The Paleo diet is a nutritional approach that focuses on eating only foods that are high in nutrients, unprocessed, and based on the foods that were available and eaten by humans in Paleolithic times.
The main idea behind the Paleo diet is that if humans were not able to consume a food thousands of years ago, before industrial agriculture, the domestication of animals, and modern food processing existed, than humans should not consume these types of foods today.
Farming changed what people ate and established dairy, grains and legumes as additional staples in the human diet. This relatively late and rapid change in diet, outpaced the body's ability to adapt. This mismatch is believed to be a contributing factor to the prevalence of obesity, diabetes and heart disease today.