Those interested in the study and emergence of languages should do some reading about a remarkable series of occurrences in Nicaragua during the 1970s. Students at a number of schools for the deaf there, initially staffed by teachers who did not know sign language, invented their own version, which grew in complexity over a period of years.
Ann Senghas, of Columbia University, has studied the signing capabilities of people who left the school at differing times and therefore different stages of the evolution of this language. Users of the early versions of the language, for instance, could not describe whether something was on the left or right side of a photograph; users of later versions could do so.
Perhaps the most interesting questions raised by this situation relate to the nature of human cognition where it comes to language. For instance, it makes one wonder about the degree to which people are instinctually provided with mechanisms for both the comprehension and development of language.
More information is in this Wikipedia entry.
I remember reading a bit of Noam Chomsky on innate language acquisition when I was an undergraduate. It was quite interesting, but I don’t have time to follow up everything that’s quite interesting…
You may be interested in looking up Al-Sayyid Bedouin Sign Language. Only 150 people speak it, in Israel. Unlike most sign-language, it is used between individuals who are able to hear, as well as among the deaf.
A fictional sign language employed in similar fashion is ‘jive’ from a number of William Gibson’s books, notably: “Neuromancer.”