iO9 has a great article about Ridley Scott and his "Space Jockeys" (pictured above). They made their first mysterious appearance in "Alien" (1979), and are starting to pop up in promotional materials for "Prometheus". So since this is the internet, they decided to dig up every reference that exists to the creatures to try and make sense of how they fit into the universe.
Needless to say, this is an internet wormhole of the highest order, and if you choose to go down it, we at Bloody Good Horror are not responsible for where you end up. Enjoy.