Xenos existed before Prometheus

I have yet to see the movie.... Sad day in Mudville. However, I have been reading and thinking about a few scenes and of course the Xeno connection, although I could care less about seeing the original alien. There is apparently the scene with all the engineers leaving and they all die but one. It has been said the elder engineer goes ito hyper sleep and his friends are killed, but nothing explains how. We see piles of dead engineers, head decapitated, etc. My theory is this: the engineers bring humans back to the planet. Something goes wrong and an Alien is created, which tears through the engineers. There is a mural of the fully formed Xeno. But we don't see the Xeno. So, the Xeno must have been created in the past or else we wouldn't have the mural. How does the Xeno get created? Humans were on the planet. They didn't have a space ship, so the engineers must have brought them. hmmm, or, the engineers DNA is the same as our, so maybe they resulted in the Xeno creation. Anyway, I don't think we need a cause and effect in this film to show how Xenos are created. It seems a lot of people are looking to see how Xenos originated, and they are getting disappointed. I think the movie clearly shows that Xenos existed before the events of Prometheus, so we should NOT be expecting an Alien origins story.