I have this theory. It is normal for any civilization capable people to bounce back from a dark ages. If you can build society you will build society. North American civilization collapsed (within the continental United States) around 600 AD and never bounced back. The never bouncing back part is what’s weird. The theory is that a mass genocide occurred of their more capable people.

It’s a scary thought because society can collapse and it can collapse permanently. Depending on what selection pressures look like during that collapse what we know of human ability may never be seen again.