Drake proposed in his books that ancient civilizations were colonies of extraterrestrials who eventually lost their technological secrets and became indistinguishable from humans, and/or that many other ancient civilizations were the result of human contact with aliens.
Drake suggested in his books that ancient civilizations could be colonies of extraterrestrials who eventually lost their technological secrets and became undetectably human-like, or that many other ancient civilizations were the outcome of human contact with aliens.