It's also very possible that human language started as a purely internal thinking mechanism, and only later became adopted for communicating with other humans.
This seems very counter-intuitive initially, but makes more sense when you think about how much of your use of language is addressed to other people, and how much is used simply for thinking to yourself.
I'm multi-lingual and very few of my thoughts are in any "language" unless I'm intending to communicate them with others. When I only spoke English, I don't think my thoughts were in English either, but it was harder to be aware of that fact until I could ask myself which language I was thinking in. The same is true of my dreams. I don't dream in any particular language, other than an occasional word here or there.
Interesting! Myself, I have a native language and I'm also fluent in English, and basically all of my thoughts are expressed conversationally (I also have conversations in most of my dreams, though of course they have many other components).
I'm curious - if you don't think in language, do you have any way of explaining how you think about abstract concepts?
This seems very counter-intuitive initially, but makes more sense when you think about how much of your use of language is addressed to other people, and how much is used simply for thinking to yourself.