At one time I did think that it was possible that we had been visited by people from outer space. Now I don't think so, reason being, if visitors from outer space had the technology to get here, that would point to a far superior knowledge base. If they then, as some believe, bred with the most humanoid animals on earth. Should there have been a bigger jump in human comprehension, should we have seen almost modern day understanding even back then. As they would have been able to travel intergalactic to get here.
Makes sense to me but I have had a few.
TC