The Xenomorph evolves based on the type of creature it grows from. The Black Goo, (it not the movies goal to explain the Black Goo and more than any Alien(s) movie has explained how they can survive with molecular acid blood, or survive even extreme environments), from what we can tell, breaks down the body and was used to seed other planets with DNA. Alien Transenspermia is a s real world "theory", not one created by the movie, and I don't think that was the point of the movie, just the lead up to what occurs.
It's not too far of a logical stretch to assume that the black goo could have been used to seal the early stage xenomorphs in, as their instinct is to impregnate other creatures with themselves, in some sort of suspension.
Altering the atmosphere, (or maybe the robot accidentally or purposefully activated them when he opened the door), probably broke that suspension.
So, let me give everyone that's not getting the movie a quick run through.
A pair of scientists/archaeologists discover a painting with the same symbolism as other throughout the world, but this one predates them by a long time. The repeated symbolism was a cluster of stars, and particularly interesting, a system that none of those ancient peoples would have any means to know about, but now, in the future, humanity does have a method to travel to. There is a single planet that might be able to harbor life and try to get funding to go explore, believing it will be the greatest finding of all time.
Unable to get the funding the want, they instead get it from the Weyland (spelling) corp, who secretly has an agenda. Or a few really. The guy that okayed it, sadly has passed on by the time they wake up from hypersleep, only to find his less sympathetic daughter in charge and changing the deal. They had also hired a variety of other experts in fields to go too, (in the Aien(s) verse, deep space explorers tended to be the ones that didn't have friends and family, as their career involved them hypersleeping for months to years there and then the way back).
Anyway, they get there, fully believing that they are going to the Engineers home world, and find that something has basically exterminated them, (that's what they think).
Finding signs of construction (Nazca Landing Strips), they go explore.
There are monsters and mysteries.
A lot of people die.
Turns out a lot of people have different agendas and it looks like the Engineers where not benevolat.
Also turns out old man Weyland has been alive the hole time, and has programmed his son, the robot, to do everything he can to find a cure for old age/death.
His daughter, a real girl (and willing to prove it) doesn't like that her father has always cared more about the robot, and has been waiting and waiting to inherit it all.
All the other scientists are there to get paid.
They finally find an actual living Engineer, find out the entire planet was just a weapons storage (for the Aliens), and the caves they are in are actually a space ship. A lot of people die. Engineer attacks. More people die.
The last survivors think they know the true Engineer's homeworld and make their way back to the ship and fly away.