I have been up all night thinking about how languages started especially in written form and I have been confusing myself and can't sleep. I've especially been thinking about the Gaelic language since I'm from Scotland and wondering how all the clans spoke the same language since I imagined they were mostly isolated from one another. Anyway that's another question for another day, unless you know the answer.
This post might answer your question:
/u/mr_tenugui and /u/farquier both provide insight.