Some minor thoughts to attach by little pieces of string to Jay's scaffold:
I continue to be bothered in these discussions by the assumption that
chimps and other primates represent some kind of earlier development - are
they not contemporaries on a different evolutionary branch - which means we
need to look at what their ancestors could do/did if we want clues as to
our own phylogenetic origins
Jay's ontogenetic thoughts bring to mind the 'conversations' that parents
have with new-born babies, in which they carry on a conversation as though
it were two sided, when the baby's only contribution, if any, is to look
attentive. By 2-3 months the baby may contribute slight noises. Studies of
these proto-conversations show that parents use more adult-like words in
these conversations when the child is new-born, and as the child gets
older, the parents contributions get more like the child's, i.e. the parent
uses more baby-like noises in the conversation.
Which brings to mind Eleanor Och's study of early language development in
village life in Western Samoa. Her data makes it clear that the patterns of
adult-child interactions that are familiar in Western cultures are not
present among Samoan families. But I don't recall if she recorded any of
these proto-conversations between Samoan parents and babies.
Cultural differences aside, maybe the ontogenetic context is one of voice
sounds and patterns before gestures, unless eye-contact, smiling, among
very young babies is semiotic?
It's a fascinating area for seeking, puzzling about, origins.