It's often said that people with bad eyes or extremely crooked teeth would have been weeded out by natural selection in the past, and such people are only proliferating today because we're able to correct these problems with glasses and braces.
"A number of researchers have hypothesized that the advent of agriculture, which led to diets consisting of softer foods that required less chewing, led to modifications in the lower jaw, either through natural selection or from developmental changes caused by the way we use our jaws beginning in infancy. But evidence from ancient skeletons has been limited. To test the hypothesis, Noreen von Cramon-Taubadel, an anthropologist at the University of Kent in the United Kingdom, looked at skull and jaw shape in 11 populations, six of which live by farming and five of which are hunter-gatherers. The populations included people from Africa, Asia, Australia, Europe, and the Americas.
In the first part of her study, von Cramon-Taubadel measured the shapes of 322 crania and 295 jaws from museums, representing the 11 populations. She found a significant correlation between jaw shape and how each population made its living. Thus hunter-gatherers tended to have longer (more jutting) and narrower lower jaws, whereas those of farmers were relatively shorter and wider."
What jumped out at me was that the lead researcher didn't equate this change with natural selection. It might be due to a change in the environment – people's jaws might not grow the same way when they're brought up on softer foods. She cited data from animals that shows this is quite plausible. People who needed braces are not, then, an example of devolution of the human race.
This brings up the even more often-cited example of devolution - nearsightedness. I've long been skeptical that this has much to do with genetics or natural seletion. I wasn't born that way. I used to have 20/20 vision when I was in about 2nd grade and now I can't see the big E on the eye chart. Without my glasses, I wouldn't even know there was an eye chart in the room.
Would that have happened to me if I'd spent my early years running around in the forest, focusing on distant objects rather than reading book after book after book?
I had the chance to pose this question to Harvard evolutionary biologist Daniel Lieberman, who said nearsightedness has spread too fast to be due to a change in the gene pool alone. There may be genes that predispose people to nearsightedness, but those genes might only cause a problem under certain conditions – such as a childhood and adolescence spent reading books.