Here is a paper 'On The Origin Of The Early Indian Scripts: [1] A Review Article
by Richard Salomon of University of Washington, if you have not seen before.

"The development and early history of writing in India of the historical period (i.e. after the time of the Indus Valley Civilization) has long been a controversial problem. Basically, arguments turn around three main issues:

The sources and origins of the Indian scripts of the historical period, i.e. Kharo.s.thî and especially Brâhmî.
The date at which these scripts, or their prototypes, first came into use.
The relationship, if any, of the historical scripts to the writing of the proto-historic Indus Valley Civilization and the explanation of the long gap between them during which writing appears to have fallen out of use in India.
The principal reasons that these issues, particularly the second, are so problematic are:

There are no securely datable specimens of writing from the historical period earlier than the rock inscriptions of Ashoka from the mid-3rd century BC. Other early inscriptions which have been proposed by various authors as examples of pre-Ashokan writing are of uncertain date at best.
The external testimony from literary and other sources on the use of writing in pre-Ashokan India is vague and inconclusive. Alleged evidence of pre-Mauryan writing has in the past been found by various scholars in such sources as later Vedic literature, the Pali canon, the early Sanskrit grammatical treatises of Pâ.nini's and his successors, and the works of European classical historians. But all of these references are subject in varying degrees to chronological or interpretive problems."

For more: http://www.ucl.ac.uk/~ucgadkw/position/salomon.html

Th earliest Tamil-Brahmi writing are supposed to be The two rock-inscriptions of Netunceliyan at Mangulam. 3rd -1st Cent. B.C. Asoka's Brahmi introduced around ca. 250 B.C. into the Tamil country. Adapted between 250-220 to Tamil. From: http://www.geocities.com/Athens/5180/kamil.html

Here is one possible tree on the evolution of Indian scripts: http://www.angelfire.com/co/malaiya/images/brah11.gif

Further, the present Tamil script was revived and reformed by Veeramaamunivar (Robert Beshchi) in the 17th century. It went through a few iterations, including the Periyar corrections. For instance we used to write 'lai', 'nai' and such words with a kombu in front, before the reforms to use the same prefix character that we use for say 'kai'. This helped in standardization, but increased the length of words. Shortening the length of words was a concern in the days when writing was on olai and hence the need to shorten characters with the use of 'kombu' and such superscripts and subscripts. This is no longer the case. This feature is very common in the Devanagari script used for Hindi.

Malayalam, evolved from the old Tamizh vatta ezhutthu, that was used even for the Grantham script that had extra characters to write both tamil and Samaskitam.