The opening of the movie showed the empty, barren earth before life existed. He drank what was presumably the same liquid contained in the bio-phials contained in the egg-vase and began to disintegrate to the molecular level.

We’re meant to imply (due to the zoom-in of the DNA and such) that these molecules were guided to evolve into the complex chains of amino acids > proteins > RNA > DNA etc. that would eventually progress back to themselves.

This race gave up on us for an unknown reason and set out to destroy us (possibly due to a failure in the evolutionary experiment – perhaps we didn’t return to a satisfactory version of their form, in their eyes). They changed their mind, though the one in the movie obviously was unaware.

My theory is that this liquid was meant to be a type of panspermia that would be able to generate their master race anywhere in the galaxy, but that the coding was imperfect.

When a human ingested this, it didn’t dismantle them as it should have, but rapidly mutated them on a genetic level. Before this could set in, the guy got his girlfriend pregnant, passing along a genetic code that the Engineers never meant to exist (hence why, even though the human form of the male was lost, it still survived).

I’m assuming that because the evolution was guided that the mutated genetics were still guided towards a direction that could be SUSTAINED, thus viable as an individual entity capable of survival.

This sole creature developed the ability to asexually reproduce to protect its survivability. Eventually, this species would develop much in the same way: a singular alien will convert into a queen and lay the facehugger eggs that implant an egg into a host that will assimilate its DNA to generate a form similar to itself but wholly xenomorph.

It connects to the AL|EN series but also stands completely alone and has the potential to spin-off into sequels completely unrelated to AL|EN at all.