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 broader evidence, including skeletal and dental evidence of working activity; weapons, art, and spiritual artifacts; and lifestyle patterns of human cultures. There is also evidence that horses were kept as meat animals prior to being trained as working animals.
Attempts to date domestication by genetic study or analysis of physical remains rests on the assumption that there was a separation of the genotype or phenotype of domesticated and the 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). Further, all modern horse populations retain the ability to revert to a feral state, and all feral horses are of domestic types; that is, they descend from ancestors that escaped from captivity.
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 domestication of the horse. The date of 4000 BC 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, 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 BC, as seen in horse remains found at the site of Csepel-Haros in Hungary, a settlement of the Bell Beaker culture.
Regardless of the specific date of domestication, use of horses spread rapidly across Eurasia for transportation, agricultural work and warfare. Possibly as early as 3500-3000 BC, and certainly during the period 2500-2000 BC, human reliance on domesticated horses spread across Eurasia for transportation 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 horse collar that arose several millennia later.
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 or were hunted out by humans, particularly in North America, where the horse became completely extinct.
Classification based on body types and conformation, absent the availability of DNA for research, suggest that that there were roughly four basic wild prototypes, thought to have developed with adaptations to their environment prior to domestication. There are competing theories, some argue that the four prototypes were separate species or subspecies, while others suggest that the prototypes were physically different manifestations of the same species. However, like Equus ferus, these animals probably were able to crossbreed with each other, thus were not completely separate species. Other theories hold that there was only one wild species and all different body types were entirely a result of selective breeding after domestication.
In any case, the most broadly accepted classification system based on body types and conformation, sometimes called the "Four Foundations Theory," describe the four base prototypes as follows:
Only two never-domesticated "wild" groups 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, Przewalski's horse is endangered; it became extinct in the wild during the 1960s, but was re-introduced in the late 1980s to two preserves in Mongolia. Although researchers such as Marija Gimbutas theorized that the horses of the Chalcolithic period were Przewalski's, more recent genetic studies indicate that Przewalski's horse is not an ancestor to modern domesticated horses. However, other subspecies of Equus ferus, appear to have existed and could have been the stock from which domesticated horses are descended.
Even though horse domestication was widespread in a short period of time, it is still possible that domestication began with a single culture, which passed on techniques and breeding stock. It is possible that the two "wild" subspecies remained when all other groups of once-"wild" horses died out because all others had been, perhaps, more suitable for taming by humans and the selective breeding that gave rise to the modern domestic horse.
Evidence for the domestication of the horse comes from three kinds of sources: 1) changes in the skeletons and teeth of ancient horses; 2) changes in the geographic distribution of ancient horses, particularly the introduction of horses into regions where no wild horses had existed; and 3) archaeological sites containing artifacts, images, or evidence of changes in human behavior connected with horses.
Archaeological evidence includes horse remains interred in human graves; changes in the ages and sexes of the horses killed by humans; the appearance of horse corrals; equipment such as bits or other types of horse tack; horses themselves interred with equipment intended for use by horses, such as chariots; and depictions of horses used for riding, driving, draught work, or symbols of human power.
Few of these categories, taken alone, provide irrefutable evidence of domestication, but combined add up to a persuasive argument.
In all of the dated chariot graves, the heads and hooves of a pair of horses were placed in a grave that once contained a chariot. Evidence of chariots in these graves was inferred from the impressions of two spoked wheels set in grave floors 1.2-1.6m apart; in most cases the rest of the vehicle left no trace. In addition a pair of disk-shaped antler "cheekpieces," an ancient predecessor to a modern bit shank or bit ring, were placed in pairs beside each horse head-and-hoof sacrifice. The inner faces of the disks had protruding prongs or studs that would have pressed against the horse’s lips when the reins were pulled on the opposite side. Studded cheekpieces were a new and fairly severe kind of control device that appeared simultaneously with chariots.
All of the dated chariot graves contained wheel impressions, horse bones, weapons (arrow and javelin points, axes, daggers, or stone mace-heads), human skeletal remains, and cheekpieces. Because they were buried in teams of two with chariots and studded cheekpieces, the evidence is extremely persuasive that these steppe horses of 2100-1700 BC were domesticated. Shortly after the period of these burials, the expansion of the domestic horse throughout Europe was little short of explosive. In the space of possibly 500 years, there is evidence of horse-drawn chariots in Greece, Egypt, and Mesopotamia. By another 500 years, the horse-drawn chariot had spread to China.
Entire herds of horses were slaughtered by the Botai hunters, apparently in hunting drives. The adoption of horseback riding might explain the appearance of specialized horse-hunting techniques and larger, more permanent settlements. Domesticated horses could have been adopted from neighboring herding societies in the steppes west of the Ural Mountains, where the Khvalynsk culture had herds of cattle and sheep, and perhaps had domesticated horses, as early as 4800 BC.
Other researchers have argued that all of the Botai horses were wild, and that the horse-hunters of Botai hunted wild horses on foot. As evidence, they note that Zoologists have found no skeletal changes in the Botai horses that indicate domestication. And because they were hunted for food, the majority of the horse remains found in Botai-culture settlements indeed probably were wild. On the other hand, any domesticated riding horses were probably the same size as their wild cousins and cannot now be distinguished by bone measurements. They also note that the age structure of the horses slaughtered at Botai represents a natural demographic profile for hunted animals, not the pattern expected if they were domesticated and selected for slaughter. However, these arguments were published prior to the discovery of a corral at Krasnyi Yar and mats of horse-dung at two other Botai sites.
The regular use of a bit to control a horse can create wear facets or bevels on the anterior corners of the lower second premolars. The corners of the horse's mouth normally keep the bit on the "bars" of the mouth, an interdental space where there are no teeth, forward of the premolars. The bit must be manipulated by a human or the horse must move it with its tongue for it to touch the teeth. Wear can be caused by the bit abrading the front corners of the premolars if the horse grasps and releases the bit between its teeth; other wear can be created by the bit striking the vertical front edge of the lower premolars, due to very strong pressure from a human handler.
Modern experiments showed that even organic bits of rope or leather can create significant wear facets, and also showed that facets 3mm deep or more do not appear on the premolars of wild horses. However, other researchers disputed both conclusions.
Wear facets of 3mm or more also were found on seven horse premolars in two sites of the Botai, Botai and Kozhai 1, dated about 3500-3000 BC. The Botai culture premolars are the earliest reported multiple examples of this dental pathology in any archaeological site, and preceded any skeletal change indicators by 1000 years. While wear facets more than 3mm deep were discovered on the lower second premolars of a single stallion from Dereivka in Ukraine, an Eneolithic settlement dated about 4000 BC, dental material from one of the worn teeth later produced a radiocarbon date of 700-200 BC, indicating that this stallion was actually deposited in a pit dug into the older Eneolithic site during the Iron Age.
Around 3500-3000 BC horse bones began to appear more frequently in archaeological sites beyond their center of distribution in the Eurasian steppes and were seen in central Europe, the middle and lower Danube valley, and the North Caucasus and Transcaucasia. Evidence of horses in these areas had been rare before, and as numbers increased, larger animals also began to appear in horse remains. This expansion in range was contemporary with the Botai culture, where there are indications that horses were corralled and ridden. This does not necessarily mean that horses were first domesticated in the steppes, but the horse-hunters of the steppes certainly pursued wild horses more than in any other region. This geographic expansion is interpreted by many zoologists as an early phase in the spread of domesticated horses.
European wild horses were hunted for up to 10% of the animal bones in a handful of Mesolithic and Neolithic settlements scattered across Spain, France, and the marshlands of northern Germany, but in many other parts of Europe, including Greece, the Balkans, the British Isles, and much of central Europe, horse bones do not occur or occur very rarely in Mesolithic, Neolithic or Chalcolithic sites. In contrast, wild horse bones regularly exceeded 40% of the identified animal bones in Mesolithic and Neolithic camps in the Eurasian steppes, west of the Ural Mountains.
Horse bones were rare or absent in Neolithic and Chalcolithic kitchen garbage in western Turkey, Mesopotamia, most of Iran, South and Central Asia, and much of Europe. While horse bones have been identified in Neolithic sites in central Turkey, all equids together totaled less than 3% of the animal bones. Within this three percent, horses were less than 10%, with 90% or more of the equids represented by onagers (Equus hemionus) or another ass-like equid that later became extinct, Equus Hydruntinus. Onagers were the most common native wild equids of the Near East. They were hunted in Syria, Anatolia, Mesopotamia, Iran, and Central Asia; and domesticated asses (Equus asinus) were imported into Mesopotamia, probably from Egypt, but wild horses apparently did not live there.
A further expansion, into the lowland Near East and northwestern China, also happened around 2000 BC, again apparently in conjunction with the chariot. Although ‘’Equus’’ bones of uncertain species are found in some Late Neolithic sites in China dated before 2000 BC, ‘’Equus caballus’’ or ‘’Equus ferus’’ bones first appeared in multiple sites and in significant numbers in sites of the Qijia and Siba cultures, 2000-1600 BC, in Gansu and the northwestern provinces of China. The Qijia culture was in contact with cultures of the Eurasian steppes, as shown through similarities between Qijia and Late Bronze Age steppe metallurgy, so it was probably through these contacts that domesticated horses first became frequent in northwestern China.
When Suvorovo graves appeared in the Danube delta grasslands, horse-head maces also appeared in some of the indigenous farming towns of the Tripolye and Gumelnitsa cultures in present-day Romania and Moldova, near the Suvorovo graves. These agricultural cultures had not previously used polished-stone maces, and horse bones were rare or absent in their settlement sites. Probably their horse-head maces came from the Suvorovo immigrants. The Suvorovo people in turn acquired many copper ornaments from the Tripolye and Gumelnitsa towns. After this episode of contact and trade, but still during the period 4200-4000 BC, about 600 agricultural towns in the Balkans and the lower Danube valley, some of which had been occupied for 2000 years, were abandoned. Copper mining ceased in the Balkan copper mines, and the cultural traditions associated with the agricultural towns were terminated in the Balkans and the lower Danube valley. This collapse of "Old Europe" has been attributed to the immigration of mounted Indo-European warriors. The collapse could have been caused by intensified warfare, for which there is some evidence; and warfare could have been worsened by mounted raiding; and the horse-head maces have been interpreted as indicating the introduction of domesticated horses and riding just before the collapse.
However, mounted raiding is just one possible explanation for this complex event. Environmental deterioration, ecological degradation from millennia of farming, and the exhaustion of easily mined oxide copper ores also are cited as causal factors.
At S’yezzhe, a contemporary cemetery of the Samara culture, parts of two horses were placed above a group of human graves. The pair of horses here was represented by the head and hooves, probably originally attached to hides. The same ritual—using the hide with the head and lower leg bones as a symbol for the whole animal—was used for many domesticated cattle and sheep sacrifices at Khvalynsk. Horse images carved from bone were placed in the above-ground ochre deposit at S’yezzhe and occurred at several other sites of the same period in the middle and lower Volga region. Together these archaeological clues suggest that horses had a symbolic importance in the Khvalynsk and Samara cultures that they had lacked earlier, and that they were associated with humans, domesticated cattle, and domesticated sheep. Thus, the earliest phase in the domestication of the horse might have begun during the period 4800-4400 BC.
These analyses showed that modern domesticated mares have almost as much genetic variation as fossil wild mares. In contrast, similar analyses of mitochondrial DNA had shown that modern individuals from cattle, sheep, water buffalo, and pig breeds are much less genetically diverse than their ancient forebears. Thus, it appears that a large number of wild lineages were involved in the equine matriline most recent common ancestor (mt-mrca) of domesticated horses, many more than in any other domestic mammal studied to date. This probably means either that mares were domesticated in many different places or that the gene pool of the domesticated population was continuously refreshed by the capture of wild mares.
On the other hand, the patriline (Y-mrca) of modern domestic horses shows greater homogeneity. Genes located on the Y chromosome, which is inherited only from sire to male offspring, show much less variation in modern domestic horses. This probably means that relatively few stallions were domesticated, and that the male offspring of unions between wild stallions and domestic mares were not as frequently retained as breeding stock. In any case, the patrilineal most recent common ancestor seems to have emerged more recently than the matrilineal most recent common ancestor. This parallels human genetics, where "Y-Adam" is considerably younger than "mt-Eve".
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 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 a dispute over what domesticated means. One theory suggests that domestication 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 in order to hypothesize how domestication occurred. For example, while Native American cultures captured and rode horses from the 16th century on, 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."
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.
On the other hand, others argue that evidence of bit wear does not necessarily correlate to riding. Some theorists speculate that a horse could have been controlled from the ground by placing a bit in the mouth, connected to a lead rope, and leading the animal while pulling a primitive wagon or plow. Since oxen were usually relegated to this duty in Mesopotamia, it is possible that early plows might have been attempted with the horse, and a bit may indeed have been significant as part of agrarian development rather than as warfare technology.
While riding may have been practiced during the 4th and 3rd millennia BC, and the disappearance of "Old European" settlements may be related to attacks by horseback-mounted warriors, the clearest impact by horses on ancient warfare was by pulling chariots, introduced circa 2000 BC.
Horses in the Bronze Age were relatively small by modern standards, which led some theorists to believe the ancient horses were too small to be ridden and so must have been driven. Herodotus' description of the Sigynnae, a steppe people who bred horses too small to ride but extremely efficient at drawing chariots, illustrates this stage. However, as horses remained generally smaller than modern equines well into the Middle Ages, this theory is highly questionable.
The Iron Age in Mesopotamia saw the rise of mounted cavalry as a tool of war, as evidenced by the notable successes of mounted archer tactics used by various invading equestrian nomads such as the Parthians. Over time, the chariot gradually become obsolete.
The horse of the Iron Age was still relatively small, perhaps 12.2 to 14.2 hands high (1.27 to 1.47 meters, measured at the withers.) This was shorter overall average height than modern riding horses, which range from 14.2 to 17.2 hh (1.47 to 1.78 meters). However, small horses were used successfully as light cavalry for many centuries. For example, Fell ponies, believed to be descended from Roman cavalry horses, are comfortably able to carry fully grown adults (although with rather limited ground clearance) at an average height of 13.2 hands (1.37 m). Likewise, the Arabian horse is noted for a short back and dense bone, and the successes of the Muslims against the heavy mounted knights of Europe demonstrated that a 14.2 hand horse can easily carry a full-grown human adult into battle.
Mounted warriors such as the Scythians, Huns and Vandals of late Roman antiquity, the Mongols who invaded eastern Europe in the 7th century through 14th centuries AD, the Muslim warriors of the 8th through 14th centuries AD, and the American Indians in the 16th through 19th centuries each demonstrated effective forms of light cavalry.