Re: Punjabi/Urdu/Hindi/Sansikrat
It is one of the possible hypothesis. The discovery in Tamil Nadu of a late Neolithic (early 2nd millennium BC, i.e. post-dating Harappan decline) stone celt allegedly marked with Indus script signs has been considered by some to be significant for the Dravidian identification.
The language being unattested in any readable contemporary source, hypotheses regarding its nature are reduced to purported loanwords and substratum influence . The Elamo-Dravidian hypothesis proposes that the extinct Harappan language (the language or languages of the Indus Valley Civilization) may also be part of the same family. As with any hypothesis, there are both supporters and opponents of this theory. I just proceeded in this discussion by supporting the theory. I may be wrong.
The origins of the Dravidian languages, as well as their subsequent development and the period of their differentiation are unclear. The Dravidian family has defied all of the attempts to show a connection with other tongues, including Indo-European, Hurrian, Basque, Sumerian, and Korean except Uralic where there has been observed some similarities. Dravidian is one of the primary language families in the Nostratic proposal, which would link most languages in North Africa, Europe and Western Asia into a family with its origins in the Fertile Crescent sometime between the last Ice Age and the emergence of proto-Indo-European 4–6 thousand years BC. However, the general consensus is that such deep connections are not, or not yet, demonstrable. This is one reason that some scholars hypothesize that the Harappan languages were Dravidian in origin. I am just accepting that for the sake of the discussion and it does not in any way mean that I am right. 
If U check the time line of Indus Valley Civilization, it started around 3300 BC. So the info below makes some sense on Harappans being Dravidian Lang speakers:
The circumstances of the advent of Dravidian speakers in India are shrouded in mystery. There are vague linguistic and cultural ties with the Urals, with the Mediterranean area, and with Iran. It is possible that a Dravidian-speaking people that can be described as dolichocephalic (longheaded from front to back) Mediterraneans mixed with brachycephalic (short-headed from front to back) Armenoids and established themselves in northwestern India during the 4th millennium BC. Along their route, these immigrants may have possibly come into an intimate, prolonged contact with the Ural-Altaic speakers, thus explaining the striking affinities between the Dravidian and Ural-Altaic language groups. Between 2000 and 1500 BC, there was a fairly constant movement of Dravidian speakers from the northwest to the southeast of India, and about 1500 BC three distinct dialect groups probably existed: Proto-North Dravidian, Proto-Central Dravidian, and Proto-South Dravidian. The beginnings of the splits in the parent speech, however, are obviously earlier. It is possible that Proto-Brahui was the first language to split off from Proto-Dravidian, probably during the immigration movement into India sometime in the 4th millennium BC, and that the next subgroup to split off was Proto-Kurukh-Malto, sometime in the 3rd millennium BC
Source:Britannica Article on Dravidian
Edit:
Dravidian hypothesis
The Russian scholar 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.
The Finnish scholar Asko Parpola led a Finnish team in the 1960s-80s that vied with Knorozov’s Soviet team in investigating the script 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.
Dravidian shares strong areal features with the Indo-Aryan languages. Dravidian languages show extensive lexical (vocabulary) borrowing, but only a few traits of structural (either phonological or grammatical) borrowing, from Indo-Aryan, whereas Indo-Aryan shows more structural features than lexical borrowings from the Dravidian languages. The Dravidian impact on the syntax of Indo-Aryan languages is considered far greater than the Indo-Aryan impact on Dravidian grammar. Some linguists explain this asymmetry by arguing that Middle Indo-Aryan languages were built on a Dravidian substratum
Source: Wikipedia