More like an assumption made by the Directors of the sequels and fans themselves, I'd say. There's nothing ever mentioned in any of the movies that suggests that the Alien was the result of natural evolutionary process.
No one ever asked Scott about it, I guess. After all, this
thing was his in the first place and it does not look natural:
That thing looks manufactured rather than the result of a natural process to me. I'm happy to see Scott address how the Creature was 'made'. I'm hoping he doesn't mess things up. Fingers crossed and anxiously awaiting more information about these movies.
I'm happy to see the origin of the Alien explained if Scott is the one to do it. I'd hate to entrust this difficult task to anyone else. There's a whole series of missing links that he could explore that would dovetail into what he intends to do with these movies. Why does its larval stage fit neatly upon a Human face with its long appendages wrapped around its victim's head? Why is the embryo so compatible with a human host? Why is the adult bipedal and possesses the basic morphology of a human being..? From what we have seen in 'Prometheus' - and what Scott has hinted at - it certainly does look like the Alien was a manufactured bio-weapon purposefully constructed to eradicate Humans. BUT WHY?
There are more important questions that Scott is posing to us. Why would someone make it such a personal affront to Humanity? Was it
really the Engineers who made this thing to exterminate
us ... and why Us? Why make it so personal? What have we done to deserve this thing cast upon us?
WHY..This is the intriguing path I am hoping that Scott hopes to take us down if this is his intention to explain the Alien's purpose and its relationship to us. I'm thinking this is something Scott has spent quite some time thinking about and hopefully he will deliver answers that don't end in a massive face palm.
-Windebieste.