (2012) pointed to horses having been domesticated around 3000 BC in what is now Ukraine and Western Kazakhstan.[6]
Genetic evidence indicates that domestication of the modern horse's ancestors likely occurred in an area known as the Volga–Don, in the Pontic–Caspian steppe region of eastern Europe, around 2200 BC. From there, use of horses spread across Eurasia for transportation, agricultural work, and warfare. Scientists have linked the successful spread of domesticated horses to observed genetic changes. They speculate that stronger backs (GSDMC gene) and increased docility (ZFPM1 gene) may have made horses more suitable for riding.[3][4]
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A 2005 study analyzed the mitochondrial DNA (mtDNA) of a worldwide range of equids, from 53,000-year-old fossils to contemporary horses.[11] Their analysis placed all equids into a single clade, or group with a single common ancestor, consisting of three genetically divergent species: the South American Hippidion, the North American New World stilt-legged horse, and Equus, the true horse. The true horse included prehistoric horses and the Przewalski's horse, as well as what is now the modern domestic horse, belonged to a single Holarctic species.[11]
The true horse migrated from the Americas to Eurasia via Beringia, becoming broadly distributed from North America to central Europe, north and south of Pleistocene ice sheets.[11] It became extinct in Beringia around 14,200 years ago, and in the rest of the Americas around 10,000 years ago.[12][13] This clade survived in Eurasia, however, and it is from these horses which all domestic horses appear to have descended.[11] These horses showed little phylogeographic structure, probably reflecting their high degree of mobility and adaptability.[11]
Therefore, the domestic horse today is classified as Equus ferus caballus. No genetic originals of native wild horses currently exist. The Przewalski diverged from the modern horse before domestication. It has 66 chromosomes, as opposed to 64 among modern domesticated horses, and their Mitochondrial DNA (mtDNA) forms a distinct cluster.[14] Genetic evidence suggests that modern Przewalski's horses are descended from a distinct regional gene pool in the eastern part of the Eurasian steppes, not from the same genetic group that gave rise to modern domesticated horses.[3][14] Nevertheless, evidence such as the cave paintings of Lascaux suggests that the ancient wild horses that some researchers now label the "Tarpan subtype" probably resembled Przewalski horses in their general appearance: big heads, dun coloration, thick necks, stiff upright manes, and relatively short, stout legs.[15]
The horses of the Ice Age were hunted for meat in Europe and across the Eurasian steppes and in North America by early modern humans. Numerous kill sites exist and many cave paintings in Europe indicate what they looked like.[16] Many of these Ice Age subspecies died out during the rapid climate changes associated with the end of the last Ice Age or were hunted out by humans, particularly in North America, where the horse became completely extinct.[17]
Classification based on body types and conformation, before the availability of DNA for research, once suggested that there were roughly four basic wild prototypes, thought to have developed with adaptations to their environment before domestication. There were competing theories: some argued that the four prototypes were separate species or subspecies, while others suggested that the prototypes were physically different manifestations of the same species.[15] However, more recent study indicates that there was only one wild species and all different body types were entirely a result of selective breeding or landrace adaptation after domestication. Either way, the most common theories of prototypes include four base prototypes:[15]
Two "wild" groups, that were believed to be never-domesticated, survived into historic times: Przewalski's horse (Equus ferus przewalski), and the Tarpan (Equus ferus ferus).[18] The Tarpan became extinct in the late 19th century and Przewalski's horse is endangered; it became extinct in the wild during the late 1960s, but was re-introduced in the early 1990s to two preserves in Mongolia. Although researchers such as Marija Gimbutas theorized that the horses of the Chalcolithic (Copper Age) were Przewalski's, more recent genetic studies indicate that Przewalski's horse is not an ancestor to modern domesticated horses.[14][4]
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Equidae died out in the Western Hemisphere at the end of the last glacial period. A question raised is why and how horses avoided this fate on the Eurasian continent. It has been theorized that domestication saved the species.[70] While the environmental conditions for equine survival were somewhat more favorable in Eurasia than in the Americas, the same stressors that led to extinction for the mammoth had an effect upon horse populations. Thus, some time after 8000 BCE, the approximate date of extinction in the Americas, humans in Eurasia may have begun to keep horses as a livestock food source, and by keeping them in captivity, may have helped to preserve the species.[70] Horses also fit the six core criteria for livestock domestication, and thus, it could be argued, "chose" to live in close proximity to humans.[71]
One model of horse domestication starts with individual foals being kept as pets while the adult horses were slaughtered for meat. Foals are relatively small and easy to handle. Horses behave as herd animals and need companionship to thrive. Both historic and modern data shows that foals can and will bond to humans and other domestic animals to meet their social needs. Thus domestication may have started with young horses being repeatedly made into pets over time, preceding the great discovery that these pets could be ridden or otherwise put to work.
However, there is disagreement over the definition of the term domestication. One interpretation of domestication is that it must include physiological changes associated with being selectively bred in captivity, and not merely "tamed." It has been noted that traditional peoples worldwide (both hunter-gatherers and horticulturists) routinely tame individuals from wild species, typically by hand-rearing infants whose parents have been killed, and these animals are not necessarily "domesticated."
On the other hand, some researchers look to examples from historical times to hypothesize how domestication occurred. For example, while Native American cultures captured and rode horses from the 16th century onwards, most tribes did not exert significant control over their breeding, thus their horses developed a genotype and phenotype adapted to the uses and climatological conditions in which they were kept, making them more of a landrace than a planned breed as defined by modern standards, but nonetheless "domesticated".
Driving versus riding[edit]
A difficult question is if domesticated horses were first ridden or driven. While the most unequivocal evidence shows horses first being used to pull chariots in warfare, there is strong, though indirect, evidence for riding occurring first, particularly by the Botai. Bit wear may correlate to riding, though, as the modern hackamore demonstrates, horses can be ridden without a bit by using rope and other evanescent materials to make equipment that fastens around the nose. So the absence of unequivocal evidence of early riding in the record does not settle the question.
Thus, on one hand, logic suggests that horses would have been ridden long before they were driven. But it is also far more difficult to gather evidence of this, as the materials required for riding—simple hackamores or blankets—would not survive as artifacts, and other than tooth wear from a bit, the skeletal changes in an animal that was ridden would not necessarily be particularly noticeable. Direct evidence of horses being driven is much stronger.[72]
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