The paleo diet consists of meat, seafood, vegetables, fruit, nuts and seeds. It is based on the research of Dr. Loren Cordain and made wildly popular by his student Robb Wolf‘s book The Paleo Solution: The Original Human Diet.
Because the Paleo diet consists of meat, seafood, vegetables, fruits, nuts and seeds (and their products i.e. avocado oil or coconut milk), it does not include some large groups of food you may be used to eating on the Standard American Diet like dairy, grains, and legumes. The diet is not devoid of carbohydrates however, of which fruits and vegetables are a great source. The simplest explanation as to why these are not part of the diet is because they can have an inflammatory effect on the gut that are linked to a host of other health issues from lethargy and depression to MS and diabetes. Obviously some people are more sensitive than others, so we all need to assess and do what we feel is best for ourselves.
The idea that this is based on the diet of those from the paleolithic era is because it is thought that humans didn’t previously consume dairy, grains, and legumes (whether in large quantities, high frequency, or at all). From an anthropological perspective, humans shifted to an agricultural based culture about 10,000 years ago during the Agricultural Revolution. Before that, which is 99.5% of human existence, humans did not have grains, for example, in their diet. The theory is that some people’s systems maybe have not adapted to the introduction of grains.