Where Xenos humans first encountered species?

I was wacthing Aliens last night and noticed something peculiar. While Ripley was giving the marines the briefing about the xenomophs towards the beginning , one of the marines say's something like (and this is not word for word) "Oh cute, she saw and Alien" Almost like aliens are a common occurrence for humans at the time or at least they've fought them before. another example of this is in another scene one of the marines say's "So is this going to be another bug hunt?" Further suggesting they have battled with extraterrestrials before and they've been insect like similar to the xenomrphs. I know they never encountered xenos before, but have marines gone to war with or maybe even wiped out entire alien species. Or even better, if you believe that that predators are in the same continuity, have humans come in contact or even formed alliances with predators, seeing predators are also somewhat bug like all the aliens humans have fought seem to also be insectoids? Tell me what me what you think.