ASH: It's almost primordial. There's inert nitrogen, high concentration of carbon dioxide crystals, methane. I'm working on the trace elements.
The keyword in this quote is "primordial," a word that means "pertaining to the origin or beginning of something." By this, Ash means that it represents a time when Earth was first forming and when the nature of our planet was at its most violent. By our standards, it was a time when nature was not compatible with human life.
KANE: A cave. A cave of some sort. I don't know but it's like the goddamn tropics in here.
The cave Kane is discussing is hot, humid and layered with mist. It brings to mind instances of other works of art dealing with an entrance into violent nature and away from civilization—like, say, Joseph Conrad's Heart of Darkness.
DALLAS: Haven't seen anything like that except molecular acid.
BRETT: Must be using it for blood.
PARKER: It's got a wonderful defense mechanism. You don't dare kill it.
Talk of "defense mechanisms" makes us think about Charles Darwin's idea of natural selection. We humans have been the dominant species on our planet for a decent chunk of time. But here, the human characters are playing the evolutionary equivalent of an away game.
DALLAS: Anybody want to say anything?
Silence. No prayers, no chants, no religious ceremonials of any type. In Alien, there is no recognition of a God that cares especially for humanity. Man is an animal, a part of the natural world, and stands against the alien alone.
BRETT: This is an ordinary prod, like a cattle prod. It's got a portable battery. It's insulated all the way up to here. Just make damn sure nobody touches the end of it. Shouldn't damage the little bastard unless its skin is thinner than ours but it will give him a little incentive.
With the alien now aboard the ship, the crew turns to the one thing they have always used to get nature to behave: technology. Hey, it worked on cows, right? Unfortunately, Brett is in for a nasty surprise.
RIPLEY: Okay, what about temperature. What happens if we change it?
ASH: Let's try it. Most animals retreat from fire, yes?
Again, the crew returns to humanity's pre-historical roots: fire. Unfortunately, although the alien represents nature, it doesn't represent nature as it is known on earth, meaning fire isn't a 100% fully refundable guarantee.
ASH: You still don't understand what you're dealing with, do you? The perfect organism. Its structural perfection is matched only by its hostility.
LAMBERT: You admire it.
ASH: I admire its purity. A survivor—unclouded… by conscience… remorse… or delusions of morality.
Notice that the qualities that the alien lacks, which keep it "unclouded," are the qualities we tend to extol in humanity. To Ash, the perfect organism is perfectly anti-human.