Bone bits and horse remains in human graves found in the Eneolithic (fifth millennium) Samara culture at the middle Volga, a culture suggested as corresponding to the Proto-Indo-Europeans in the Kurgan hypothesis championed by twentieth century Lithuanian-American archeologist Marija Gimbutas, represent the earliest archaeological evidence of horse domestication.
It cannot yet be determined decisively if these horses were ridden or not, but they were certainly used as a meat-animal.
Recent discoveries concerning Botai culture suggest that Botai culture settlements in the Aqmola Province of Kazakhstan are the location of the earliest domestication of the horse.
However, an increasing amount of evidence supports the hypothesis that horses were domesticated in the Eurasian Steppe (Dereivka, centered in Ukraine) approximately 4000-3500 BCE.
The date of 4000 BCE is based on evidence that includes the appearance of dental pathologies associated with biting, 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.
