Vatteluttu "na"
In southern India, the Tamil people first used the Tamil-Brahmi script, and then over time, developed the Vatteluttu (also called Vattezhuttu) script. From what I can tell, this was an evolutionary change and not a sudden invention.
Vatteluttu was used from about 500 AD to about 1500 AD, mostly used to write Tamil and sometimes Malayalam. Tamil-Brahmi was also widely used to write Tamil and sometimes Malayalam. This multiplicity of scripts was able to happen because there was not a centralized government to impose a practice — either by edict or by custom — of using one or the other script.
Eventually, two different scripts displaced both Vatteluttu and Tamil-Brahmi, one for Tamil and one for Malayalam.
There were two regional variants of Vatteluttu: Kolezhuthu and Malayanma. There was almost no difference between the scripts, except that Kolezhuthu script didn’t allow endings in a, e, or u.
Links: Wikipedia, Ancient Scripts, C. Radhakrishnan
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