Some questions about the plot

1) Engineers on a budget (i.e. lots of extras who looked as much like engineers as they could do en masse).

2) We've only seen a small area of the planet (i.e. the city and a few kilometers away from it - they run from the landing site to the city gates). What we know though is that the pathogen spread from the delivery site to destroy or corrupt all animal life on the planet in the course of ten years.

3) The eggs in ALIEN are not carbon dated. The engineer corpse in Prometheus is carbon dated to two thousand years before. However, in the ampule chamber there with the corpse's head, sealed for those two millennia, is a depiction of a Xenomorph and it's life cycle (including facehuggers, etc) on the wall. David says in Covenant that he created a what he thinks of as a "perfect organism", which is the (much more organic) version of the Xeno that we see. However, he lies on numerous occasions, forgets that Shelley, not Byron, composed "Ozymandias" and comes off as generally delusional AND he's had ten years to study everything the engineers left behind. He was in the pyramid on LV-223 and saw everything there, even figuring out how to operate their mechanisms right away. He would likely have figured out at some point that the Engineers considered the Xeno their "ultimate weapon" and sought to *replicate* it, then taking credit for a design he merely *copied*. In any event, some version of the Xeno, facehuggers, etc existed before, and what he created was his own version, which is not *exactly* like the classic version we're familiar with.

They seemed like engineers to me, they have that same deep, powerul voice when they realize something is wrong

It matches that tone of voice of the engineer from Prometheus.

Let me offer a different theory that I really hope will turn out to be true.

the humanoids in the city are NOT engineers, and the planet is not the engineers homeworld. they are just another form of humans the engineers created in they're image, they live in one city and serve and worship they're gods the engineers. 

This is why they seem to be farmer like, and technologicaly simple.

this is also the reason all of them gathered around the center arena to witness the return of they're gods after 2 millennia (no reason for super advanced life creating species to be at awww about a space ship landing).

the city itself and the landing hub was built by the engineers for the humanoids in order for the humanoids to serve and worship them.

also this could explain why the true engineers wanted to wipe out humans on earth about 2000 years past, that's about the time humans stopped worshiping many gods (the engineers) and turned to a monoteistic religions. The engineers see themselves as gods because they create life in the galaxy in they're image, and when their creations on earth turned their backs on them that was blessfemy and so the earth humans must die.

also that is the reason why the engineer in promethius wanted to destroy David and kill his maker, only they are true gods and only they can create life , so Davids existence is an offense to them and an abomination.  

@Julius  I couldn't agree more. The civilization we see in AC is remarkably different from the Engineers we see in Prometheus, be it the 'muscular/military' type, or even the elder Engineers from the opening scene full cut. Plus, that seems to be a very small city or colony. I imagine the homeworld of the Engineers to be a vast cultural and technological, well organized, society.

You pointed out a descent reason for the hatred of the Engineers towards humanity. However, for some reason, I've been lately thinking about the possibility that David sees Engineers and Humans as a whole single thing, and that, maybe, the firts scene of Prometheus was indicating an attempt of the Engineers to spread their DNA through the right use of the black goo, and endure their race against extinction. Maybe humans and that civilization from AC are just possible results of it, and David knows that and is willing to wipe them all out - Engineers and humans, as they're "the same". (I'm thinking about starting a thread about it, but lately this website is getting full of them and most of the times, there's not even time for people to notice that you started one).

The planet in 'A:C' is the same planet in the beginning of 'Prometheus', so the humanoids on Planet 4 that are wiped out by David were created by the combination of the black liquid, the Engineer's DNA and falling into the water at the beginning of 'Prometheus'.  At least, that's my theory and why I think they look different.  I believe we still have yet to see more Engineers and their homeworld.

However, if those were Engineers and they look that different between 2 movies made only 5 years apart, shame on Ridley Scott and all involved for the ridiculous lack of continuity!

Juius in the original Alien they performed no carbon dating on anything...they simply stated that it appeared old and fossilized...so it leaves wiggle room for them to work around the ancient angle as just the "space truckers" werent sure what they were looking at and diagnosed it as ancient when it really wasnt

But Julius i fully agree that on an entire habitable planet these beings only lived in one solitary spot seems odd

also i dont believe the beings that David bombed were the same beings from Prometheus. They are far too different in appearance. They are maybe just another faction of humanity that has evolved or perhaps the beings on LV223 were genetically superior beings or an entirely different class of humanoid, but i believe they werent the same. Ridley Scott above all else is a stickler for visual detail, if they are meant to be the same race of beings as LV223 they would have appeared as such....i believe there is something to this, that hopefully will be revealed in future installments.

Whoa @Membrane, that would be a nice tie in! I wish I could ask Sir RIdley right now because the diffference is shocking. My current thinking is that the Engineers were just part of the military/science division so they all had to be the ebst of the best physically.

Also, I would like to say that the mural in Prometheus is not a xenomorph. I mean, the crucifixion looking one isn't. The weird bird looking thing may be a xenomorph, but it is impossible to really tell. Still so much to decipher from Prometheus.