In the beginning of Prometheus (the opening scene and the briefing scene), the film conveys that the Engineers did something on Earth that either started life itself on Earth, or started human life on Earth. Does the movie make clear which of the two alternatives it actually tries to convey?
The only thing I found to be somewhat related to that question is the biologist in the briefing stating (I'm paraphrasing here) that he isn't ready to throw out 300 years of Darwinism for some cave drawings. Of course, that comment doesn't make much sense in either scenario (theory of evolution doesn't concern itself with abiogenesis at all, and a single species being added externally to an already existing ecosystem doesn't contradict the theory either), so it doesn't help to decide in either way.