I woke up in the middle of the night awhile back with this running through my head, and my brain refused to shut up about it til I'd written it down. Didn't have anywhere to put it then, so its just been languishing in a text file on my desktop. Its not terribly ground-breaking or original, but - though I was reading Richard Dawkins at the time - I should point out that it came to me before I got more than a chapter or two into The God Delusion, and he hadn't got around to talking about the origins of religion yet.
Belief as an emergent property of intellignce and communication.
Belief acts as a saftey-valve; it stops over-thinking from getting in the way of survival.
Belief systems are built up around things analysed by intelligence to allow re-use of applied intelligence. Thus, I _believe_ that this ball will fall back down if I throw it up, because a collection of previous experiences and the application of my intelligence at an earlier time tells me that it is so, but I don't actually work it out every time - I just believe it. Its belief based on a solid basis of logic and analysis, but once we've done the maths once, it falls back on belief to avoid having to do it again.
Also, belief in combination with the human knack for communication allows transfer of belief systems to others / offspring. So the children who believe their parents when they say its not safe to leave the cave and Smilodons are afraid of fire are more likely to survive. And once we have transfer, natural selection applies as to what belief systems survive, and we have memes. And in that case the usual rules apply for the evolution of a fertile platform for the memes; a certain amount of mutation (new ideas) is good, but too much is actually bad, and a certain amount of inherent resistance to mutation is also good. So people have actually evolved to naturally accept the memes they have created or had transferred to them from trusted sources, and to resist questioning them by reapplying intelligence or logic. This is mostly natural and good - it allows the re-use case to function. Only a few people - or more people under limited circumstances - will break this mold by re-visiting the underlying thought.
So we were pretty much doomed to a planet full of unthinking unquestioning automatons from day 1 at "bang the rocks together".
Of course two of the most successful types of memes are:
1) the sort which allow you to accurately predict the world around you - symbiotic memes which help the host survive. I shall call these predictive memes "science".
and 2) memes which cause the host to actively spread the meme (proselytize), create a social hierarchy in which those who spread them most actively are granted special status, and cause the hosts to attack hosts infected with competing memes in the same niche. These parasitic memes primarily look out for themselves, and any benefit they bring to the individual host brain appears to be largely circumstantial. I shall call this last category "religion"... though "nationalism" would work just as well.
(Though there is a case to be made for specific religions creating a social structure which advances the survival of the individual member... particularly in an environment full of hostile competing religions. Obviously a meme of this sort will only survive if its hosts survive long enough to pass on the meme - which is why suicide cults don't tend to last long. But as it is this category of meme which creates the hostile environment in the first place, its difficult to go overboard with the survival value of the meme to the host; prolific breeding and the indoctrination of children is good for both the religion and the host, but religious wars, prejudice, and self-sacrifice are good for the religion at the _expense_ of the survival of the individual host.)