When I was 15 years old, I encountered Zecharia Sitchin's The Wars of Gods and Men which depicts ancient Sumerian religion as actual accounts of the acts of ancient alien visitors, called the Nephilim, who came from Nebiru, the 12th planet of this System, colonized Earth and created Humanity.
He also argues that the asteroid belt was once a planet which the Sumerians called Tiamat.
Nibiru (the planet associated with Marduk in Babylonian cosmology) is a central element of Sitchin's theory. He claims it was a tenth planet (twelfth to those who included the Sun and Moon) which followed a long, elliptical orbit, reaching the inner solar system every 3600 years.
According to his theories of Sumerian cosmology, Nibiru was the twelfth member in the solar system family of planets (which includes 10 planets, the Sun, and the Moon). Its catastrophic collision with Tiamat, a planet that was between Mars and Jupiter, would have formed the planet Earth and the asteroid belt and comets. It was the home of a technologically advanced human-like alien race Anunnaki of Sumerian myth, who, Sitchin claims, survived and later came to Earth. According to Sitchin, they subsequently genetically engineered our species, originally as slave animals to work in their gold mines, by crossing their genes with those of Homo erectus.
Tiamat, as outlined in the Enûma Elish, is a goddess. According to Sitchin, however, she may have been what we now know as Earth, when one of Marduk's (Nibiru) moons struck Tiamat and broke her into two separate pieces, on a second pass Marduk himself struck the broken pieces and one half of Tiamat was crumbled and became what we now know as the asteroid belt. The second half being struck again by one of Marduk's moons was pushed into a new orbit and created what we now know as Earth. Although most scientists argue that the scenario is improbable, Sitchin's supporters argue that it would explain how the Earth's continents were divided and would also explain why the Earth is layered in sediments.
Sitchin claims that artifacts and documents from the great library of Ashurbanipal at Nineveh and other sources, show technology and advanced scientific knowledge out of place for their time period. This he claims as proof for the following theory.
According to Sitchin in his book The 12th Planet, the Annunaki were extraterrestrials related to the Biblical Nephilim. He claims that they first arrived on Earth probably 450,000 years ago, looking for minerals, especially gold, which they found and mined in Africa. According to Sitchin, the "gods" of the Anunnaki were the rank and file workers of the colonial expedition to earth from the 12th planet, also known later, through the Babylonians, as Marduk. Sitchin claims that ancient records report that a human civilization in Sumer of Mesopotamia was set up under the tutelage of these "gods" and human kings were inaugurated as go-betweens, foremen of the human populations answering to the Annunaki.
The Nephilim "gods" were the commanders of the operation. The Anunnaki performed the menial labor, mining ores and building bases, while the Nephilim issued the orders setting these tasks into motion. It was only due to an uprising by the Anunnaki against the Nephilim in protest of these conditions that the Anunnaki 'workers' revolted against their overseers. Because of this the Nephilim and Anunnaki came together in a project to blend the DNA of Homo erectus with that of their own, thus giving rise to the Homo sapiens. He proposes that fallout from the nuclear weapons used during this struggle was the "evil wind" that destroyed Ur c. 2000 BCE, as recorded in the Lament for Ur
Text Source: http://en.wikipedia.org/wiki/Zecharia_Sitchin
It is quite an interesting read... though considered by many of the scientific community to be mere science fiction, the translation of the Mesopotamian mythologies are indeed accurate and based on actual research which give adequate cause for the argument of probability.