At the end of Prometheus, we are lead to believe that the Engineers were planning on using the black goo as a biological weapon to end human life on Earth. At the beginning of the movie, black goo is used to create the DNA that would lead to human life. Humans don't seem super aggressive compared to the life forms created by the black goo in the airlock room. Is this the same black goo used to create human life in the beginning of the movie, as the black goo that created the hyper-aggressive worm and the giant squid Dr. Shaw gave birth to? Or, did the Engineers try to "tweak" this goo to perform different actions on DNA?
This interview with director Ridley Scott and writer David Lindelof indicates that it is the same liquid, but their film is intentionally ambiguous about what exactly the liquid does.
So there’s a speculative part of it – the question becomes, “What does the black goo do?” That is the question that you’re supposed to be asking coming out of this movie.
The movie demonstrates what it does in certain circumstances. So, here’s what it does if it gets on worms; here’s what it does if it gets on your face; here’s what it does if someone just puts a little bit of it in your drink. Now we see that lots of this is headed to Earth. Now, you used the word “weapon” – you’re extrapolating that based on the theory Janek [Idris Elba] has, because it looks like a payload to him; all these ships are loaded with this stuff, and they’re headed for Earth. The intent has to be to wipe us out, or is it to evolve us, or is it for something else?
These are all hopefully questions and points of debate – frustrating for some – but ultimately the kind of science fiction. Why the two movies that Ridley did decades ago are still being discussed is this idea that when you walk out of the cinema that you have to go into a community and start to discuss.
For his part, Scott quips that the goo does three things: cleans your teeth, acts like Viagra, and causes a meltdown in the morning!