To some degree literally all of it. My monkey brain was designed to handle at most 150 people, wandering around all day searching for food, unprocessed food, using my body, having a close community I trust, relationship with nature, extreme knowledge of a small amount of things, and an uninterrupted sleep cycle powered by the son.
In a way it designed itself over time. I am a collection of accidentally acquired traits that happened to survive more often in the world that used to be. Mercifully it appears that I am somewhat adept at living in this world, but damn does it feel like I am a fish out of water being in this world.