My theory is that aliens came thousands of years ago and took away some humans, the humans joined their civilisation and live among them as possibly equals. They then come back in flying saucers to watch over this planet and learn from its cultural and biological ecosystem. In this way mankind would be saved in the event of a catastrophe but the Earth would not. So if you see a real flying saucer it could well be piloted by 100% genetic humans.
Just a thought.