Personally I don't believe in a God, admittadly this is mainly on the evidence presented by "organised" religions, and as such its more accurate to say I don't believe in a God that has been presented by main religions.
Do I believe in a being that created the earth? I've not heard anything that has caused me to think "Actually, yeah, there IS a god." As such I would say I do not hold religious, nor spiritual, views. I do believe in science and the theory of evolution though, due to the evidence that I have seen for it (I took evolution modules during university) and my own personally observations and experience has caused me to think "Yeah, this seems legit."
No... you see, hundred-dollar bills exist, you know they do. You've probably held one in your hot little hand. Where they are and where they go are mundane matters of location.
Now, could you believe in something that doesn't exist? Something not made up of pieces and bits of things that already exist in your world (i.e., science fiction), but something that bears no relation to reality as you know it? Could you believe in the existence of something to which there is no correlative in your world?
And now assume that you're a prehistoric man who has no written language, no concept of the world outside of his own immediate area, no real way of remembering the past or imagining the future... how is that person going to make up a thing that doesn't exist? And then pray to it? I don't think it can be done.
I think that prehistoric man instinctively felt that there is Something Out There; and for him to feel that, I believe it had to have been there. Whether that Something was God, or Alpha Centaurians, or transdimensional beasties, or even just the ghosts of dead humans, I can't say.
I choose to believe what I believe, as do you. That's what a belief is, a choice you make in the absence of evidence. We don't like not knowing things, so we fill in the blanks with beliefs. Unless you refuse to alter your beliefs in the face of solid empirical evidence, or try to force others to believe what you believe, there's really neither good nor harm in it.
And I promise not to belittle your beliefs, swear to Whomever.
A few points on your theory that jumped out at me; Is there evidence for pre-historic mans belief in diety/ies? I'm assuming (so correct me if I'm wrong) that by pre-historic you mean either mans pre-evolutions, such as Homo Erectus and Homo neanderthalensis or what we refer to as Homo Sapien, but before historical records. So generally I assume you mean before historical record and as such we can't really say what they believed.
The paragraph before your last (beginning I choose to believe) is a great point, and one that I feel is the reason for many peoples beliefs. They don't know hwo the world works and here's this book, or books, that tells you how everything became and even how you should live your life. Its also a reason why I don't believe in any religion or God. Just because we haven't discovered why doesn't mean we wont nor does it mean that a wizard must have done it.
Yes, I believe God exist.
We are too wonderfully made, our world/universe is too complicated, and we interact with our environment too well to have all just happen. Evolution doesn't even come close in explaining the whats, hows, and whys. There has to be a master engineer as the cause of it all.
We interact with our environment quite horribly actually. We are very slowly destroying it via pollution, logging, clearing rainforests to make room for cattle for meat, causing the environment to change quicker that ity should, overly farming fossil fuels, cutting down forests to make room for farms to grow plants to make "eco friendly" bags with, making products that need large land fills. We're the single worse species on this planet for interacting with our environments.
Which brings me to the point of how things aren't as perfect as people suggest. If we were perfect beings we would be able to interact and take advantage of much more of our world. We can't survive on the majority of the planet due to it being water, then there's the part where not all of the none watery parts is livable either due to being too cold, too hot, having volcanoes that will cover you in ash and lava, mountains that are too slanted, areas where you wouldn't be able to get water or food to live off etc, etc. I suppose some would say that with all that against life, life MUST be a miracle, but personally the world just looks too chaotically put together for a creator.