All of this has happened before. All of this will happen again.
2,000 years ago, on the planet Kobol, there were the 12 Lords of Kobol... some of whom were actually Ladies, but you get the idea. Just like female military officers are addressed as "sir."
They spawned the human race, and lived on Kobol with them in peace.
At some point, for an uncertain reason, the humans left Kobol and established the 12 Colonies (one for each Lord, presumably). But, legend has it that there was a 13th Tribe who left Kobol before and went to a far-off world called Earth. It seems this 13th Tribe felt they needed to be kept separate from the other 12.
In time, the humans created an artificial race called Cylons to act as servants. The Cylons evolved, gained sentience, and rebelled. After a period of war the Cylons went away and continued to evolve, until they managed to create bio-mechanical bodies and a subset of the Cylon was established that "look and feel" human.
There are, coincidentally, 12 of these "skinjob" Cylon models. Five of them are so secret the other seven don't know what they look like... but for some reason those five seem inextricably tied up to human mythology... are they, in fact, somehow, the Five that the Temple of Five is dedicated to?
Why is the 13th Tribe so well secluded from the others?
What if there is a temporal loop here?
At some future time the Cylons and Humans are cast back into the very distant past, and settle on Kobol in peace together.
The 12 Cylons are the 12 Lords of Kobol... the founders of the 12 Colonies are their descendants.
The 13th Tribe... perhaps they were always, to preserve causality, kept separate from the other 12... because the other 12 Tribes are, in turn, their ancestors!
It is eventually decided that the humans must leave Kobol to prevent a temporal paradox from eventally happening. They head out to the most distant part of the galaxy... a planet in a far off spiral arm... a planet they call Earth. Once there, they eschew all technology and, in time, forget, and become us.
Some time later the children of the 12 Cylons also leave Kobol and establish 12 Colonies. But they too forget. All they have is a cloudy record of the ancient past that contains the enigmatic phrase, "All of this has happened before. All of this will happen again."