Try looking into Joshua Tenenbamu's cognitive-science research. As I recall, he's a big Bayesian (so LW will love him), and he published a paper about probabilistic learning of causality models in humans. If I had to bet, I would say that evolution came up with a learning system for us that can quickly and dirtily learn many different possible kinds of causality, since the real thing works too quickly for evolution to hardcode a model of it into our brains. Also, the real thing involves assumptions like The Universe Is Lawful that aren't even evolutionarily useful to non-civilized pre-human apes -- it doesn't look lawful to them!

We could then have evolved language out of our ability to learn models of causality, as a way of communicating statements in our learned internal logics. This would certainly explain the way that verbal thinking contains lots more ambiguity, incoherence and plain error than formalized (mathematical) thinking.