What a great question! Why didn’t all the nearsighted cavemen, with blurry distance vision, get eaten by saber-tooth tigers and other predators before they could reproduce? The answer is most likely that these conditions are largely acquired rather than purely genetic.
Once a controversial theory, there is gathering evidence that children spending so much time indoors and performing near vision tasks (such as reading) may be a significant cause of the increase of myopia seen all across the world. Myopia (nearsightedness) is typically caused by having eyeballs that are larger in diameter and length than is congruent with the focusing system at the front of the eye (the corneal + natural lens mostly). This larger size eyeball changes the default focal distance of eye from “out there” to “up close” (depending on how nearsighted you are). So myopic eyes can read and perform near work more easily than emmetropic (20/20 naturally at distance) eyes. It is possible (but not definitive) that by continually correcting developing myopia in children with glasses and contact lenses, we may be inadvertently stimulating further inappropriate growth of the eyeball, creating ever higher degrees of myopia.
This default focus at near could have been quite useful in the past for scholars, priests, and a literate elite. But it would have been a serious disability for hunters and gatherers who would spend most of their time looking out for threats, opportunities, and prey off at a distance.
_Written by J. Trevor Woodhams, M.D. – Chief of Surgery, Woodhams Eye Clinic