There probably was no invasion, but according to hard scientific facts there was a fairly large migration from Central Asia to India. The Central Asians migrants are classified as Caucasian and they spoke a Indo-European tongue. The indigenous Indians were not of the same stock as the Central Asians, and even today Indians have physical features more akin to the indigenous people. Only in the deep North (Punjab and above) can we see the Central Asian-looking Indians, and they happen to be a minority. A Central Asian migrant would look like a Persian, not even an Arab, whom usually have Negroid admixture, or Afghanis whom have an even smaller Mongoloid admixture. The language of Indus Valley is unknown according to Western Scholars, however we do know that the chances of Harappan language being IE is low, and Dravidian being much more likely. However this doesn't mean that the IVC tongue was related to Sumerian, Austro-asiatic or even Sino-Tibetan.