The ancient extraterrestrial hypothesis developed from centuries-old theory that life is present on other planets and that humanity and extraterrestrials have crossed paths before. The theme of human-alien relationship was pushed to the forefront in the 1960s, influenced by a trend of UFO sightings and popular films like 2001- A Space Odyssey.
The space program also played a role: If mankind could go to other planets, why not a visit by extraterrestrials to Earth?