There are some theories that fall into a third category of "unlikely, hard to prove, probably false but intensely fascinating." A good example is Julian Jaynes's The Origin of Consciousness in the Breakdown of the Bicameral Mind [1], which Richard Dawkins describes as:

"A book that is as strange as its title suggests. It is one of those books that is either complete rubbish or a work of consummate genius, nothing in between! Probably the former, but I'm hedging my bets."[2]

By the way, the Wikipedia page about Jaynes' book uses part of that quotation, but cuts off the last sentence.

Dawkins provides a good summary of the book:

"Jaynes notes that many people perceive their own thought processes as a kind of dialogue between the 'self and another internal protagonist inside the head. Nowadays we understand that both 'voices' are our own - or if we don't we are treated as mentally ill...

"Jaynes's suggestion is that some time before 1000 BC people in general were unaware that the second voice - the Gilbert Pinfold voice - came from within themselves. They thought the Pinfold voice was a god: Apollo, say, or Astarte or Yahweh or, more probably, a minor household god, offering them advice or orders. Jaynes even located the voices of the gods in the opposite hemisphere of the brain from the one that controls audible speech. The 'breakdown of the bicameral' mind was, for Jaynes, a historical transition. It was the moment in history when it dawned on people that the external voices that they seemed to be hearing were really internal. Jaynes even goes so far as to define this historical transition as the dawning of human consciousness. There is an ancient Egyptian inscription about the creator god Ptah, which describes the various other gods as variations of Ptah's 'voice' or 'tongue'. Modern translations reject the literal 'voice' and interpret the other gods as 'objectified conceptions of [Ptah's] mind'. Jaynes dismisses such educated readings, preferring to take the literal meaning seriously. The gods were hallucinated voices, speaking inside people's heads. Jaynes further suggests that such gods evolved from memories of dead kings, who still, in a manner of speaking, retained control over their subjects via imagined voices in their heads. [2]

[1]: http://www.amazon.com/Origin-Consciousness-Breakdown-Bicamer...

[2]: http://books.google.com/books?id=yq1xDpicghkC&printsec=front... (page 392)