I think this is what happened.
Earth WW3 they used Nuclear Weapons (As evident with the radiation detector in the last episode) this might have been due to Human vs Human or Human vs Cyclon
They left Earth to find other planets to live upon, and along the way they left tracks to find their way back.
They found a planet (The one where that priestess woman dies) can't remember the name.
After 100s maybe 1000s of years on this planet war again Human vs Human or Human vs Cyclon
They set of again into space to other planets, this time they founded the 12 Colonies. After 100s or 1000s of years fact become mixed with fictions such as Earth becoming the 13th colony instead of the planet they left.