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 Daniken 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.