One my first papers as an undergrad actually dealt with this. It has been theorized that language first evolved as series of iconic hand gestures accompanied by gutteral vocalizations. These sounds later developed into the spoken language we have today. The theory would explain why both spoken and sign languages follow the same syntatical rules, as well as why deaf babies "babble" with their hands.
In my Master's program, I did a literature survey on neo-Darwinist approaches to the evolution of communication. If I have time, I'll see if I can dig up the reference list.
Thank you for your informed reply.
And, by the way, although I fiercely defend the religionists against the left-wing secularists, I am your screen name.