One thing nature shows us is that the most hostile environments create the most hostile organisms. In order to survive an organism must be able to adapt to its eco-system, predators, and prey.
The alien is extremely hostile - both in action and physiology. It is also has great strength, and a remarkable ability to adapt. My personal belief is that if the alien evolved it evolved on a world that wad so extremely hostile that no life form that we could currently conceive could exist on it.
It's exoskeleton can withstand extreme heat and cold as well as tremendous pressure... or the lack there of. Personally I think it would be a hot planet - mainly because the alien always seems to nest in hot areas (near the reactor core of the Nostromo (if I remember from the book correctly), the processing station on LV-426, in the basement of the mining facility on Fury 161 (presumably some of those pipes carried hot water), and near the reactor core of of the Auriga). And if the alien could withstand molten lead I'd wager it's home planet was hotter than the processing station on LV-426. It also doesn't appear to need O2 to survive, so the atmosphere could most likely be toxic to your run-of -the-mill carbon-based life form.
...The blood, now this is an interesting trait. My guess is that on a home world the alien would not be at the top of the food chain, but perhaps somewhere in the middle... why else would it evolve something like reactive blood? Numerous plant and animal species on Earth have evolved to incorporate some sort on noxious defense against predators so that they taste bad... I see no other reason for the alien's circulatory fluids to be acidic if it didn't have to fend off a predator. Being in the middle of a food chain might also explain the alien's need to be extremely prolific: the more offspring there are the better the chances the species would survive.
But there's a problem here. There are several unanswered questions - such as its adaptability, the fact that it doesn't seem to find a balance with its environment (it just seems to take it over), its ability to exist in extreme cold and low pressure environments, and the fact that its seems unfazed by lack of atmosphere. If we look at these as well as it's reactive blood, it's ability to withstand heat and pressure, and the fact that it's exoskeleton has enough tensile strength to stand up to small arms fire at close range (Gormon in the air shaft) I can't help but think that no one environment could exist that would present the need for each of these traits. But rather maybe these traits were created to so that the alien could exist in almost any environment. I.E. - perhaps the alien was bioengineered for a purpose, rather than having evolved on its own.
