For example: First scene in the movie - barren planet, presumably Earth, spaceship drops off a humanoid alien who drinks some goo, dies horribly and disintegrates. His DNA is seen entering the ecosystem and crumbling into bits. Inference is that this act seeded pre-prehistoric Earth with the beginnings of life.
Flash-forward - on the alien planet, the scientists anaylyse samples from the miraculously preserved alien head - OMG, the DNA is an exact match for human! Ummm, no its not, otherwise the aliens wouldn't look like odd giant humans, they would look exactly like humans! Implication is that the alien DNA seen in scene 1 caused life to spring up on Earth, then evolved into humanity, which ended up with exactly the same DNA as the original aliens. I've heard of convergent evolution, but that is ridiculous!