> I think they would have to evolve together to some extent. As the language
> becomes more complex there would have to be ways of controlling that
> complexity in interest of understandibility.
I’m sorry about this, but basically everything about this is wrong. There’s no reason to think that the complexity of language has changed in the past 200,000 years. There are no “primitive” languages. All languages have approximately the same complexity, although that complexity is expressed differently in different languages. It’s possible that earlier in the evolution of the hominids (several million years ago) there was something different than language (using sounds, gestures, facial expressions, etc.) which was simpler than present-day language because the users of it were less intelligent, but there’s no way to know for sure.
There’s no sense in which a written-down grammar controls the complexity of language. Written-down grammars didn’t exist before Panini, so there were tens or hundreds of thousands of years of human language before anyone attempted to write down grammar. Indeed, there’s no need to control the complexity of language. Language is not unstable and thus not in danger of becoming too complex. There are natural parts of the evolution of spoken language which can both make the grammar more and less complex, and this evolution doesn’t go too far in one direction or the other.