So I'm finally getting some time to watch the Expanse and it is interesting to see the physiological differences of people living in low gravity. Reminded me if a what if I had where ancient humans developed space faring tech, maybe do a significant fraction of light speed (0.3 or so), but not enough to experience major relativistic effects. They send a seed vessel out carrying a viable population to live and die for thousands of years while they go settle another system.
They then have myths and legenda about Earth, and having been living solely in space have really focused on this tech and solved a lot of the challenges related to light or FTL travel, decide to come back to Earth.
They are now slender, tall, and gray with big eyes. We call their ships UFOs and they are aliens.
I am confident I am not the only person to think that one up, but it is an interesting thought. For the record I don't see any evidence to make me think that is what actually happened, but it is fun to theory craft.