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.
In his writings, Drake proposed that ancient civilizations were colonies of extraterrestrials whose technology was lost over time, making them indistinguishable from humans, or that numerous ancient civilizations originated from interactions with aliens.