So.........the cave drawings

(sorry if this has been discussed - I did a brief scan & didn't see it) So we're told that the Original Engineer landed on Earth, then drank his drink and sacrificed his himself in order to begin life here and human life evolved from that event. So, at what point did the cavemen who drew the cave paintings contact the Engineers in order to be able to draw them? The only Engineer there (it seems) disintegrated & fell into a waterfall. Plot hole? And for what reason did they draw them? Intentionally as an "invitation" as Shaw theorized, or maybe as a warning? Overall I LOVED this movie, I actually love how everything wasn't explained. So many movies seem to feel like they need to wrap up everything nicely in a bow. I just watched it for the first time on Vudu three nights ago and I can't think about anything else.