Paleocontact theory is a term used to describe the hypothesis that intelligent extraterrestrial creatures visited Earth in the distant past or at the dawn of human civilization. Carl Sagan, I.S. Shklovskii, and Hermann Oberth are three notable scientists who have seriously considered the possibility of extraterrestial visitations in Earth's past.
Ancient astronaut theory is a term used to describe the theories of ancient extraterrestrial contact being involved in some way with the origin or development of human culture.
Most notably popularized by authors such as Erich von Däniken and Zecharia Sitchin, this theory is an expansion or elaboration of the more basic Paleocontact theory.
These theories often claim that human beings are either the descendants or creations of aliens who landed on Earth millennia ago — ideas that are commonly dismissed by the scientific community. Another element of this view is the suggestion that much of human wisdom or religion was given to man by extraterrestrial visitors in ancient times. This possibility has
been considered by some scientists, including Carl Sagan and I.S. Shklovskii, but the lack of hard evidence and the nature of the concept makes it unpopular among the scientific community.