Oh but there is a 4th option: A prior Earth civilization. One of the ones that existed prior to 12800 BC when there was a period of 100 years of comet debris strikes that changed the climate worldwide, wiped out all of the mega-fauna, and ushered in the Younger Dryas Event that lasted 1000 years of the coldest time on Earth, then ended with more comet debris hits. they fled elsewhere, leaving stone monuments around the world which we cannot duplicate.
They are just returning home. Get used to it.
I absolutely agree that they could be from a prior civilization... but they never left... They have always been here. We cannot consider time as linear with this situation.