A recent study by Warmuth et al. goes some way to resolving the contested theories on the origin of the domestic horse. Earlier scenarios diverged: archaeological studies found domestication was a ripple effect from a central domesticated group in the Western steppe (Ukraine, southwest Russia and west Kazakhstan); however DNA studies concluded that pockets of domestication occurred separately across Europe and Eurasia.
In this reported study researchers computer modelled DNA samples from horses in 8 countries in Europe and Asia; they found that domestication occurred around 6,000 years ago in the Western steppe (Ukraine, southwest Russia and west Kazakhstan), where wild mares were taken to restock domestic herds because, it’s suggested, it was difficult to breed captive, domesticated horses. This last point underlines one of the criteria for domestication which is that the species must breed easily in captivity. In the early domesticated herds this would have required selective breeding from the more docile members.
Reference (online): Warmuth, V. et al., (2012). Reconstructing the origin and spread of horse domestication in the Eurasian steppe. PNAS 2012 : 1111122109v1-201111122