Prior to the agricultural revolution which took place about 10,000 years ago (that's ~500 generations) man only ate stuff that didn't need a lot of processing - meat, fish, fruit and veg. With the advent of farming, grains were cultivated and ground and assumed much greater importance in the diet. Moreover, with the domestication of animals, humans had much greater access to the milk of other animals than previously.
This was a huge shift in diet. However some geneticists suggest that our bodies have not had time so far to evolve and adapt to this shift. Moreover, the archeological record suggest that people got shorter and sicker as the quantity of grains in the diet increased.
Followers of the Paleo Diet try to eat as their paleolithic ancestors would have. That means no grains, potatoes, pulses, dairy, refined sugar and processed foods. It's not Atkins - you are encouraged to eat as much fruit and veg as you want. To me it just makes a lot of sense.