doppelganger
Through the Looking Glass
That said, I would certainly say that the greatest of these developments would have to be language. I am not saying that animals do not have language, but simply that ours has developed in such a way to allow the easy transport and exchange of ideas, upon which no other invention would have ever been possible.
That's why I was more specific about it, because non-human animals do have language. The peculiar ability to interact with one's environment through abstract worlds of thought is the big development. And the cornerstone of that is dualistic language structures - starting with 'yes'/'no', but really taking off in terms of its usefulness with the first acquisition of common nouns.