As far back as the 3rd or 4th millennium B.C. and probably much earlier still, India was in possession of a highly developed civilization with large … - Kingsley Davis

Although there were mass conversions, the country was too vast, the invaders too few, and the volume of immigration too small to change the social complex… India, therefore, never became a Muslim nation, but remained simply a Hindu country in which Muslims were numerous.

So putting the evidence from archaeology, literature, and history together, we reach the conclusion that before the Christian era India had a substantial population, first because of its advanced technology and second because of the fertile environment of the application of this technology.

So in India some three to seven thousand years ago there were peoples possessing a technology sufficiently advanced to support a dense population.