What the Additions Are Evidence For

Three added letters and a different treatment of the inherent vowel amount to a claim about how writing spread through the subcontinent. A script transmitted as a finished object, copied and applied, would show the same inventory everywhere and would leave the languages of the far south either badly served or unwritten. What the Tamil-Brahmi material shows instead is a script being taken up by people who understood the principle behind it well enough to extend it, adding signs where their language needed them and altering the fundamental vowel convention where the inherited one did not suit.

That is a different kind of borrowing from the copying of shapes, and it is the reason Tamil-Brahmi is treated as a field with its own conventions rather than as a regional variant of the northern script. Editions of the southern material use their own transliteration practice, and the standard handbooks treat the two bodies of evidence separately.

It also bears on the older argument about where Brahmi itself came from. A script that its earliest users were confident enough to restructure for a different language family is not obviously a script that had just been received passively from outside, though the inference is suggestive rather than decisive and epigraphists continue to argue the origin question on other grounds.