This is based on our ancestors that were hunter gatherers, before the advent of agriculture. Their primary intake was protein which included high quality grass fed meat. There were some vegetables, fruits and nuts as well as regional foods based on the climate.
In a nut shell, you would take in more protein, less carbohydrates, and some healthy fats. The food would be fresh so you would have a lower sodium or salt intake which would help to balance out the body.
Paleo, as it’s been termed, was developed by Dr. Loren Cordain. For more information check out http://thepaleodiet.com/