Zeddoo covered some of this, but not all. Major timeline conflict. Spoiler! Some think this explains the origins of the aliens prior to the first Alien movie, however, this does not explain the origins. If you think back, Aliens vs Predators accounts for aliens thousands of years in the past, long before the alien seen in Prometheus. It may have explained how they were created, this is certainly not the true origins of the species. Now a question I am wondering about is the first scene with the Space Jockey drinking that stuff and disintegrating into the water. Was this intentional or did he have an alternative purpose and the creation of life was simply an accident that was not intended? The later would certainly hint at why they would want to destroy earth. It may have answered some questions, but it left a whole lot unanswered.