I recently found this excellent video by Mark Bittman, an American food journalist, author and former columnist at the New York Times. He is currently a fellow at the Union of Concerned Scientists.
He succinctly describes the origin and evolution of the Standard American Diet (SAD) in a provocative and entertaining manner. If you eat, you should listen to his 20 minute description of the heart of what we eat and why. The video was filmed in 2007, but still remains true today.