> Nicaraguan Sign Language (ISN; Spanish: Idioma de Señas de Nicaragua) is a sign language largely spontaneously developed by deaf children in a number of schools in western Nicaragua in the 1970s and 1980s.
Similar story, in some ways: In Nicaragua, the kids were taught a wholly deficient sign language that wasn't sufficient to the needs of actually using it as your mother tongue. They improved upon it by inventing grammar (not just words, but actual rules) and generally bringing it up to the standards required of a real language. Same kind of independent discovery and invention.
In a linguistic sense, there appears to be a minimum complexity a language must have before it's worthwhile, and humans will spontaneously invent complexity that is missing. This also completely debunks the idea that grammar is dying: Humans can't exist without grammar. They can, however, exist without your preferred grammatical and stylistic conventions. (Grammar is observed, as part of a science; style is dictated, as part of fashion.)
In a broader sense, humans desire a certain minimum level of complexity in their lives and will get bored and invent complexity if it is lacking. We aren't cattle.