Retroflex Approximant zh:
The retroflex approximant zha is as of now still pronounced by majority of Malayalis and few Tamils (Bhadriraju Krishnamurti)*. It is a rather intriguing sound found in old literature of SD languages. Some of my observations.
1. Present day written and spoken Telugu employs retroflex Da, Kannada employs retroflex La and Tulu employs alveolar(trill or tap ... no idea) ra
2. But many Nilgiri dialects and Malayalam spoken in Kasaragod and many Tamils employ palatal ya.
3. Generally, Telugus use Da, where you find SD-I speakers use La or Ra.
kOLi(Ka)kOri(Tulu)kODi(Te)kOyi (Kasaragod Malayalam)kOzhi(Standard Malayalam) (=chicken)
4. Present day Malayalis turn 'r' sound in foreign words into 'zh' sound.
eg. 1. Parsee is written/pronounced as Pazhsee.
2. University is written as Univezhsity.
Then what could have been the original sound? I used to think, because of my Kannada background, that is basically an exotic La because 'La' is used in Kannada where Tamil/Malayalam use 'zha'. However, I was told by a linguist that *zh sound must be constructed in SD/PD as a rhotic. Of course, this was before I carefully listened to Malayali pronunciation of 'zha'. Malayali pronunciation is exotic 'r'. And according to Wikipedia this is equivalent to present American English retroflex approximant 'r'(in some places). But in my opinion, Malayali pronunciation is retroflex palatal approximant(or exotic 'rya').
It appears AE retroflex r is an allophone of alveolar approximant r (Scottish English). Last night I was listening to this "500 miles" song(which kindled this entry). I thought "Lord" was pronounced like "Loyd" with palatal approximant y.
Listen to Petey, Paul and Mayi singing with the palatal approximant.
My construction of PD sound:
The original PD sound could have been alveolar approximant r. This we can observe in Tulu where alveolar trill or tap r is used. This alveolar approximant r gave rise to retroflex approximant r(zh) (in my opinion, retroflex palatal approximant r) which in turn gave rise to palatal approximant y. Therefore, all three are allophones.
alveolar approximant r -> alveolar trill/tap r (Tulu)
alveolar approximant r/alveolar trill/tap r -> retroflex approximant r/retroflex palatal approximant r -> palatal approximant y (Malayalam,Tamil, few other SD-I tribal languages)
During SD-I and SD-II split, alveolar approximant r, might have given rise to an allophone alveolar plosive d. Which became retroflex plosive in SD-II languages. But I think this should have happened with a big chunk of SD-I speakers too.
alveolar approximant r -> alveolar plosive d -> retroflex plosive D
The influence of geography changed retroflex plosive D into retroflex plosive L in Proto-Kannada.
retroflex plosive D -> retroflex lateral L
Migration to Tamil region:
Based on the above arguments, I would say the youngest Dravidian region, Tamil Nadu, had two defining migration patterns. The Proto-Tamils were mix of Proto-Malayalis (retained in allophones zh, y) and Proto-Kannadigas (retained in L). Standardization of written script probably happened in a region where Proto-Malayali migration was dominant. This script was probably became model for Proto-Kannada script too.
* There is a hilarious story about Malayali zh and Tamil zh. A southern Malayali chieftain was troubled by an unending Tamil migration to his region. He was scared that Malayalis would become minorities in that land. However, it was tough to block the migration as there was a lot of to-and-fro movements from Malayalis too. At the entry point of Tamil/Malayalam regions he created posts and put a 'chicken' test to incoming people. Every person who wants to enter Malayalam region had to pronounce kOzhi. According to the story, Tamils would pronounce it as kOLi (like Kannadigas) and would be turned back.
My wife(the zha Malayali) who doesn't think 'lord' is 'loyd' in that song.