Earth has been in existence for millions of years now and intelligent life on earth has been there for around 10,000 years. Many of the writers have proposed that our planet, sometimes in the past, was visited by intelligent extra-terrestrial beings. This past although refers to antiquity or prehistory when the aliens or Ancient Astronauts, as they can be called, came in contact with the humans. Not only this, the writers believe that the ancient astronauts shaped the course of development of humans as a whole whether it is the culture, whether it is technology or religion. The most common theory, which has been put forward by a majority, is that the deities present in all the religions that are present since ancient times are nothing but the portrayal of the aliens who visited at that time.
Everyone interpreted their advanced technology as divine powers.
This proposed theory became popular mainly in the latter half of the last century with Erich von Däniken, Robert K. G. Temple, David Icke and Peter Kolosimo being the key writers in this regard. But, on the contrary, the idea of extra terrestrials visiting in prehistory and ancient astronauts is given little or no importance by the academics.
Not only this, in the peer review studies and theory, this idea got negligible amount of attention. Only science fiction has shown any major interest in the idea of ancient astronauts. Advocates of this theory believe that we humans are descendants or creations of these ancient astronauts. Another idea that is associated with the same is that all of the human knowledge, culture, religion etc. was inherited from the aliens thousands of years ago. The proponents also believe that thousands of years ago, the ancient monuments like the pyramids of Egypt or the Moai Stone Head structures and similar structures, which seem impossible to build considering the non-availability of technology and modern machinery at that time, were built by these aliens or they helped the humans to build them.
The gaps present in the historical and archaeological records related to various ancient monuments and artefacts are enough proof for the proponents that ancient astronauts existed. Also, the incomplete explanations of the certain data related to history and archaeology further strengthens the argument, according to the advocators of the theory of ancient astronauts. Certain artwork and artefacts clearly can be interpreted as either a contact to the extra-terrestrials or the usage of their technology in front of the humans. But certain academics have responded in a different way to this saying that incomplete explanations should not be concluded as the ancient astronaut theory. One theory, by the founder of the double helical DNA structure Francis Crick, states that our planet was seeded with life in the form of blue algae by the intelligent aliens in order to ensure the continuity of life. Another theory is that earth was accidentally hit by a garbage form the outer space supposedly belonging to some form of extra-terrestrial spaceship and because of that pile of waste, life on earth started spreading.