“In the early period of human civilization, people lived and died by their ability to organize things spatially in their minds, tasks necessary when it came to constructing a shelter. We can thank our ancestors for that ability because nearly anyone can mow a lawn, though that deceptively simple task actually takes a lot of brainpower.
Then, with the advent of agriculture and cities, that ability became less necessary. Because humans were increasingly reliant on one another, natural selection made it less necessary for humans to weed out harmful mutations.”
Read more at http://www.medicaldaily.com/articles/13084/20121112/research-suggests-humans-becoming-more-stupid.htm
That explains a lot. But it has been said before…
16 Signs That People Are Becoming Stupider