How we have systematically bred the nutrition out of our food.
The ancestor of corn had ten times more protein than the corn we eat today. That's just one example.
Over the millennia, farmers have selectively bread food crops for better taste. In the industrial era, we got really efficient at it and bred food that worked best in the industrial food system and contributed to better "food products."
Nutrition took a back seat to all this selective breeding. As a result, our food is nearly devoid of nutrition compared with what our ancestors ate. http://www.nytimes.com/2013/05/26/opinion/sunday/breeding-the-nutrition-out-of-our-food.html