Alright, I've thought about your interpretation of Ouramancy, Misa, and come up with an alternative:

You say that it is an evolutionary response, something to allow these families to survive their multiple encounters with Spirits and Magical Beings... And I don't quite agree. To me, the biggest distinction between Ouramancers and the common people is exactly what Hanged has just said: [i]"Every other kind of Spirit is as you think - [b]only an Ouramancer can communicate and interact with them.[/b]"[/i]

So, there are gods that have an interest in this world, there are the spirits of ancestors and of the simple dead, and they have a single conduit for effective communication with the physical plane: Ouramancy. It isn't an evolutionary response on the part of the humans, it's a path laid down by the [i]Divines themselves[/i]. They have crafted Ouramancers as their connection to the physical world, gifting what would have been ordinary magestravi with these altogether unique abilities. After all, how could a human, magical or no, learn to speak with the [i]gods[/i], if that knowledge didn't come from the gods themselves?