one timestamp to look for is end of biocorp, per wiki 189 years ago. Afaik we dont know when lemurians got gassed before that or when split in station zero happened. Given shadowlith influence it was likely within 1st generation already? If they emerged while biocorp was still a thing they would likely come back to finish their job.
Shit, where did you get that Station Zero and Serpent backstory? It didn't even connect with the gassings on my end. I've read the terminals and talked to that deranged person before killing him for his chip.
Station Zero was dying due to inbreeding, AFAIK? Nothing about the monolith. Damn.