Brahmins in South India:
I had few thoughts about migration of Brahmin communities to South India. This was rekindled by my discussion with Srikanth of Srican regarding development of caste system in South India.
The popular notion is that Brahmins were imported from North-West of India by South Indian kings to perform religious duties. This could be true for some Brahmin families. But in my opinion there were Brahmins in South India before that period.
South India's religious history:
It's tough to demarcate religious character of South Indian society over the period. The common population most probably worshipped phallus, goddesses, folk heroes and nature(trees, snakes etc...) without any Vaidik touch. However, common people were irrelevent(no democracy!) during those days. Therefore, we have to consider only the religion of the rulers and the elite classes.
The ruling and elite classes of South Indian society was either Buddhist or Jain or non-Vaidik Shaivite. Of course, Brahmins were there, but just as another religious group. The ascendency of Brahmins was started mostly around 6-7th century CE and was complete over all of South India by 12th century CE. As it's obvious, this change in the religious character was brought about by the transition of South Indian kings and elite classes from Buddhism, Jainism and non-Vaidik Shaivism to Brahmanical Hinduism or the caste system. However, the general population was affected by this transition mainly because of the common base of both Brahmin religion and local religions as I have discussed before.
1. I suppose the recorded history of Brahmins' migration to South India as priests of kings was around 5-6th century CE.
2. It seems Vaidik gods like Indra, Varuna, Semitic god Brahma were mentioned in old Tamil literature( 2nd century BCE -2nd century CE) along with non-Vaidik gods. Since we know Brahmin caste first created in North-West India before 5th century CE(Buddha's period), some of this caste people might have already migrated to various parts of India by that time(As far as I know, they were prohibited only from crossing the sea).
3. It should be noted here that Brahmins in South India used to claim they worship higher forms of Shiva and Devi(goddess) and the general population(shudras) worship the lower forms. No, these two groups didn't borrow anything from each other, just that Brahmins had literate touch to their religion(okay, that means lot). However, there were contradictions to this claim if you consider India in general. I suppose Kali is lower form and Vishnu is higher form in South India, whereas, in Bengal, Kali is traditionally worshipped by the upper castes and the lower castes are generally Vaishnavites(Note:Vishnu is just an abstraction of Dravidian folk heroes Rama and Krishna).