According to ancient alien theorists, extraterrestrials with superior knowledge of science and engineering landed on Earth thousands of years ago, sharing their expertise with early civilizations and forever changing the course of human history. But how did this concept develop, and is there any evidence to support it? Ancient alien theory grew out of the centuries-old idea that life exists on other planets, and that humans and extraterrestrials have crossed paths before. The theme of human-alien interaction was thrust into the spotlight in the 1960s, driven by a wave of UFO sightings and popular films like 2001: A Space Odyssey. The space program played no small part in this as well: If mankind could travel to other planets, why couldn't extraterrestrials visit Earth? In 1968, the Swiss author Erich von Daniken published Chariots of the Gods?, which became an immediate bestseller. In it, he put forth his hypothesis that, thousands of years ago, space travelers from other planets ...