Actually, aozilla, something vaguely similar to that
has happened, amongst
deaf children in
Nicaragua. Until the late
eighties/
early nineties, there were no
schools for the deaf, no way for deaf
children to meet up with or
communicate with each other - so they never learned
language.
About ten years ago, though, a school for deaf children was set up - and the children who went there started to make up their own (sign)
language. It evolved into a real, fully functioning language in something like
ten years. Of course, it could be argued that these children already had some idea of what language was and how it worked, and didn't have to start from scratch; still, it's enough to make me think that a real language could
evolve in no more than one or two
generations, rather than thousands of years.