Evolution theory.

I dont think the goo has anything to do with "terraforming" planets. I think the Engineers simply select worlds that already have the basic building blocks in place and seed life into these worlds using variations of their DNA forms that are suited to whatever particular planetoid/moon/whatever.

@ Corey; I heard it was used for that in an article. That the SJ would use it to "seed" new life onto barren planets, effectively terraforming them. I assumed it needed no extra DNA to begin the transformation. However, Canadaphil makes a good point, maybe the planets chosen have the beginnings of life such as bacteria or whatever. Which brings us to David8; who knows what the SJ technology is like? Maybe it can work with any matter, the fact David is synthetic is not a problem for the transformation cycle to begin and that David does in fact become a host, thus creating a new Xenomorph that is biomechanical in structure.

@Svanya: The goo may seed new life on the planet, but terraforming is a process of changing the atmosphere of a planet to make the air breathable and habitable for a human beings. I don't think the Engineers terraformed at all. I think that CanadaPhil was on the right track in saying that they selected planets with basic building blocks to seed life into these worlds. But the life that would exist in these worlds would be able to live in whatever atmosphere that world had. I think the engineers were able to modify the DNA to make whatever life form they created able to thrive in that world. I'm thinking that the Engineers came to earth once, saw how the atmosphere was, did a bunch of scientific experiments to see what would be able to live in such a world, and thus they created human beings. But I don't think they were interested in terraforming at all. I think these beings were so far advanced that they were able to create life that could be suitable to any condition. I think they did this on purpose. They would create organisms that were able live in different conditions, and they would watch and study them.