Far from being the smartest possible biological species, we are probably better thought of as the stupidest possible biological species capable of starting a technological civilization - a niche we filled because we got there first, not because we are in any sense optimally adapted to it. - Nick Bostrom
Imagine a world inhabited by a species, call them goblins, that is just below the threshold of mental capacity required to start a technological civilization. The average goblin, or tribe of goblins, is just barely too stupid for civilization. Goblins can talk, and argue, and form coalitions, and play politics and signal status, and they can look at the world around them and dream and speculate and make art. They can use technology if someone smarter tells them now, and can sometimes even make simple tools and innovations if properly trained, but they just do not have what it takes to actually start a civilization on their own. Unless they have someone smarter to steal from, their society will inevitably forget important things and regress into stone-age savagery.
However, there is a genetic variation among goblins. Sometimes, by random chance, there will be a tribe whose average mental capacity raises above the civilization threshold, for a while, until mean reversion takes them below the threshold again.
What would you observe in your world, if you were a goblin?
You would observe a world filled with the ruins of fallen civilizations. You would see the crumbling remains of great buildings and structures that nobody knows how to build. You might see that these fallen civilizations transformed the land, making roads or canals or even altering entire ecosystems to suit their needs. There would be artifacts from these civilizations, strange items that nobody knows how to make. Sometimes nobody can even guess what they are meant to be used for.
If you were part of a tribe that was clever and curious enough to translate and read texts from these ruins, you might learn their history. You would know that, sometimes, a tribe of goblins would suddenly form a civilization, gain great wealth and power, and conquer and enslave all of the surrounding tribes. But then, over time, that civilization would, for some reason, become less capable. It would coast along, accomplishing little, feeding off the riches of its glory days, until some kind of shock like a natural disaster, resource shortage, or outside invasion would destroy it and leave nothing but ruins.
If you were smart, you might wonder exactly why these great ancient civilizations were inevitably destroyed by trivial things, at a time when they had far more resources and power then they did when they were overcoming much harder obstacles, but you are probably not smart enough to ask questions like that.
If you were a goblin in the later years of one of these civilizations, what would you observe?
You would observe that your ancestors used long words you can barely understand, and sentences with grammar that you can barely parse. They would speak of concepts that mean little to you. They might be deeply concerned with things that seem bizarre or meaningless.
You would observe that goblins in other tribes outside your civilization can never seem to form or sustain a working civilization on their own, no matter how many resources or tools you give them.
You would observe your civilization slowly decaying. You would see that it takes your people a lot of time and money to do things that were once done swiftly and cheaply. You would observe that a lot of things seem to cost more, or are of worse quality. You would see things falling apart faster then they can be built or repaired.
You might observe different parts of your civilization decaying at different rates. If your civilization happens to have some kind of system that identifies the smarter goblins and collects them in special places, then those special places will function well, and may even advance, but the places that you took the smart goblins from will inevitably regress into barbarism in a generation or two.
Different factions in your civilization would all blame different things for the decay. If you were smart, you would notice that each faction blames the thing that it has always blamed for everything bad, and recommends solutions that would increase the wealth and social status of its members. But you are probably not that smart, so you accept your faction's explanation, and believe that things will be good again as soon as you gain power over the other faction and make them do what you say.