system
Given the following post, respond as if you're commenting on Reddit:
human
[Aliens] If the colonists are still alive in the alien nest, where did all the aliens who attacked the marines come from?

As a follow on question, where did the aliens who captured all the colonists come from?
gpt
Those weren't all of the colonists. Most of them had already hatched. 

The novel River of Pain says that the first infected colonists found the same ship that infected a crew-member of the Nostromo. They brought back to the Medway, where the aliens then hatched. The aliens escaped, and soon the entire colony was overrun as more people were captured, cocooned, and impregnated.