Probably, now nobody believes that tribals of South India were the original speakers of Dravidian languages. In fact, Haplogroups H and R2 are too old to be associated with Dravidian languages. I wonder what would be the language spoken by these tribals prior to their Dravidisation. I believe it should be Austro-Asiatic. However, I am not sure if Austro-Asiatic substratum could be observed among Dravidian languages. Or could it be Burushaski languages? I wonder if it is possible to detect the substratum of dead linguistic families. But before that more on movement of South Dravidian languages.
It is very evident that Proto-Kolami Parji all other tribal languages in fact branched main Proto languages for Telugu, Kannada, Tamil and Tulu. In my opinion Proto-Tamil-Kannada should be the sub-branch of Proto-Tulu-Kannada.
The language could become independent in a region owing to;
1. Sound changes due to migrations to different geographic location.
2. Adoption by non-native speakers.
-> There are two cases in this. In the first case, the natives would adopt an alien language. In the second case, migrants would adopt the local language.
Let's see if the above language tree supports the migration theory along river Godavari, then coastal Karnataka and then along river Kaveri.
South Dravidian -II(Central):
If you observed Proto-Telugu-Kui branches before Proto-Kui-Gondi. But Gondis are found in Eastern Maharashtra. SD-II might have branched from SD-1 before that region. I mean close to coastal Maharashtra. This Proto-Telugu then moved to Northern Andhra and then spread to South. If you observe except Telugu all other languages in this family spoken by only tribals. But it should be noted that Proto-Telugu does not exist as a sub-family of any of those tribal languages. In my opinion, if the Central-South Indian tribals spoke Dravidian languages first then linguistic construction must be able to trace non-tribal language to tribal languages. On the contrary here we find Proto-Gondi-Kui traced to Proto-Telugu-Kui.
Unlike SD-II, SD-I speakers didn't come across many tribals in South India. Probably, when the migration took place South India was sparsely populated. This family has four mainstream languages and few tribal languages. As I discussed above the split from SD-II should have happened somewhere in coastal Maharashtra(until this time SD-I and SD-II called number eight as enuma*).
This group must have moved along Krishna basin and coastal region. The group moved along Krishna basin became Kannadigas. The group moved along coastal region became Tuluvas.
This is the sub-group of Proto-Tulu-Kannada(the split from Proto-Tulu-Kannada could be seen in the word for number eight where it is now called entu*). This group moved along the river Tungabhadra reached the region of Kaveri river in Kodagu.
From the region of Kaveri in South Karnataka, it's the movement of Proto-Tamil people. As they moved most of tribes on trail adopted Dravidian languages which later became independent. The stretch from Kodagu to Nilgiri hills nicely shows origins of Kodava, Toda, Kota along the way. Irula was the last tribe to adopt Dravidian language. In the case, of Malayalees it's the change from one Dravidian language (Proto-Tulu) to another Dravidian language. However, Proto-Tulu was adopted by tribals like Koraga in Northern Malabar districts.
Later Kannada movements:
Kannada could have been mostly spoken in Northern Karnataka and Southern Maharashtra* initially. But later it made inroads into Southern Proto-Tamil regions along Kaveri river but lost its identity in Southern Maharashtra.
It looks like Kodavas were present in the region before Proto-Tamil speakers migrated there. This is bit difficult believe as Kodavas are supposedly later migrants to that region. If we can show that language became distinct after some of the Dravidian languages in the region next to it then we can say Kodavas are the later migrants. But this tree does not show that. However, one at Encyclopedia Britannica gives a different picture. According to it, Proto-Tamil-Kodagu branches after Proto-Tamil-Toda. This might show later non-native speakers migrating to that place adopting Proto-Tamil.
*In my opinion,archaic forms of a language could be found in a population that is;
- migrant to a different linguistic place
- uninfluenced/isolated native speakers
The Tamil Brahmins in Karnataka who migrated 800 years back from the region of Tamil Nadu supposedly exhibit archaic Tamil forms in their speech.
The Havyaka community in coastal region show archaic Kannada words in its speach. The region they inhabit is originally a Tulu speaking(Uttara Kannada) region. The region where archaic Kannada was spoken could be mainly in Northern Karnataka or Southern Maharashtra or could even be North-Western Andhra Pradesh(Krishna basin).
I felt Kurukh(belonging to North-Dravidian linguistic family) sounded like Havyaka Kannada. The Kurukh people even have a folklore that they migrated from Northern Karnataka. In all probability, Kannada took its form in Northern regions, then influenced southern regions whose proto-language could be close to Proto-Tamil.
Similarly, Telugu took its form in Godavari basin and later influenced Western Krishna basin where the population could have been speaking a language close to Proto-Kannada.
Whereas, Tulu regions were encroached by both Kannada in the North and Tamil in the South in the later period.