I agree to a certain point, but writing (invented in the past few thousand years) has not led to more complex languages. I've worked on languages for which the

I agree to a certain point, but writing (invented in the past few thousand years) has not led to more complex languages. I've worked on languages for which there was, until a few decades ago, no writing system, and they're every bit as complex as long-written languages. The qualitative differences between human languages and any animal "language" (scare quotes intentional) include recursion and breadth of vocabulary, and likely a lot of other things to. Recursion and the ability to learn a large vocabulary are probably biological and unique to us, the other things might or might not be unique to us