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.