I find it incredibly unintuitive that humans are genetically adapted to reading, and that only a rare mutation can prevent them from learning. What exactly is going on with the dyslexia genes, do they have some other advantage/disadvantage in the prehistory of mankind?

Also: suppose it were reversed and “genetic dyslexics” were 99% of the population. Would we have developed written language in some form, or does dyslexia render that impossible?