What is the basic nutritional theory behind the Paleo diet?

The basic theory is that our genes have not changed much at all since the birth of agriculture. However, since the introduction of grains (approximately 12,000 years ago), processed foods (last 100 years), and the introduction of refined sugar in the human diet (approximately 400 years ago) we have become more and more sick. The theory is that by eating foods similar to that of our early ancestors, we can lead healthier lives and prevent many of the common diseases that plague modern culture. Modern humans are still adapted to the diet of our early ancestors. The ideal diet is one that resembles the ancestral diet. We are 99.9 % the same as when humans first evolved. The only thing that has changed is our food supply. 

Is there science to support it?

There is a great deal of science that supports the paleo diet. www.thepaleodiet.com (Dr Loren Cordain, PhD) offers peer reviewed published scientific research on the paleo diet. Another great place to find published research is www.staffanlindeberg.com (Staffan Lindeberg, MD PhD).

What can one expect when adhering to the diet and who should be on it?

Expect to eat a combination of meats (pasture raised, humanely treated, and antibiotic free), seafood (sustainably raised and wild caught), healthy fats (avoiding all industrial vegetable oils and hydrogenated oils including canola oil and stick with stable animal fats and coconut oil), vegetables and fruits (local organic is best). Try to avoid packaged, processed foods and just to eating real foods, nuts and seeds in moderation-properly prepared (soaked and or fermented). Avoid grains, legumes, soy, and most dairy (some people can consume raw dairy).

"The Paleo diet is the healthiest way you can eat because it is the ONLY nutritional approach that works with your genetics to help you stay lean, strong and energetic! Research in biology, biochemistry, Ophthalmology, Dermatology and many other disciplines indicate it is our modern diet, full of refined foods, trans fats and sugar, that is at the root of degenerative diseases such as obesity, cancer, diabetes, heart disease, Parkinson’s, Alzheimer’s, depression and infertility". - Robb Wolf