Jurassic Park?

I think the plot can suit well the a darwin style explanation of the evolution. Human came from the engineers or their god(s), not from the Earth itself. I mean, in the beginning of the movie, the engineer help mother nature but it's the nature with evolution who create different species and finally us. Prometheus plot proposes a sci fi explanation of our origin on Earth. As an atheist I think it's a fair theory, because it's a explanation of how life have been created on this planet, without refering to any human religion.

So humans suddenly emerged. No other species on earth?

When you see this scene at the beginning of the movie, it's not clear if what the engineer did has brought life in Earth or just some kind of human DNA that directly create human. I prefer to believe he simply brings life to Earth even if it's more relevant to think for the plot that he just brings human DNA to colonize Earth. I think this is really confusing, I hope we will have answers in Prometheus 2 if it happens. Or maybe in the blu ray commentary.

Humans evolved from something that lived in the ground at the time of dinosaurs, not from dinosaurs. That's why the meteor didn't stop us, and we continued to evolve using - according to the film - Engineer DNA as a template. They wouldn't need to see it coming. And I've assumed since I saw the film that Earth wasn't the only planet they seeded anyway. Why put all their eggs in one basket?

well acctually looks like life as we know it could have been kick-started by a meteor falling on earth...so not far from what is explained in the movie