According to Zecharia Sitchin’s interpretation of Sumerian tablets and images, the Anunnaki, “Those who from Heaven to Earth came”, are the inhabitants of the twelfth planet in our solar system (given the name Nibiru in the Sumerian texts) and the creators of human civilisation.
The claim goes that before 4 billion years ago, our solar system comprised of a star with eight orbiting planets:
Approximately 4 billion years in the past, a rogue, Mars like planet, freed from its home system elsewhere in the galaxy tore through our own Solar System. During its passage along a path that took it between Mars and Jupiter, the Free planet, through its gravitational influence, knocked moons from the gas giants into positions as Pluto and Charon before ploughing into the Uranus-sized (yet rocky mantled) Tiamat.
Tiamat was destroyed in the collision, some of the debris reforming into the planet Earth and its moon, much of the rest settling into the annihilated planet’s old orbit between Jupiter and Mars to become the Asteroid Belt.
The rogue planet was captured by the Sun and curved back around into the Solar System again to begin a retrograde orbit much like that a comet usually takes, and at its closest to Earth reaches a point near to the Asteroid Belt every 3,600 years.
Having spent time in Interstellar space, the rogue’s atmosphere was diminishing and only gold, through some very advanced process that Sitchin and similar thinkers don’t go into, could stop it from becoming entirely uninhabitable.
Around 450,000 years ago, many Nibiru inhabitants (the Sumerians later called Anunnaki), being capable of space travel, left their home world and splashed down in the Persian Gulf on a long term gold mining mission. A city was constructed to act as a gold ferrying port on the nearby planet Mars, and we know this site today as the Cydonia region (famous for the “Face”, trick of the light or not).
Work went ahead as planned for 200,000 years. At that point mine workers rebelled and Anannuki rulers on Earth decide to replace their own workers with a simpler slave species. Trials with native Earth life were unsuccessful, so genetic engineering was used, splicing the native Homo Erectus genes with the more complex Nibirian genes. It was a success and the resulting Homo Sapiens was used in mining and construction of technologically advanced Anannuki cities around the world.
However, the human race soon became unacceptably numerous, their knowledge of Nibirian secret sciences too great and interbreeding between the two races began to take place. Anannuki lord Enlil deemed the Homo Sapiens immoral and was to have their planet devastated by enormous tidal waves, but Enlil’s brother, sympathiser and designer of the Humans, Enki, taught the secret of building a boat that could withstand the disaster to Noah (Sumerian Upa-Napishtim), to save the seed of Mankind.
Homo Sapiens survived, with remnants of Anunnaki mathematics and science, but tension grew between the brothers. The bulk of the story ends in roughly 2,000 B.C.E. in the fall of Sumerian civilisation after the warring of Enlil and Enki caused the destruction of the Earth’s major spaceport on the Sinai Peninsula, along with cities such as Sodom and Gomorrah using, literally, nuclear WMDs.
It would seem the Sitchin story stops abruptly here, explaining nothing about where the Anunnaki and the planet Nibiru disappeared to in the last 4,000 years. It's an interesting version of human history though, Crackpot or not...