Globular Amphora culture
The mobility of the Kurgan culture facilitated its expansion over the entire region, and is attributed to the domestication of the horse and later the use of early chariots. The first strong archaeological evidence for the domestication of the horse comes from the Sredny Stog culture north of the Azov Sea in Ukraine, and would correspond to an early PIE or pre-PIE nucleus of the 5th millennium BC. Subsequent expansion beyond the steppes led to hybrid, or in Gimbutas's terms "kurganized" cultures, such as the Globular Amphora culture to the west. From these kurganized cultures came the immigration of Proto-Greeks to the Balkans and the nomadic Indo-Iranian cultures to the east around 2500 BC.