The basic idea is that people of the Indus Civilization were the very distant forefathers of the present – day Dravidians and that they had subsequently moved away, for whatever reasons, from the Valley almost enmasse and settled down in the south. The Brahui phenomenon may be looked upon as a tell-tale part of that exodus.
This theory, considered very probable by some scholars for many years now, has of late received a shot in the arm from the striking research carried out mainly by a group of Scandinavian scholars, notably Parpola of Helsinki. Parpola has argued that the language used by the Harappan people in their writings was much closer to Tamil than Sanskrit. (Statesman, Aug. 8)

