About so called caste formation in Tamil Nadu and Kerala. It is not a separate process, most sociologists and linguist call the shared cultural region as Tamilakam. In Wiki we have to rewrite the article to reflect this reality. If you read the Kerala and Malayalam entries, and www.kerlahistory.com site this new idea is brought out. Hence Kerala and Tamil Nadu are hitched at the hips up-until the 10 century regarding their cultural development. 

Caste itself is not a proper word to describe the social condition of South Asia. What is more accurate is Jati and Varna.  According to linguists like Proff Hart of Berkeley and others, indigenous societies specifically in South India there already were societal cleavage based on certain lineages that were slowly cornering certain economic productive mechanisms such as cultivation, fishing, security, drumming, bards etc. This was in response to a move away from hunter gatherers to slash and burn to settled agriculture and cattle herding. 

The Sangam literature which is a kaleidoscope of the social conditions prevalent in ancient Cera, Chola and Pandya countries describes a situation where there was a lot of flux and violence used to maintain the economic advantages. In this milieu the northern ideas and people were also added their system such as the Varna. But in reality what the Brahmins  did was to become just another Jati to fit the local model already prevailing and added religious sanction on top of the violence that is needed to maintain the Jati system . Thus  Brahminism, Jainism and Buddhism were  very important in pacifying the populace that lead to the development of later great empires. 

The word Pulaiti and Pulayan are used in Sangam literature to describe people serving chiefs. They were attached to the wealthy because apparently they were dispossessed or had nothing to independably live on. The name of the Kannadiga caste is Holeya. Where Hole in archaic Kannada means menstrual blood thus indicating that even before Tamil and Kannada became independent certain lineages may have become debased. As you know that’s a long time ago:)) 

Why is ancient Tamilakam important as opposed to rest of India to understand the unique caste system. Because it was the last frontier to succumb to Indo Aryan population and cultural advance into South Asia. Further it maintained its cultural independence just enough to record (via the Sangam literature) the prevailing conditions before culturally becoming part of rest of India after the Bhakthi period. Today so called separate Keralite and Tamil cultures are just another appendages of a greater Indian culture with minor regional variations.