10,000 yrs ago some prehistoric savants were drawing on the sides of caves. Before that they might have been drawing in the mud or on the side of rocks, who knows? When picasso first saw these rediscovered drawings he was aghast - "we haven't learned anything!" he said. What he meant, all the techniques he thought so novel had already been put into practice, by some caveman no less - before painting got sophisticated. If Og and Ug had managed to invent oils and canvas they might even have got round to knocking out some rembrandts/
So why did Ug and Og start drawing. Perhaps they thought it contained magic (or that was what they told the rest of the tribe). No doubt, deep down, they started drawing for the same reason we do it today. Because when you draw you learn to look, and when you look you start to see, and when you see you start to understand. Looking at those prehistoric drawings we understand what a bison IS, what it means, even more than if we were looking at the actual animal. The essence has been distilled.
In the same way, great writers write about the human experience in order to comprehend it. Their prime motive isn't narrative or entertainment (although these may be happy by products) but understanding. It is this main drive that seperates art from entertainment (novels from books, Battleship Potemkin from Transformers 2). In the same way, purpose, and the nature of the impetus behind it seperate art from the artistic, from craft, design or decoration.
Its more complicated than that, perhaps. Drawing is a two way process. We look, see and take in information. We try to understand. When that information comes back out it takes a bit of ourselves with it. This gets mixed up with the final product. So, the process of creation also gives people the chance to understand themselves. How well the artist handles this two way process will determine how 'successful' the work is.
A lot depends on their in built capacity to absorb stimuli in the first place. Its true, I think, that art is rarely a product of the heart, many times a product of the mind, but mostly a product of the nervous system. Thats why Ug and Og started having fits and visions, and the rest of the tribe left them alone to doodle on the walls.