I'm really simply amazed at everything this movie did. There was so much there that me and my friends spent the whole ride home dissecting it!
Some of us came out wondering if there was some kind of cycle occurring, life creating life creating life. An interesting proposition that pokes fun at the notion of a god. God created us, but what created god?
What really impressed though, was that all the questions you guys are asking, could be logically answered based on what we've seen in the movie, without things being spelled out. That we're able to sit here, analyze the evidence and come up with theories that generally point in the same direction just goes to show how well crafted a movie this was.
People giving the movie serious knocks for small points in the plot that could have been done better are paying to much attention to things that ultimately don't matter a whole lot in the grand scheme of the film.
Things of note:
-Engineers seed life, seemingly on purpose.
-Engineer fluid reacts to different life forms differently. It reacts to the worms differently than it reacts to the Engineers, and the humans and even sperm.
-Lets note that sperm don't exactly have the same genetic stuff as a full human.
-The implication that the xenomorph started out as a sperm retroactively makes Alien that much better! It strengthens all the sexual themes and implications of rape throughout the entire series!
-The 'tentacle thing' at the end was the first face hugger. That much is pretty obvious, especially considering what follows.
-While this wasn't the same planet as in Alien, we can conclude that a similar thing occurred. Th Engineers being of the same, or mostly the same, DNA can be presumed to generate the same kind of alien when mixing with the fluid in odd ways. Thus causing xenomorphs to come about despite the difference in location.
-That the aliens were not the original weapon, as presumed by many, is a great twist on things.
-The Engineers are spreading life for some reason, and that reason seems to be hinted at. They are clearly seeking to create weapons for war. The fluid is clearly a weapon of war. How do they make the fluid? And in such large amounts as they had? My friends and I came to the conclusion that they seed life to break it down into the fluid. Life is, for the Engineers, a thing to be harvested at a certain point, and not allowed to rival them. They're trying to eliminate whatever is threatening them, not seed more creatures that could possibly threaten them. Hence their wanting to go to earth and break down life. Answering the invitation was the cue to go harvest them.
-The thing this implies is that the Engineers are fighting something. THAT is the only thing that can't really be answered based on the movie alone. They could be fighting themselves, or something we don't even know about yet. You gotta be making weapons to use against something after all.
Other notes:
-Weyland puts special emphasis on David being like a son, and overlooks his daughter a whole lot. That's an interesting and subtle thing I didn't realize till after the movie.
-The scene where they awaken the Engineer is great. They all yell at him to ask their question. Then you don't get to know what he asks him, which in and of itself builds on David's character, and the ramification of the question is the head getting ripped off. What seems to have happened is the Engineer realized that, in keeping with the above, it was harvest time. Something along the lines of 'how beautiful what we have created, time to get rid of it'.
-The end is a fantastic setup. Head of super smart robot and Noomi Rapace flying through space trying to find the root of all things. If they made a sequel based on that premise, it would be a true space odyssey. Amazing potential.
And then this doesn't even get into what the murals in the one room meant, and what that translucent green thing was. We had no idea and have resolved to go see the movie again to try and figure it out.
But ya, the concern going into this was 'they're going to ruin the Jockey mystery by not answering the mystery in a good manner'. They answered the Jockey mystery amazingly, and have purposely replaced that riddle with a new, and equal, one. That is an astounding achievement.