New Ridley Scott movie "Prometheus" is exactly dealing with this topic.
The history of Sumerians is highly interesting and do show a glimpse of something unearthly. It seems like both Abrahamic religions (Judaism, Christianity and Islam) and Indo-Europeans religions (European Paganism, Hinduism, Buddhism) have their roots from exactly the happening that occurred in beginnings of Sumerian communities…if one could follow the birth of both branches they would probably trace you back at some point to Sumerians or at least their geographical proximity. It's even incredible that Sumerians had better insight in medicine, mathematics, astrology then Medieval populations of Europe, Asia and Middle east together.
I have been reading on internet on numerous conspiracy theory websites…it is mentioned that Nibiru X (the hidden 10th planet of our System) passes close to Earth every 3600 years and that's when the so called Anunnaki visit the earth and supposedly that time is near.
By the story the Anunnaki (who we proclaimed as Gods or Human reffer to them as such today in numerous religions) when first came onto earth used the mentioned Iggi (other beings they created) as slaves or workers for their digging operations on earth. During some period the Igigi rebelled and that's the moment when Anunnaki created Humans to help them instead of rebelled Igigi beings. Anunnaki created human beings in their picture, some suggest with mixing the developed primates and their DNA, producing Homo-Sapiens (thus the missing link in evolution chain). Anunnaki are said to be human like only of larger proportions and double our size, both muscular and more intelligent. Anyway it seems the circle is soon to be repeated and Anunnaki will visit earth soon due to planet Nibiru X will be in earths proximity "soon".
So the conspiracy theory suggests we were so to speak genetically engineered on this planet as helpers of Anunnaki to help them dig out ores for them. They are an Alien humanoid beings that live on that other planet Nibiru X that comes close to our earth every 3600 years and that's the time when they use it to dig ore out of Earth. So for them if it would be true we're nothing else but genetic-made tools for grinding the ore they need so what we call Gods and worship might actually be Aliens that just use us same as we would in soon future use robots to help us with things we don't like to do or can't do. it would be kind of depressing to know if your whole point of existance was just actually that you were made to do someones filthy work. And the first Sumerian cities were actually the first stations where Anunnaki created humans and used them as workers, thus the first civilized cities on earth there all out of sudden. Interesting theory guess we'll never know