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The history of horse domestication has been subject to much debate, with various competing theories over time about how domestication of the horse occurred. The main point of contention was whether the domestication of the horse occurred once in a single domestication event, or that the horse was domesticated independently multiple times. The debate was resolved at the beginning of the 21st century using DNA evidence that favored a mixed model in which domestication of the stallion most likely occurred only once, while wild mares of various regions were included in local domesticated herds.
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In the 20th century, various theories were postulated. One set hypothesized multiple ancestral body types of the single species Equus ferus or the original wild horse, each adapted to a given environment. Another hypothesis held that prototypes originated from a single wild species and that all different body types were entirely a result of selective breeding after domestication. Yet another proposed that the theoretical prototypes were each separate species or subspecies. These theories were all based on body types and conformation, prior to the availability of DNA for research, and have since been superseded by modern studies.
Theories
In the past, several theories were proposed about the origin of the domesticated horse and how the variety in horse breeds developed. They generally can be subdivided in two categories, single origin versus multiple origins.
Single origin
The single origin theory holds that domestication occurred once, after which all breeds arose through selective breeding.
Multiple primitive types
A theory associated with James Cossar Ewart in Scotland and Johann Ulrich Duerst in Germany postulated three primitive horse types, considered subspecies of Equus caballus, as ancestors of modern breeds. They were:
To these Elwyn Hartley Edwards adds a fourth, the "Tundra Horse", supposedly ancestor of the Yakut pony, and "largely unconsidered by hippologists".
A later theory associated with European scholars such as Jimmy Speed, Ruy d'Andrade, Hermann Ebhardt and Edward Skorkowski, postulated four basic body types, which were not considered to be named species. They were:
American paleontologist Deb Bennett postulated that the early form of E. caballus developed into seven subspecies, of which four supposedly contributed most to the ancestry of the domesticated horse, both directly and via assorted crossbred lineages between them. These were:
The other three proposed subspecies were:
Current theory
Modern genetic evidence now points at a single domestication event for a limited number of stallions, combined with repeated restocking of wild mares into domesticated herds. This suggests that different body types might be a combination of both selective breeding and semi-feral landrace traits.
A study in 2012 that performed genomic sampling on 300 work horses from local areas as well as a review of previous studies of archaeology, mitochondrial DNA, and Y-DNA suggested that horses were originally domesticated in the western part of the Eurasian steppe. Both domesticated stallions and mares spread out from this area, and then some additional wild mares were added from local herds; wild mares were easier to handle than wild stallions. Most other parts of the world were ruled out as sites for horse domestication, either due to climate unsuitable for an indigenous wild horse population or no evidence of domestication. It remains possible location that a second, independent, domestication site might exist in the Iberian Peninsula, but the study could neither confirm nor disprove that hypothesis.
The Przewalski's horse (now Equus ferus przewalskii) is currently believed to be unrelated to the modern domestic horse, though studies using DNA have had varied results. Recent mitochondrial DNA analysis suggests that the Przewalski and the modern domestic horse diverged some 160,000 years ago. Studies using DNA have been inconclusive. A 2009 molecular study using ancient DNA (that is DNA recovered from archaeological finds like bones and teeth) places the Przewalski's Horse in the middle of the domesticated horses, These difficulties exist in part due to crossing domestic horses into the Przewalski's Horse as well as the limited genetic variation present in the founder population of the modern Przewalski's Horse.