Here is what I am pondering about the Engineers in Prometheus

Please note: if your answer to any of the below is "movie magic", fair enough, point made. But what I am really interested in is any cool explanations and speculations for how it might actually work as a story, sci fi or not.

Question 1 (has many parts). Did the Engineers pop along to earth 3.5 billion years ago to start all of life on earth.. or 85 million years ago to start just the primates.. or between 400,000 and 250,000 years ago to start homo sapiens?

How close was the the DNA match between humans and engineers?

How quickly does DNA in general change with evolution? I am wondering this because if the story is asking us to believe we are an exact DNA match with the Engineers, then there is a big puzzle regarding how we turned from crumbling Engineer who drank the black juice to a new race a) within a short enough time that our DNA stayed the same and b) how we managed to "inject" ourselves into the fossil records?

I can imagine that they popped along to earth at any of the times listed above and that their tech is somehow so super smart that it could have even have triggered homo sapiens at any time in the chain - maybe it was like some sleeping function in the DNA that waited for the right conditions before working its magic?

Question 2. Why did they trigger the cave paintings to be made? (And was it indeed an invitation or something else?)

Question 3. Of course, the third question is the same question asked in the movie: why did they change their minds and decide to destroy us with alien black goo?

My pet theory for 2 and 3 is that they were truly interested in creating life and leaving a sign to see if we could grow up enough to follow it. But then over the proceeding thousands of years, their society changed and became more aggressive, more warlike. Perhaps they suffered a terrible war? Perhaps they even had a war with another species they created? Perhaps they have been seeding the galaxy for millennia before they popped along to earth and one of their earlier creations came to fruition, found the Enigneers and started a huge war that was so bad that the Engineers got scared and decided they should destroy their other creations before the whole thing could happen again?

What do you think?

I posed this question on IMDB too. :)