Are we looking for something that is only used by human species? Because, for example, tools are used by a variety of animals to varying extent. So... a leverage is most likely not an actual man-made invention, though it
could be, but more likely the use of tools (of which most are based on leverage OR a inclined surface, though the most simple is of course a rock with which to beat things open) was passed on to human species from their ancestors.
Communication would also be quite an impressive feat, but alas, that also isn't limited to humans and was most likely herited from pre-human primates - with Homo Sapiens Sapiens it just evolved onto current level, through natural selection of course. Better means of communication, better means of survival. I wouldn't, however, give the mankind the whole credit of "inventing" speech as we know it...
Well, of course we come then to the idea of making notes, which is actually a sample of binding complex concepts onto abstract images. Which is of course the same thing as what happens with representing art. The cave paintings are probably the first examples (preserved to these days) of making notes of How To Make the Hunt Go Well and such things. These probably described the important events in the tribe's history as well, serving as the first "chronicles" - though they probably also had some spiritual meaning and more likely than not only served as back-up for oral folklore transferred from grandfathers to fathers and so on. Most likely after the concept of presenting abstract things in images was widely used by drawing maps on birch bark or similar substances, and VERY likely large-scale hunts were planned using a "terrain box", a small area of ground made to resemble the surroundings and so on.
So, inevitably these images were soon caricaturized and they evolved into marks for words rather than being accurate images of things themselves - a system still widely used in many East-Asian languages. After this, it wasn't of course a long way onto finding a way to write things down accurately to transfer and store information effectively and securely.
Coupled with the discovery of agriculture these things made cultural evolution faster than any other species had ever* demonstrated on Earth, for good or for worse.
So I'd have to go with
art and/or the skill of writing, which almost certainly derived from art. Art probably came first as an idea of describing things with something else than pointing at it or saying the word for it. And, from an artistic presentation it's a small way to a cartoon, from which it's a small way to including the thing onto single picture (word mark for subject) and the thing it's doing onto another picture (word mark for predicate) and voilá, we got the first system of written language, though it's surely very simple. But why do you thing cave paintings are stylized? I don't think for a moment that they
couldn't have made the animals realistic if they had wanted. No, they made them caricaturized because they had a reason to do it that way, and most likely it was that it was easier and had the exact same information as a photorealistic 1:1 fresco on the wall of a cave.
