Lol about the movie
Referencing earlier in the discussion about the Colonial Marines, references (the Colonial Marines Tech Manual) point them out to be formed in 2101, after Prometheus is set.
My take on the matter is that with this, Ridley doesnt need to include the Marines, and in-universe, quite possibly colonisation is still in its early stages in 2085, with those set up still under the flag of Earth rather than being independant and needing that body to be formed. I also think at such an early date based on real-life colonisation deadlines, we wont be out of our own Solar system by then, which also makes me think the starcharts in the archeological dig will also be found alongside blue-prints for a drive that is capable of humans to make such a drastic trip that time dilation is a factor.