Comment (by Thiyagu):

The archaeological excavations at harrapan site have already disproven any vedic links to it and there is complete lack of evidence of any vedic rituals happening there. The vedic people didn't have the gods that we have today. They imposed there vedic rituals in the Ancestral gods of the indigenous population. The Sanskrit (categorized as Indo-European) came in with the Aryans and it apparently became of the rituals, because they are the ones who introduced the rituals. I am not saying Tamil is older/greater than any language. But it is quite evident the history of Tamil predates that of Sanskrit in India. The antiquity of any language can only be measured by the availability of the oldest script available and Sanskrit has none of it. The most recent dna analysis done at the Indus site has confirmed that the there is no presence of the vedic people (steppe pastoralists). However, the uppercastes brahmins In North india have the r1a Gene which links them to the steppe pastoralists. The Gene found in the remains is found more relatable to Ancestral South Indian. What this means is that, the Aryans came in to India when the indus valley civilization was already in decline.    (Ref.:  Thiyagu's  comment, "Sanskrit Is Older Than Tamil.." Oct. 15, 2019, http://creative.sulekha.com/sanskrit-is-older-than-tamil_107509_blog?c=2468471#2468471)

Response (by Subhash Sharma):

r1a gene, in the above, could easily be a long-term cold weather genetic effect / influence on people living currently in Steppes and north India, and nothing more.

Moreover, the same  gene (r1a) not noticed in south Indians currently might be the result of hot weather (not conducive to r1a gene) in south India throughout, whereas the absence of same gene in Indus valley samples from long ago might be due to the fossilized remains not being perfect (having even dissipated / depleted the r1a genetic material in the samples over time).

As for the religious rituals among Indus valley people long ago, there were plenty according to the archaeological studies (including ancient fire pits etc.). Here is evidence of it (rituals) below from one of my blogs,

'(f)  Harappan evidence for agriculture. The archaeological findings from the ancient Harappan (Indus Valley) Civilization also put a great emphasis on agriculture and farming. Ref. (11), related to ‘Cultural History of India’ by Om Prakash (on page 89 of the book) states, “A sealing from Harappa shows on the obverse a nude female figure, turned upside down with outspread legs and a plant issuing from the womb. On the reverse of this sealing is depicted a man with a sickle in his hand and a woman seated on the ground with hand raised in the posture of prayer.”

'The above ancient Harappan seal also appears to depict and celebrate the importance and reverence for farming and agriculture long ago.  A plant sprouting from the womb of an upside-down woman in the seal is probably a reference to the mother earth, symbolized as a woman and bearing the life-sustaining plants and crops. The reverse side of the seal seems to show a family (man and woman) offering a prayer at harvest time. The woman is sitting on the ground and has her hand lifted in prayer while the man (perhaps her husband) stands nearby holding a sickle in his hand. He appears to be ready to harvest the crop. Note, these types of pre-harvest rituals (prayers and worships) by farmers and their families (engaged in farming) are common even today in many places and cultures. This seal presents the archaeological evidence that the custom of praying by humans (farmers etc.) before harvest, especially for a good harvest, is very ancient.' (from  "Farming and philosophy in India during ancient times" by Subhash Sharma, 2011, http://creative.sulekha.com/farming-and-philosophy-in-india-during-ancient-times_527016_blog)

Moreover, seeing something scribbled (etched) on stones / rocks long ago which might have some similarity in shapes to a few letters in scripts of several languages these days (including Tamil) cannot be considered as belonging to one language only (Tamil alone).

In any case, if there are not many symbols on these ancient rocks that look similar to the letters in the current Devanagari script for Sanskrit, that does not mean the ancient version of Devanagari script for Sanskrit (with symbols similar to that on the ancient rocks / stones) did not exist long ago (when the symbols were etched on stones) or that Sanskrit was not there as a language long ago (at the time of symbol drawing on stones); perhaps Sanskrit has been there as a language for a very long time, using the older version of Devanagari script (similar to the symbols on ancient stones) previously and with a modified / updated version of Devanagari script later (as currently).