Digestive System

Digestive tracts vary considerably because animals have evolved different processes to convert foods to essential molecular building blocks. Differences in digestive strategies distinguish, for example, foregut and hindgut fermenters, and animals utilising different dominant food types, for example herbivores, carnivores, and folivores. Neither the modern human diet nor the size and proportions of the human gut resemble those of other primates. The human digestive system has evolved and diverged in response to introduction of new food types and food preparation techniques. For example, persistence of lactase activity into adulthood occurred in populations that maintained cattle to harvest milk. Humans have utilised non-thermal food preparation for over 2 million years and cooking for 300,000–400,000 years. For most extant humans, prepared food comprises over 70% of the diet. The modern human digestive system is suited to pre-prepared food because of its smaller volume, relative to other species, and because of differences in dentition and masticatory muscles that results in lower bite strength. Adaptations of human digestion in response to diet involved genetic selection over thousands of years. However, transmissible changes linked to diet occur in a single generation. These are best documented for epigenetic changes related to obesity, and are maladaptive in some cases. Diets for most humans have changed substantially in the last half century, too rapidly for evolutionary change in digestive physiology. The capacity to adapt to recent dramatic dietary changes has proven insufficient to avoid deleterious effects leading to obesity, diabetes, fatty liver disease, and metabolic syndrome.