The modern domesticated horse (Equus caballus) is today spread throughout the world and among the most diverse creatures on the planet. In North America, the horse was part of the megafaunal extinctions at the end of the Pleistocene. Two wild subspecies survived until recently, the Tarpan (Equus ferus ferus, died out ca 1919) and Przewalski's Horse (Equus ferus przewalskii, of which there are a few left).
Horse history, especially the timing of the domestication of the horse, is still being debated, partly because the evidence for domestication itself is debatable. Unlike other animals, criteria such as changes in body morphology (horses are extremely diverse) or the location of a particular horse outside of its "normal range" (horses are very widespread) are not useful in helping resolve the question.
Horse History and the Evidence for Horse Domestication
The earliest possible hints for domestication would be the presence of what appear to be a set of postmolds with lots of animal dung within the area defined by the posts, which scholars interpret as representing a horse pen. That evidence has been found at Krasnyi Yar in Kazakhstan, in portions of the site dating to as early as 3600 BC. The horses may have been kept for food and milk, rather than riding or load-bearing.
Accepted archaeological evidence of horseback riding includes bit wear on horse teeth--that has been found in the steppes east of the Ural mountains at Botai and Kozhai 1 in modern Kazakhstan, around 3500-3000 BC. The bit wear was only found on a few of the teeth in the archaeological assemblages, which might suggest that a few horses were ridden to hunt and collect wild horses for food and milk consumption. Finally, the earliest direct evidence of the use of horses as beasts of burden--in the form of drawings of horse-drawn chariots--is from Mesopotamia, about 2000 BC.
Horse History and Genetics
Genetic data, interestingly enough, has traced all extant domesticated horses to one founder stallion, or to closely related male horses with the same Y haplotype. At the same time, there is a high matrilineal diversity in both domestic and wild horses. At least 77 wild mares would be required to explain the diversity of the mitochondrial DNA (mtDNA) in current horse populations, which probably means quite a few more.
A 2012 study (Warmuth and colleagues) combining archaeology, mitochondrial DNA, and Y-chromosomal DNA supports the domestication of horse as occurring once, in the western part of the Eurasian steppe, and that because of the horse's wild natures, several repeated introgression events (restocking of horse populations by adding wild mares), must have occurred. As identified in earlier studies, that would explain the diversity of mtDNA.
Three Strands of Evidence for Domesticated Horses
In a paper published in Science in 2009, Alan K. Outram and colleagues looked at three strands of evidence supporting horse domestication at Botai culture sites: shin bones, milk consumption, and bitwear. These data support domestication of the horse between about 3500-3000 BC sites in what is today Kazakhstan.
Horses skeletons at Botai Culture sites have gracile metacarpals. The horses' metacarpals-the shins or cannon bones-are used as key indicators of domesticity. For whatever reason (and I won't speculate here), shins on domestic horses are thinner--more gracile--than those of wild horses. Outram et al. describe the shinbones from Botai as being closer in size and shape to those of Bronze age (fully domesticated) horses compared to wild horses.
Fatty lipids of horse milk were found inside of pots. Although today it seems a bit weird to westerners, horses were kept for both their meat and milk in the past--and still are in the Kazakh region as you can see from the photograph above. Evidence of horse milk was found at Botai in the form of fatty lipid residues on the insides of ceramic vessels; further, evidence for consumption of horse meat has been identified at Botai culture horse and rider burials.
Bit wear is in evidence on horse teeth. Researchers noted bitting wear on horses' teeth--a vertical strip of wear on the outside of horses' premolars, where the metal bit damages the enamel when it sits between the cheek and tooth. Recent studies (Bendrey) using scanning electron microscopy with energy dispersive X-ray microanalysis found microscopic-sized fragments of iron embedded on Iron Age horse teeth, resulting from metal bit use.