Okay, so the Great Disaster happens and humanity is forced to flee underground to survive. There are the same elites and working class people there were in the old era and the same class structure. The elites are incurious and concerned only about their status and luxury. The workers explore their new underground world and toil to keep their society functioning, they make the scientific discoveries that are necessary for life to function underground but the elites demand that only they get to enjoy the luxuries of the old era in their new underground domain. This causes a divergence in the species.

The working class people denied the luxury of a world as close to the above ground era as possible evolve into a species that is adapted to the harsh life of a world underground. But adapt they do and they thrive as they toil. The elites lead pampered, idle lives simply expecting luxury as their entitlement. They don't care about the science that provides them with this luxury and they don't care about the philosophy that permits society underground to function. They just want their lives of ease and comfort without having to think or worry about how it is delivered to them and they don't want the ones who toil to have it because to them the workers just don't deserve it.

Generation upon generation comes to pass and the worker scientists learn that the world above ground has recovered, life once again thrives upon the face of the earth, but it is a world that will be forever shut off to them because they have evolved into a species that cannot live above ground, but the elite have not evolved at all. They are the same as humans before the Great Disaster because they never faced the same ecological pressures the workers did but mentally these humans have regressed because intellectual challenge was seen as a burden of the working class, not something the elite class should have to endure.

The worker class scientists realize they can be rid of the elite class that has ruled over them simply by returning them to the surface. They can now toil for their own benefit and live free from the oppression of an idle and incurious elite. But they also realize that if these elite propagate upon the surface of the earth then they will destroy the beautiful bio-diversity that nature over many thousands of years has returned to the earth. The elite will need to be monitored and culled from time to time when their numbers become too many. This is necessary to prevent humanity's sins of the past from once again destroying the earth. This is necessary to prevent the elite from returning underground to rule over the working class. So they devise a signal that will return some of this elite class back underground so they can be culled, but not enough that they can assert dominance over the worker class. And, so as not to be wasteful the culled elite's protein will be harvested to supplement the diets of the working class.

Order is maintained. Life thrives upon the surface world. The workers thrive beneath the surface free from the shackles of their oppressive overlords and the elite frolic in incurious bliss and luxury upon the surface, managed so they can never again harm the planet. Earth is a paradise for everyone.

Then some fucking Victorian twat in a home made time machine shows up and ruins it all.