we evolved on a different planet, and eventually had the technology to leave or maybe had to because it was dying or some other natural disaster was occurring. Through various wars/disagreements, space plagues?, accidents, etc., our population dwindled and when we got to our new planet, we were so fractured and set in our beliefs and opinions, that we all disperse to separate areas, fighting for resources and territory. These primal goals reignited our primal instincts, and all someone would have had to do is leave with the ship and we’ve got no technology, no way to leave, no history… on a whole new planet with no evidence of our great heritage and accomplishments! This could totally be what happened to us… it would explain inherited memories of fantastical, technologically advanced cultures (Atlantis, for example, could be in ruins on a whole other planet), cities, civilisations, the appeal and pull of the stars, aliens… & how people way back then built so-called ‘alien’ things here, pyramids, etc… they stilled remembered how (or perhaps built them when they first arrived- before the "fall", so to speak), because they had grown up with the technology, and maybe even kept some of it for themselves. But their descendants would only know the stories… and they would fall into myth, of course, because what evidence supports it? None here….
As postulated by the Ancient Astronaut theory, we also could be the mixed descendants of the indigenous species and the alien race that came to Earth…