That really depends on your definition of Folk music. I've just read a really interesting book about the origins of 'Folk music' which confirmed a feeling I'd had that the term is really a construct of our modern musical tastes and philosophical ideals.

If you think 'Folk music' is music that is transmitted orally and socially, and practised by the common people in their everyday lives and in social gatheringsthen the first folk singers were probably the Neandertals as explained above. On the other hand if you think it is defined by the body of music commonly held as 'traditional' by folk club practioners today then it is a construct of the enlightenment and the first folk singers were those collecting, and preparing for performance, popular songs of the day.

I blame Rouseau myself.

(The book was, by the way, The Invention of Folk Music by Mathew Gelbert; a thoroughly good book, if cluttered a little by post-modernism. I would recommend getting it from the library however as, like all good academic text books, it is rather expensive.)