I have discussed about religious practices of South Indians before brahmanical Hinduism. Mostly, elites were Buddhist, Jain or non-Vaidik Shaivites. The common people might be worshipping all kinds of spirits. Among elite religions Shaivism was the religon of common people too.
We have already seen that Kashmiri Pundits and Konkani Saraswat brahmins were once considered pure Shaivites. Also, many South Indian brahmins were considered as Shaivites. I have already argued that most of these bramin castes were Dravidian priestly class basically and merged with Indo-Aryan and Semitic priestly class and established caste system. Sheer dominance of Dravidian religious deities in Hinduism is the direct result of this Shaivite Dravidian priestly class.
South Indian Shaivite priestly class:
The caste system was created in North-West of India, the meeting and melting point of Dravidians, Indo-Aryans and Semites. Though North India had a stable society with political entity before South India, the known South Indian kingdom of Pandyas was as back as 6-5th century BCE. That is almost a millennium before recorded Brahmin migration to South India. But it won't be surprising if there were brahmins in 6-5th century BCE.
So what happened to South Indian priestly class? Did they merge with brahmin priestly class as north Indians? Most plausibly many of them did. However, many continued to be Shaivites. That is non-Vaidik Shaivites.
Shaivites in Karnataka and Tamil Nadu:
There is a big group of Non-Vaidik Shaivites known as "VeeraShaivas" or "Lingayats" in Karnataka. This religious movement was started by Basavanna in 12th century CE, who was by birth a brahmin. The curious point about him is that he renounced brahmanical Hinduism denouncing all its practices including caste system and started a movement with total devotion to Shiva. This Veerashaivism converted many brahmanical Hindus to its fold. However, there still exists a section of Shaivites in Karnataka who consider themselves purer than these Veerashaivas since they were the oldest and natural Shaivites and not converted Shaivites. I have heard there is a section of Shaivites in Tamil Nadu also which considers itself as Non-Vaidik Shaivite. Most probably, these Shaivites(in Karnataka, they call themselves, before Basavanna) were dominant in South India before the rulers turned their back to non-Vaidik Shaivism. Similar is the case with Jains and Buddhists. But one hardly finds any Buddhist in South India indicating that they were not as numerous as Jains.
All these facts again prove that, a big chunk of Indo-European speaking people, from brahmins to non-brahmins and all Dravidian speaking people were assimilated ethnically and culturally after their migration to the subcontinent. All the so-called European admixture is due to the Western Eurasian females who form 10% of our population; since we have already seen that male Indo-Aryan genetic marker was already present in India. Anyway, admixture analysis using autosomes is still controversial.