I personally think the "it has happened before and will happen again" is important. I think on Cobal the dominant race created its own cylons and they were humans. There was obviously a war on Cobol and that war was the 12 or 13 tribes or "human" servant types rebelling against the Cobal "gods" who created them. The humans win the revolt and leave Cobal for the colonies. 1 type or tribe goes to earth. Flash forward a thousand or more years and the humans create thier own cylon servants who then revolt. Either the 12 humaniod cylons could have always been around but in hiding as left overs from the original Cobal types or the cylons were just stupid enough to make brand new humanoid types. New revolt of cylons against humans just as occured on Cobal. And now of course there is the brand new repeat again internal cylon war.

Bits and pieces of Cobal history all through the seasons. Mainly that there was war on Cobal. Caprica 6 talks about it to Boltar when they find the skulls. It seems as if there was a huge ancient war and the inhabitants fled the ruins to found the 12 or 13 colonies. Its just my guess that the war was the result of a human versus cylon type conflict. I base it on the whole "it has happened before and will happen again" they keep saying.

I just think the writers are going for that whole cycle of creation and war and destruction theme. Cobalese creates servants - cobalts posibly interbreed with them - servants revolt and flee to form colonies. Earth is founded by the remnants of the original pureblood Cobal people. (which would explain the cylon disease - the fleeing remnants designed defenses but the defenses dont work against the humans because the humans are actually hybrids of Cobalese and their servants). The Colonists later invent or re-invent their own servants the cylons - the cylons improve themselves and revolt and colonists are forced to flee just like what happened on Cobal. The cyclons create their own humanoid forms (under the urging of the final 5? who might be hidden remnants of the original cobal servants). The cylons then suffer internal revolt as well which is what is happening now.

Its just a guess on my part but I think the fact that the old half breed ship brain that you see in Razor says Starbuck will destroy human kind is actually a clue that the colonist arent pure humans but are instead the offshoot of Cobal and original cylons and that earth is inhabited by pure humans. and if Starbuck brings all the shit to earth it will mean the end of pure humans.

Another wild ass guess could be that they will find earth but that all these events were in ancient times so the fleet hits earth in say prehistory. Thus giving birth to earths whole mythology and the cycle of creation - war - death continues on. (remember nowhere have they ever said what time span all this is happening in from our reference point).