How can this question possibly be answered? Seeing as the invention would have occurred in pre-history (clothing, a building material, a weapon, a tool, etc.), it seems to me the answer is by definition not answerable by rigid historical methods.

More than that, I could easily imagine the first intentional invention being the result of tweaking an unintentional one. e.g. after the proto-human discovers that a rock makes a good hammer he sets out to find a better rock that would make a better hammer. Is this an "intentional invention"?

I feel like this question is very "gray" and fully unanswerable.