Ancient Astronaut was the title of a series of books published in the 20th or 21st century, which promoted the theory that extraterrestrials visited Earth during Humanity's pre-history, and contributed to the advancement of the species and its culture. These books were presented as history, but were in actuality fiction. Admiral James T. Kirk initially compared the book Strangers from the Sky to these titles, calling it an "artful fabrication." (TOS novel: Strangers from the Sky)

Interestingly, Kirk had discovered "proof" of "ancient astronauts" visiting Earth in the TOS episode: "Who Mourns for Adonais?", and other subsequent stories.
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