[sddsdd]

What is unique about Nicaraguan sign is that the entire process of evolving from a loose invented communication system with lots of pantomime and little regular linguistic structure into something with grammar and visual "phonemes" and a fixed core vocabulary was observed in real time over several generations of speakers.

As successive group of users were introduced to it as new native speakers they evolved it adding more and more structure, it doesn't start out fully language like at first. I can't think of any other case where that process has been documented in action.