The modern populations in South Asia and Europe were both shaped by two successive migrations to resident hunter-gatherer (H-G) populations. The first migration was from the Near East after ~9000 years ago (#1 on the map). This migration brought farmers who mixed with local H-Gs. The second migration was from the steppe after around 5000 years ago (#2 on the map). This migration brought pastoralists who likely spoke Indo-European languages (this is contested of course), who then mixed with the local farmers they encountered along the way. The resultant mixture of these groups then formed two respective gradients of ancestry in Europe and India. 

~Excerpted from Who We Are And How We Got Here by David Reich