The dwellers of Indus valley were Dravidians. The existence of the Brahui tribe in Balochistan, Pakistan who speak a Dravidian language like South Indian Tamil, gives evidence that a migration did occur.
The Harappa religion has many similarities with elements of Hinduism, popular in the present Dravidian culture. The analysis of the signs on the inscriptions found in the IVC has led several scholars to the view that the language is not of the Indo-European , or the Sumerians ,Elamite, nor the Munda languages of India. If it is related to any modern language family it appears to be Dravidian akin to Old Tamil
Sir John Marshall, was the first scholar to suggest that the language of the IVC was Dravidian. Piero Meriggi in his book “ Zur_Indus Scrift “ (On the Indus Script) was of the opinion that Brahui, a Dravidian Language spoken in Balochistan, must be the original language of the Harappans
Brahui Research Institute