ABSTRACT: For about 65 million years our primate ancestors consumed plant-dominated diets. Humans developed the technological ability to consume meat-based diets only about 2 million years ago, so at least 98 percent of the evolution producing our basic primate physiology occurred before this nutritional pattern emerged. Ninety-eight percent of the human genome is identical to the nearest primate relative, chimpanzees, who eat a 95 percent plant diet. Recent hunter-gatherers consume up to 20 times more meat than chimpanzees on a percent energy basis, a substantial deviation from the primate baseline. I show that, similar to other primates, humans retain many physiological and behavioral features displaying adaptation to a plant-based diet, some of which are potentially maladaptive for diets supplying a high proportion of energy from meat, dairy, eggs fat, or refined carbohydrates. Recent hunter-gatherers and pastoralists appear protected from maladaptive responses to such diets by their baseline body composition, ecological context, and evolved non-nutritive ingestive behaviors. Modern people adopting meat-based ‘paleo-facsimile” diets differ from recent hunter-gatherers in body composition, ecological context, and non-nutritive ingestive behaviors. Referring to specific cases, I show how, in part due to these differences, some modern people may develop disorders involving physiological congestion and stagnation when consuming diets supplying a high proportion of energy from meat or fat. Plant and animal foods generally have opposite yet complementary nutritional characteristics. I present an integration of Chinese medical yin-yang theory with Western nutrition that can enable us to understand the relation each type of food (plant or animal) to modern diseases of affluence, and can help guide us to identify an appropriate dietary plant-animal ratio for any individual.