The exact timeline and mechanisms of the domestication of the horse have been subject to significant academic debate. Although horses appeared in Paleolithic cave art as early as 30,000 BCE, these were wild horses and were probably hunted for meat. Early discoveries in the context of the Botai culture in the Akmola Province of Kazakhstan had strongly suggested that Botai settlements (dated to 3700–3500 BCE) were the location of the earliest domestication, citing evidence such as bit-related tooth wear and horse milking.
However, more recent archaeological and genetic evaluations have challenged this timeline. Taylor and Barrón-Ortiz (2021) argue that Botai findings only reflect intensive exploitation of wild horses, possibly involving some level of management, herding, or seasonal capture, but not full domestication in the way we see in later horse-using societies. While earlier studies by Warmuth et al. (2012) pointed to horses having been domesticated around 3000 BCE in what is now Ukraine, Russia, and Western Kazakhstan, archaeozoologist William T. Taylor argues that true domestication did not take place until around 2000 BCE.
Though previously thought to have been domesticated in Central Asia, recent genetic studies have shown that all modern domestic horses were exclusively domesticated in the Pontic–Caspian steppe of Eastern Europe. Genomic evidence indicates that the source of this domestication occurred around 2200 BCE in the Volga–Don region of modern-day southwestern Russia, rather than earlier, broader suggestions. From this core area, the domestic lineage rapidly expanded across Eurasia for transportation, agricultural work, and warfare, completely replacing other local horse populations. Scientists have linked the successful spread of domesticated horses to observed genetic mutations. They speculate that stronger backs (the GSDMC gene) and increased docility (the ZFPM1 gene) made these horses uniquely suitable for riding, providing the clearest evidence for their widespread use as a means of transport in chariot burials dated c. 2000 BCE.
Background
The date of the domestication of the horse depends to some degree upon the definition of "domestication". Some zoologists define "domestication" as human control over breeding, which can be detected in ancient skeletal samples by changes in the size and variability of ancient horse populations. Other researchers look at the broader evidence, including skeletal and dental evidence of working activity; weapons, art, and spiritual artifacts; and lifestyle patterns of human cultures. There is evidence that horses were kept as a source of meat and milk before they were trained as working animals.
Attempts to date domestication by genetic study or analysis of physical remains rest on the assumption that there was a separation of the genotypes of domesticated and wild populations. Such a separation appears to have taken place, but dates based on such methods can only produce an estimate of the latest possible date for domestication without excluding the possibility of an unknown period of earlier gene flow between wild and domestic populations (which will occur naturally as long as the domesticated population is kept within the habitat of the wild population).
Whether one adopts the narrower zoological definition of domestication or the broader cultural definition that rests on an array of zoological and archaeological evidence affects the time frame chosen for the domestication of the horse. The date of 4300-4000 BCE is based on evidence that includes the appearance of dental pathologies associated with bitting, changes in butchering practices, changes in human economies and settlement patterns, the depiction of horses as symbols of power in artifacts, patterns consistent with long-distance raiding and the appearance of horse bones in human graves. On the other hand, measurable changes in size and increases in variability associated with domestication occurred later, about 2500–2000 BCE, as seen in horse remains found at the site of Csepel-Haros in Hungary, a settlement of the Bell Beaker culture.
Use of horses spread across Eurasia for transportation, agricultural work and warfare. Horses and mules in agriculture used a breastplate type harness or a yoke more suitable for oxen, which was not as efficient at utilizing the full strength of the animals as the later-invented padded horse collar that arose several millennia later.
Predecessors to the domestic horse
A 2005 study analyzed the mitochondrial DNA (mtDNA) of a worldwide range of equids, from 53,000-year-old fossils to contemporary horses. 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 many breeds of modern domestic horses, belonged to a single Holarctic species.
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. It became extinct in Beringia around 14,200 years ago, and in the rest of the Americas around 10,000 years ago. This clade survived in Eurasia, however, and it is from these horses which all domestic horses appear to have descended. These horses showed little phylogeographic structure, probably reflecting their high degree of mobility and adaptability.
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. 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. 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.
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. Many of these Ice Age subspecies died out during the rapid climate changes associated with the end of the last Ice Age particularly in North America, where the horse became completely extinct.
Two undomesticated sub-species survived into historic times: Przewalski's horse (Equus ferus przewalski) and the Tarpan (Equus ferus ferus). 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.
Genetic evidence
A 2014 study compared DNA from ancient horse bones that predated domestication and compared them to DNA of modern horses, discovering 125 genes that correlated to domestication. Some were physical, affecting muscle and limb development, cardiac strength and balance. Others were linked to cognitive function and most likely were critical to the taming of the horse, including social behavior, learning capabilities, fear response, and agreeableness. The DNA used in this study came from horse bones 16,000 to 43,000 years ago, and therefore the precise changes that occurred at the time of domestication have yet to be sequenced.
The domestication of stallions and mares can be analyzed separately by looking at those portions of the DNA that are passed on exclusively along the maternal (mitochondrial DNA or mtDNA) or paternal line (Y-chromosome or Y-DNA). DNA studies indicate that there may have been multiple domestication events for mares, as the number of female lines required to account for the genetic diversity of the modern horse suggests a minimum of 77 different ancestral mares, divided into 17 distinct lineages. Studies of modern horses showed very little Y chromosome diversity, which was originally interpreted as evidence of a single domestication event for a limited number of stallions combined with repeated restocking of wild females into the domesticated herds. However, more recent studies of ancient DNA show that Y chromosome diversity was significantly higher a thousand years ago. The low present diversity may be partially explained by the popularity of Arabian and Turkoman studs, especially the three foundation stallions of the Thoroughbred breed.
A study published 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 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.
Genes located on the Y-chromosome are inherited only from sire to its male offspring and these lines show a very reduced degree of genetic variation (aka genetic homogeneity) in modern domestic horses, far less than expected based on the overall genetic variation in the remaining genetic material. This indicates that a relatively few stallions were domesticated and that it is unlikely that many male offspring originating from unions between wild stallions and domestic mares were included in early domesticated breeding stock.
Genes located in the mitochondrial DNA are passed on along the maternal line from the mother to her offspring. Multiple analyses of the mitochondrial DNA obtained from modern horses as well as from horse bones and teeth from archaeological and palaeological finds consistently shows an increased genetic diversity in the mitochondrial DNA compared to the remaining DNA, showing that a large number of mares has been included into the breeding stock of the originally domesticated horse.
Variation in the mitochondrial DNA is used to determine so-called haplogroups. A haplogroup is a group of closely related haplotypes that share the same common ancestor. In horses, eighteen main haplogroups are recognized (A-R). Several haplogroups are unequally distributed around the world, indicating the addition of local wild mares to the domesticated stock.
In 2018, genomic comparison of 42 ancient-horse genomes, 20 of which were from Botai, with 46 published ancient and modern-horse genomes yielded surprising results. It was found that modern domestic horses are not closely related to the horses at Botai. Rather, Przewalski's horses were identified as feral descendants of horses herded at Botai. Evidence suggested that "a massive genomic turnover" had occurred along with the domestication of horses and large-scale human population expansion in the Early Bronze Age.
Subsequent research showed that horse lineages from Iberia and Siberia, also associated with early domestication, had little influence on the genetics of modern domestic horses.
More than 150 scientists collaborated in gathering 264 ancient horse genomes from across Eurasia, dating from 50,000 to 200 B.C.E. In October 2021, results of the analysis were published in Nature. They indicated that domestication of the modern horse's ancestors likely occurred in the Volga-Don region of the Pontic–Caspian steppe grasslands of Eastern Europe. Both Tarpan and Przewalski's horse were related to different ancestral populations than those underlying the modern domestic horses (DOM2).
In addition, researchers were able to map population changes over time as modern domestic horses expanded rapidly across Eurasia and displaced other local populations, from about 2000 BCE onwards. The genetic profile for DOM2 horses is associated with horses buried in Sintashta kurgans with early spoke-wheeled chariots, and with horses in Central Anatolia where two-wheeled vehicles were depicted. DOM2 horses also occur in some areas prior to the earliest evidence for chariots, suggesting that both horseback riding and chariot use were factors in expansion.
Genetic data may also provide clues as to why this particular domestication event had far more widespread impact than other domestication events in Botai, Iberia, SIberia and Anatolia. The genetic lineage that leads to modern domestic horses shows evidence of strong selection for locomotor and behavioural adaptations. Changes relate to the GSDMC gene and the ZFPM1 gene. The GSDMC gene is linked to back problems in people, and scientists speculate that changes may have made horses' backs stronger. The ZFPM1 gene is related to mood regulation, and scientists speculate that this may have made horses more docile and easier to tame and manage. Strength and docility would have made horses more suitable for riding and other uses.