Have you ever thought about soup? It's fairly common to all cultures, it's tasty, and it's one of the most efficient ways to cook, since none of the nutrients that seep into the water get thrown away. Not only that, it's communal. If you divide a chicken eight ways, somebody's going to get a cut they don't like, or one person will pull at a drumstick, and tear off half the bird. Eight bowls of soup, though, are roughly identical. (Note to self: investigate ascendency of wontons in correlation with the development Chinese hierarchies.)
So who invented it? Once you've learned how to cook things in fire, at what point do you decide to suspend a pot of water above it? For that matter, early water supplies would be wells or rivers, and the chance of someone tossing in a few potatoes to make river water taste better seems slim. Tossing a cow down the well? An important discovery, but it's not soup.
Perhaps it was from sauces. Start with a simple mash of berries that you're intending to slather on the pig. Not enough berries? Try adding some water to thin it out... but that still seems like a bit of a stretch. Besides, if you're mixing sauces for your roast pig but still haven't progressed beyond campfire cooking, you might have skipped a few branches on your tech tree.
I think a more likely answer is that the original liquid in soup must not have been water. It was probably some other liquid that had to be contained separately, like milk. Then, the milk goes bad, and you throw in a few herbs to make it taste better. After that, it's only a matter of time before someone warms it up on the fire, and from there, simple curiousity can lead to adding a few tubers. Voila -- soup's on!
Sorry, but I've been spending the long weekend sick in bed, rustling through my shelves for as many liquid dinners as I can find. I haven't really had all that much else to entertain my stream of consciousness, and so, I concentrate on the cultural impact of soup.
Healthier posts to come later this week.