Golden Rule = Iterated Prisoner's Dilemma.
It isn't so much that it is self-evident as it is that it is rational behavior. The rational behavior in this case has long-term benefits to all parties, whereas irrational behavior has long-term negative consequences. Most people figure this out subconsciously pretty early on, and behave rationally without consciously thinking about the rationality their behavior. A small fraction of the population IS aware that good behavior is optimal and beneficial (independent of any religious system) and behave accordingly. The combination of positive and negative feedback create a system with a fairly strong vector towards behavior that is "good". The word for people that don't learn these patterns or figure it out is "sociopath".
I am glad to see you are a thoroughgoing realist. You accept a readily available rational standard of behavior that is common to all. I agree with you. However, I cannot agree that this state of affairs could come about through purely naturalistic events. I fail to see how rational thought and a free will which enables one to act on that thought can come about from chaos. Furthermore, if it does come from the random workings of matter- why is worthy of trust? In accepting the rationality of morality, you are accepting that it follows consistent norms that can be followed- but why should we have this assumption if we are merely at one point on an ever evolving naturalistic path? Would it not be the case that if morality is simply the result of evolutionary tendencies- that certain people could be more or less morally evolved? Would this not entail different standards for different peoples and different times? Different levels of moral responsibility? I believe it does. and furthermore that it is absurd. The morality theists believe in is universal, unchanging, and eternal. I don't think you can hold to that claim- your metaphysical commitments do not allow you to.