I think it was the natural progression from the guttural grunts that animals made. As humans evolved, and the brain became larger, the noises turned into recognized sounds, which would then turn into a language over time.
But, the study of how languages develop is utterly fascinating. I recommend a book called "Mother Tongue", which charts out the progression of
modern languages especially English and European, from ancient ones. I'm afraid I don't oknow the author's name. I borrowed the book some time
[Edited on 3-5-2004 by Zzub]