Zecharia Sitchin (July 11, 1920 – October 9, 2010) was a Russian-American author who proposed an explanation for human origins involving ancient astronauts.
The following hypothesis is a modernized conception of Sitchin’s ideas:
The Earth was created when the planet, called Nibiru, inhabited by the Anunnaki, entered our solar system, causing massive disruption to the existing pattern of planetary interactions. After Nibiru collided with Uranus, some of the disturbed planetary matter coalesced in a different location, forming the Earth. For millions of year, all Extraterrestrial species never set foot on Earth due to not knowing the life that was to appear there. For millions of years, dinosaurs roamed the earth, after the dinosaurs died out, the caveman evolved over 60 million years later. One day the Anunnaki, The Greys, Reptilians and Zeta Reticulans landed on Earth in spaceships. They were interested primarily in mining Earth's gold and study the life there. After mining the Earth for some time with help from the cavemen, they all decided to genetically engineer a slave race from the cavemen to do the mining for them. They created the first human beings by splicing some of their own genetic material with that of the cavemen. After some time during which we were forced to work as slaves in the gold mines, some of the Anunnaki chose to interbreed with humans, creating a hybrid race. Not all of the Anunnaki were happy about this, however. As a result, when Nibiru was due to pass close to Earth in its predictable long-range orbit, some of the Anunnaki chose not to warn Earth's inhabitants of the possible danger. Nibiru's proximity caused the Great Flood on Earth, almost wiping the human race out. Some Annunaki were sympathetic, however and took steps to ensure that we survived. The Anunnaki and all of the other aliens then left earth and never returned, but on Earth, they came to be worshipped as gods in the religions of Earth, the region where they first established their presence on this planet.
It is presently believed by Sitchin followers, that Nibiru continues its elliptical orbit around the sun, returning every 3600 years close to the Earth where its gravitational pull can unleash cataclysms.