Vaddakkan Pattu: Collection of ballads of medieval origin. These present the saga of heroes like Aromal Chevakar, Tacholi Otenan, Tachola Chandu, Palattu Koman and heroins like Unniarcha, Mattu, etc. The oldest compositions do not date earlier than 16th but their idiom and vocabulary seem older. The Malayalam used is devoid of Tamil or Sanskrit, thus is probably close to the spoken idiom.*
In my opinion, Proto-Kannada that reached the region of Kerala and Tamil Nadu branched into Proto-Malayalam and Proto-Tamil(need not be Proto-Tamil-Malayalam). However, Tamil and Sanskrit exerted superstrate influence on Malayalam because of Tamil political rule and religion respectively.
A Dictionary of Indian Literature By Sujit Mukherjee (Google Books, limited preview).