There are a number of variations of the ancient astronauts hypothesis but some central themes are common:
- One or more alien races visited Earth in the distant past and interacted with early humans.
- Aliens assisted human evolution through genetic engineering and/or selective cross-breeding.
- Aliens helped humans develop culture, ethics, technology and rudimentary science.
- Aliens were perceived as supernatural beings or gods, laying the foundation for various religions.
- Evidence of ancient astronauts can be found in art, writings, oral tradition and physical monuments.
A variation of the hypothesis states that intelligent life may have evolved more than once on earth, and the ancient visitors were actually native to this planet. Perhaps they lived here alongside humans, or perhaps they were an older race that had previously left earth, returning for occasional visits.
Researchers such as Erich von Däniken and Zecharia Sitchin (see below) have accumulated images and historical texts from around the world to support the theory of ancient astronauts. Rock drawings, Egyptian hieroglyphs, sculptures and other artifacts are said to depict futuristic technology (examples).
Proponents of this theory also contend that ancient humans were not capable of certain achievements (such as the Great Pyramids), therefore they must have been assisted by a more advanced race.
Erich von Däniken
Perhaps the most well-known proponent of the ancient astronaut theory is Erich von Däniken. His 1968 book Chariots of the Gods? included apparent evidence of ancient aliens visiting earth. This evidence was mostly in the form of archaeological items and mythological stories re-interpreted to fit the extraterrestrial scenario. The book became a bestseller and was the basis for several television documentaries. Unfortunately for von Däniken, one of these documentaries was the 1978 Nova Documentary "The Case of the Ancient Astronauts" which proved that much of the evidence was manufactured. von Däniken rationalized his actions by saying that his theories would not be believed otherwise (which is probably a fair observation).
Despite the ensuing academic ridicule, von Däniken's theories have continued to attract support and he went on to author around a dozen more books on the subject.
Zecharia Sitchin is well-known for popularizing the "12th Planet" theory, AKA "Planet X". In a series of books called The Earth Chronicles Sitchin interpreted ancient Sumerian cosmology and claimed they knew of a 12th planet called "Nibiru" that orbits the sun once every 3,600 years. According to Sitchin, Sumerian history reveals that the inhabitants of Nibiru, known as "Anunnaki", visited earth 400,000 years ago. Anunnaki apparently used humans for labor and genetically engineered them for more advanced tasks. The Anunnaki society was later destroyed by natural catastrophe, after which they abandoned the planet.
Sitchin's theory has been widely condemned by mainstream science. In particular, his interpretation of Sumerian cosmology is said to be highly suspect (no actual Sumerian text shows any knowledge of more than five planets).