Lettuce - why?

What I want to know is, why anyone ever thought of eating lettuce.

Now in civilised societies, lettuce is an excuse for mayonnaise and vinaigrette. But what made the first lettuce eaters decide that lettuce was worth eating? I've eaten naked lettuce myself a few times: it is insipid and slightly bitter. I doubt that it's even particularly good for you, but if it is, the smart thing would be to get something else to eat it for you and then eat that creature. (I'm thinking rabbits here, not slugs.) Then you could save the vinagrette for your seafood salad and just eat the mayonnaise straight from the jar.