If you'll indulge me, I'll turn your world upside down.

I've not seen anyone else propose this line of reasoning. Here's my theory; The scene of the sacrificial Engineer at the beginning is a deliberate piece of misdirection in order to help us go along with Shaw's hypothesis about our creation and to set us up for a twist-in-the-tale in part 2 or 3. This was not the moment of Creation on Earth, this could even be a flash forward, maybe the Xenomorph home planet? Time will tell. If the engineers had created life on Earth 3.8 billion years ago, then our DNA would not now match. If it did, then all life on Earth would match the Engineers' too (which it doesn't). It is also an unfeasibly long time for the Engineers to wait and for their civilisation to last essentially unchanged. It is slightly more feasible that the Engineers added their DNA to life that already existed on Earth but not combining with it, thereby starting the genus Homo 2.5 million years ago. But I believe this is still an unfeasibly long time to plan ahead and the DNA would still not necessarily match. It was Elizabeth and her partner who came up with the hypothesis that the Engineers are our creators. Based on, as Elizabeth says; "what I chose to believe" or faith, wishful thinking, rather than any evidence. Therefore I think a more credible reason our DNA matches the Engineers is that Elizabeth got it the wrong way round. The Engineers are not our Creators but are instead the genetically altered descendants of humans that were abducted from Earth at various times over the last 35,000 years or so. This is why the cave paintings point to the stars, because that is where they were being taken. They were abducted by aliens we have not yet met. The Xenomorph depicted in the mural. In this hypothesis the Xenomorph in the mural would be a sophisticated, highly intelligent and benign race and the Xenomorph from Alien and Aliens would be an abomination of this race created by accident. The story is essentially comparing natural life that has evolved over millions of years with 'created', unnatural life that hasn't. The essential difference being the 'soul' and a moral conscience. A subject explored in Blade Runner which, according to Ridley Scott, also takes place in the same universe. In the real world we are working towards creating artificial intelligence and scientists and philosophers speculate as to whether such AI will have a soul/conscience or not. Let's say the evolved Xenomorphs, as an old, sophisticated and civilised race will have gone through the early warring and destructive phase, that we are still going through, but went on to prosper and live in a spirit of peaceful cooperation and discovery. Developing faster than light travel and exploring the universe eventually to discover primitive humans 35 to 40 thousand years ago. Not being constrained by a Star-Trek like 'prime directive' not to interfere, they did exactly what we have done, and still do, here on Earth. We have abducted animals from their natural habitat and put them in Zoos and laboratories, not because we are evil but because we are curious. The Xenomorphs did the same. Some humans were taken away for study and experimentation but most were left on Earth. The Xenomorphs, using selective breeding and genetic manipulation bred the 'Engineers', superhumans who were sent back to Earth from time to time to interact with the primitive humans, out of curiosity in the name of science. This interaction, due to the 'Engineers' size, strength and intelligence is probably the reason we developed the myths of the Titans etc. As well as breeding and genetically enhancing the Engineers, the Xenomorphs created artificial life in their own form, just as Weyland did on Earth, not as a weapon but, to quote David 'because they could'. Unfortunately the created a soulless, predator. Perhaps the Xenomorphs were destroyed by their creation or enslaved by them. If we create something more intelligent than us, stronger than us and more resistant to disease etc., then what is to stop it replicating itself and wiping us out unless we can imbue it with a moral conscience? Asimov's 'three laws of robotics' wont work, he wrote a whole series of novels proving their flawed nature. The Xenomorphs and Engineers realised too late that it requires millions of years of evolution to 'develop' a soul and there are no shortcuts. This is why the Engineer, when awoken from stasis, reacted to David the way he did. He realised he was artificial and to his way of thinking, 'an abomination'. I think the Engineers on LV223, in order to save their benefactors, the original, benevolent Xenomorphs, were working on a toxin that would destroy the artificial Xenomorphs and were preparing to leave for Earth as a safe haven (home) to work, when they were overrun 2000 years ago. The black goo is still a mystery. If it is a weapon I don't think it is intended for us.