humans did not come from Earth. Everyone came from Kobol
That has not been firmly established.
I have a really hard time believing everyone came from Kobol rather than Earth. Not with all the anthropological record we have of man evolving on Earth. It just wouldn't make any sense to me if they were to say man came from Kobol and for some reason lost or abandoned his technology. How would that explain the neanderthals and the many stages of man? How would it explain the little similarities like the neckties and suits?
It makes far more sense to me to think that thousands of years from now, humans HAVE to leave earth for some reason. Maybe we've ruined it with pollution, or climate change has made it uninhabitable, and survivors flee the planet in generational ships. They're in transit at sub-light speeds for hundreds or thousands of years. They colonize Kobol. Then, thousands of years later, for some reason, they have to leave Kobol. Most go to the closer twelve colonies planets, either no longer believing earth exists or not believing humans could survive on the earth they'd left. Maybe they created an entire race of skinjob cylons thinking the cylons could withstand whatever damage had been done to earth and make it fit for humans again. The cylons make the longer voyage back home to the abandoned Earth, a voyage that takes thousands of years. But by then, earth's just a myth to the twelve colonies.
That makes more sense to me than thinking we all came from Kobol, lost our technology, reverted to apes, evolved again, and after hundreds of thousands of years, we've reinvented the gods of Olympus and accidentally recreated the necktie.