Linguists hypothesized that Dravidian-speaking people were spread throughout the Indian subcontinent before a series of Indo-Aryan migrations.
In this view, the early Indus Valley civilisation is often identified as having been Dravidian. Cultural and linguistic similarities have been cited by researchers Henry Heras, Kamil Zvelebil, Asko Parpola, and Iravatham Mahadevan as being strong evidence for a proto-Dravidian origin of the ancient Indus Valley civilisation. Linguist Asko Parpola writes that the Indus script and Harappan language "most likely to have belonged to the Dravidian family".
Parpola led a Finnish team in investigating the inscriptions using computer analysis. Based on a proto-Dravidian assumption, they proposed readings of many signs, some agreeing with the suggested readings of Heras and Knorozov (such as equating the "fish" sign with the Dravidian word for fish "min") but disagreeing on several other readings. A comprehensive description of Parpola's work until 1994 is given in his book Deciphering the Indus Script. The discovery in Tamil Nadu of a late Neolithic (early 2nd millennium BCE, i.e. post-dating Harappan decline) stone celt allegedly marked with Indus signs has been considered by some to be significant for the Dravidian identification. While, Yuri Knorozov surmised that the symbols represent a logosyllabic script and suggested, based on computer analysis, an underlying agglutinative Dravidian language as the most likely candidate for the underlying language. Knorozov's suggestion was preceded by the work of Henry Heras, who suggested several readings of signs based on a proto-Dravidian assumption.
While some scholars like J. Bloch and M. Witzel believe that the Indo-Aryans moved into an already Dravidian speaking area after the oldest parts of the Rig Veda were already composed. The Brahui population of Balochistan has been taken by some as the linguistic equivalent of a relict population, perhaps indicating that Dravidian languages were formerly much more widespread and were supplanted by the incoming Indo-Aryan languages.