As a note, the Primal Blueprint refers to the followers of www.marksdailyapple.com. He espouses his own version of the Paleolithic diet that is entirely acceptable. There are minor differences between the two.
Thankfully, Mark has done most of the research for us so I will be linking to his website often.
The Paleolithic Diet (or Paleo Diet) is a diet that attempts to emulate the diet of our evolutionary ancestors. The closest resemblance to a human first appeared about 2.2 to 2.3 million years ago and the modern human evolved between 400,000 and 250,000 years ago (http://en.wikipedia.org/wiki/Human_evolution). There are those who hypothesize that early humans were primarily scavangers. Later on they adopted hunter-gatherer tendencies.
Roughly 10,000 years ago, the Neolithic period begins and humans adopt agrilculture. Cereal grains become a staple along with wheat and a few other grains. (http://en.wikipedia.org/wiki/Neolithic).
The point? Humans have only been eating grains for ~1% of their total existance. We did not evolve to consume Neolithic foods and the Paleolithic claim is that these foods are the culprits behind most Modern diseases.
Why Grains are Bad
Grains do not want to be eaten. Thus, they have protective measures to help themselves survive and reproduce. These measures are in the form of internal toxins. Phytates, Lectin, and Gluten. (Some may not literally label these "toxins" but I think they are "toxic" to the human body regardless).
For 99% of our species existance, carbohydrates and refined sugars were nigh impossible to find. The only carbs that our ancestors obtained were from wild fruits (which were wildly less sweet than our modern versions) and vegetables (veggies have carbs in them, but usually in the form of dietary fiber). What do sugar and carbs lead to? Insulin. Sugar is toxic in the bloodstream and insulin is released to regulate it. Modern diets of carbohydrates and refined sugars constantly bombard our system with insulin which leads to a multitude of terrible consequences.
Meat, vegetables, and fruits all have a large amount of micronutrients in them. Grains have alot less. Lectins also inhibit micronutrients from entering your system. Eating grains means you will not have A) Less gross micronutriets in your food, and B) Less that will actually be able to be used by your body. The total net loss is insane (<--personal commentary).
Now, I will vomit links.
On Grains: http://www.marksdailyapple.com/definitive-guide-grains/
On Grains: http://www.marksdailyapple.com/why-grains-are-unhealthy/
Lectin Wiki: http://en.wikipedia.org/wiki/Lectin
Phytate Wiki: http://en.wikipedia.org/wiki/Phytic_acid
On Insulin: http://www.marksdailyapple.com/diabetes/
On Sugar: http://www.marksdailyapple.com/the-definitive-guide-to-sugar/
For more information on micronutrients: The Great Cholesterol Con by Anthony Colpo - Chapter 15 "It's the Food, Stupid!"
Why Saturated Fat and Cholesterol are Good
People with a low intake of Saturated Fat and Cholesterol have a higher incidence of Coronary Heart Disease (CHD).
(I cannot truly provide enough information in this crash-course to give you a full understanding of how Saturated Fat and Cholesterol have been villified so I recommend you read the book "The Great Cholesterol Con" by Anthony Colpo. Seriously, it might save your life).
In short, there are truly zero sceintific studies that, when fully reviewed and analyzed, conclude that a higher intake of Saturated Fat correlates with a higher incidence of CHD.
Ancel Keys, the man who claimed Saturated Fat was evil, conducted a study proving the previously mentioned correlation, and he managed to find seven countries that proved a perfect linear correlation. Sadly, he actually collected data from 22 countries and conveniently left the other 15 out. When all the data is entered the correlation literally dissapears.
Similarly, there are no scientific studies with the conclusion that higher Cholesterol means a higher incidence of CHD. In fact, most studies show that people with lower levels of Cholesterol have a higher incedince of CHD.
Keep in mind that there are studies that claim these things are unhealthy, but there's a tendency for the truth to stretched or shady testing practices to be used to achieve said results.
On Saturated Fat: http://www.marksdailyapple.com/saturated-fat-healthy/
On Fats: http://www.marksdailyapple.com/fats/
On Cholesterol: http://www.marksdailyapple.com/cholesterol/
On Everything: The Great Cholesterol Con by Anthony Colpo
What to Eat
Primary: Meat and Vegetables. Corn is not a vegetable.
Secondary: Fruit, Roots and Tubers, Nuts, and Seeds.
Up For Debate: Dairy. I personally cannot eat it but I think that some people are okay with it. Listen to your body on this one (in my case, I get crazy fucking acne when I eat it -- a quite obvious response). Dairy can cause congestedness and an increase in mucus production. I was also chronically congested until I gave up Dairy. I venture to say everyone should give it up for a month to see if any positive changes occur.
Hell No: Grains, Refined Sugars, Legumes (Peanuts are a Legume) and Soy. If it's not previously mentioned than you probably shouldn't eat it. (Legumes have similar issues as grains http://www.marksdailyapple.com/beans-legumes-carbs/).
I highly recommend perusing www.marksdailyapple.com if you have interest in these subjects. He has covered many things in regards to our Paleolithic ancestors. Here are some handpicked links I think are important to read from his site:
(I think Stress is under-estimated. Stress kills slowly, but surely).
On Vitamin D:
(Most people do not get enough of this essential Vitamin. You cannot obtain enough through diet alone).
(In short, there are two major energy production systems the body uses. One for slow, leisure activity and another for fast, intense activity. There is no "moderate" energy system to provide adequate energy to jog at 10 mph for an hour. The physical body cannot handle the stress from this either. Sprinting or walking -- choose).
Summer Reading List
The Paleo Diet by Loren Cordain
The Great Cholesterol Con by Anthony Colpo
The Primal Blueprint by Mark Sisson
The Omnivore's Dilemma by Michael Pollan
As a final note, I want to reassure everyone that this is not truly as difficult a diet as it might first appear to be. The only issue I come across is a carb craving once in awhile but I usually eat a banana and I'm over it. I invite anyone to the "30-day challenge." Live and eat this way for 30 days and then make a decision whether you agree or not.
I'll leave with this: http://www.marksdailyapple.com/the-book/success-stories/.