Extensive research has been done on the subject of sounds, and studying the remainders of the bodies can tell us a lot about the sounds these "people" were able to produce. Based on that, of course, theories can be made up concerning the extend of communication.
Also, many studies have been done on the development of languages. Something that helped greatly was the research on the development of languages was the study of pidgin languages (esp. the ones who turned into creoles): [
home.bluemarble.net]
lots of material on the subject of languages and their origins can be found here:
[
www.archaeolink.com]