Ha! The first civilization which did not leave even a paragraph of writing.
There is a lot of material available on this.
There is no material connecting Indus Valley civilization with the later Vedic civilization. Or with Tamil. All theory and wishful thinking.
Religion and politics interfering in matters which should be purely academic.
Than that is India where politicians and other groups would like to believe that the Aryan Invasion is a fact where as it is still a matter of debate.
The IVC symbols have been found in some of the excavations near Coimbatore in recent times. Such relics have also been found in excavations from other parts of south India also. Iravatham Mahadevan has proposed that the IVC symbols may be linked to Tamil or some other Dravidian language. It therefore looks to me that the whole continent was populated by people who spoke some form of Dravidian language except the eastern/north-eastern parts which had people speaking varieties of Mon-Khmer languages.
The problem today arises only when it is stated that into this area some people migrated from the north-west, in may be more than one batch, separated probably by long intervals of time as well, who had very close relationship with the zoroastrians. This has now become politically not acceptable and the hindutva lobby has created its own panel of experts who propose that sanskrit emanated from dravidian/munda, from sanskrit the avestan, persian and so on emerged, thus possibly spawning the entire gamut of IE languages.
But this does not satisfactorily explain the kikkuli text and the Mitanni treaty, IMO.
While there may or may not have been an Aryan invasion — in the way in which invasion has taken place in historical times — it is not at all impossible that a set of immigrants came, started occupying living areas, hunting areas etc., thus competing for food sources also, which would most probably have invited animosity from those who were already there. When a subsequent batch of immigrants came they might have encountered similar resistance from even those who had come and settled (by somehow finding an equilibrium of sorts but not necessarily a very peaceful absorption into the rest of the population) earlier, and this could have happened many times also, during the ancient past. This hypothesis, if accepted, will help us understand many things in the rigveda like enmity towards dasas, dasyus, asuras, etc., and the "atharvans" and their 'veda' (which was not initially recognized by the vedic tribes/clans) and so on.
Indus script, therefore, represents one earlier civilization which populated much of the Indo-Gangetic plains at one point of time but which was destroyed for reasons not yet clear to us and those people spread to the south of the continent in search of new places of living.