Does anyone have an idea on how the "Proto"morph comes about?

Its one of those chicken and egg situations

Another thought is that the captain who gets facehugged is the one who unleashes the protomorph. The egg that the facehugger came from could be from a engineer who was egg morphed hence the size of the egg. We don't know how long the xenomorph can live ( I can't find any info on it) . I don't know what they need to survive specifically either. But seemed to have died out. So let's say there was an outbreak. Someone or something put it there and it spread.....at the same time with what looks like the engineers that where there were incinerated to stop the out break....someone or something did that. With it being quarantined..what's very interesting is how long the eggs can remain dormant ( 1000's of years so far what we know) but how.....the humans come along and start it up again. I'm against the fence on if the engineers are extinct 

maybe they learned there lessons ( play'n games with life)and moved away with what they had left. Maybe the Xenomorphs and engineers are tied closer together than what we know And WY finds them in alien 3 ( Neil blomkamp) and it starts up again. I'm sure BD has already come up with this conclusion added to many. ( guy is a genius ) So I apologize if it has.

Here is my thought:

The Engineer homeworld already has the proto-morph, or as I'll call it the Organic Xeno, living on the surface. The Black Goo alters DNA and therefore on the Engineer homeworld it is forbidden, hence the base on LV-223. When good ol' David and Shaw bring the Juggernaught to the homeworld carrying the Black Goo, it alters the homeworld's atmosphere (the pod spores and such) and also the organic Xeno creating something in between the organic Xeno and the classic Xeno we've all seen.

David then plays on Walter's curiosity and gets him to help in his experiments. All this leads to somehow Shaw's DNA, the Black Goo, and..Walter, being combined to create the classic Xeno.

This would tie in all the elemnets we know, plus Prometheus. The mural in the room with the ampules HAD a classic Xeno on the wall, in almost a worship-like stance. The engineers knew that if they mixed organic material and technology, this creates the beast. Therefore, they kept the Black Goo away from their homeworld, possibly because an accidental Xeno was created there and threatened their existence.

This is confusing I know, but I believe there are 3 types on Xenomorphs here, the Organic Xeno that already exists on the Engineer homeworld, the true Proto-morph which is a mixture of the Black Goo and human DNA, and the Classic Xeno which is the Goo, human DNA, and a mechanical contributor...which mark my words, will be Walter. I can see David creating the classic Xeno from Shaw, Walter, and the Black Goo, and then being killed by it, thus closing the circle on creator, creation, and death just like in Prometheus (Engineers created us, we created androids, and our creation was our downfall.)