Tamil's immediate predecessor was the
Grantha script, sharing ancestry with
Malayalam and bearing a similar appearence. One significant development of Tamil that differentiates it from other
Brahmi derived scripts is its abandonment of consonant cluster letters. In most other scripts, famously in
Devanagari, the individual signs for two consonants are combined in a new,
amalgam character for the consonant cluster, taking on its own distinctive appearence and preserving the
syllabary qualities of the
abugida. Tamil is still an
abugida, but it has taken to writing consonant clusters out with each individual consonant letter, showing a movement more towards an
alphabet.
Another notable development of the Tamil script is its use of certain ancient markers to create new sounds for borrowed foreign words. The aytam symbol can be attatched to a 'p' or 'j' to transform it into a frictive not present in the native Tamil orthography, allowing borrowed English words like ficu for fees and ziroks for Xerox. The same process can not be used to represent English vowels, however, which are instead approximated by Tamil vowels.
Information sourced from Daniels, Peter T. Bright, William. The World's Writing Systems. Oxford: Oxford University Press, 1996.