http://en.wikipedia.org/wiki/Prometheus_(film)#cite_note-Writing1-60
"Spaihts met Scott in late 2009, where they discussed Scott's desire to pursue an Alien prequel."
"Spaihts set about writing a 20-page "extremely detailed outline", and within three and a half weeks his first draft was complete, and submitted to the producers on Christmas Day, 2009. Within 12 hours, Scott returned the script with notes for changes, and Spaihts spent the Christmas holiday redrafting."
"By April 2010, the script was on the fourth draft.[57] Scott described the script, saying "we are talking about gods and engineers. Engineers of space. And were the aliens designed as a form of biological warfare? Or biology that would go in and clean up a planet?"
"In June 2010, Scott stated that the script was complete and ready for filming."
"However, Scott instead contacted Lindelof and requested that he review Spaihts' script."
"Scott liked these ideas and had them explored further in Lindelof's rewrite."
"As with David, Lindelof further expanded this facet of the character during his rewrites. Lindelof spent approximately eight months developing the script, finishing in March 2011, as filming began."