I think 2e issues, vision issues, physical challenges, personality idiosyncrasies all associated with a good chunk of the exceptionally gifted are not a coincidence. A small cultural unit healthy enough to support someone with challenges could also benefit from a bigger, creative thinker. You see medicine men, adventurers, etc. throughout cultures around the world and through time.
People come to recognize that when Grog says not to eat something they should pay attention. And Grog knew because he was constantly observing, experimenting, and following the caveman version of the scientific method. There is a natural way of learning that kids follow and more people would've in the past.
You also have oral story telling and when F'lut knows every story ever told in the village and tells them a bit better, suddenly instead of spending as much time milking her cows, someone helps her out and asks for a story. Maybe the one about the young lovers, because he has trouble with a certain girl in the village.
Once you have writing, then the kind of mind that intuitively learns to read well at three in our times would figure out writing pretty quickly at ten or eleven the first time they see it. Rumors always spread, and someone smart enough would figure out where to go meet someone more like them to learn from. People grouse about being stuck with a school nowadays, but back then a twelve year old might've set out on a ten month journey north in hopes of finding that guy who knows more about plants.
Then you have the "smart enough" people who recognize the too smart people and can capitalize on their abilities to make them advisors or such. Or play with the hypothesis that religion was invented by gifted people to protect themselves from people who couldn't intuit a moral code and as a means of storing and transmitting knowledge unbounded by mob mentality.
Look for autobiographies by naturalists like Benjamin Franklin to fill in the gaps.
