Based on the analysis of modern physiological, biochemical and cytological data, as well as ideas about the molecular and cellular mechanisms of the three main types of digestion (extracellular, intracellular and membrane), the characteristic of the classical and currently emerging paradigm of food absorption is given. This paradigm made it possible to consider the assimilator cycle as a whole from new positions. The relationships of the main types of digestion and membrane transport in different groups of organisms in connection with their systematic position and ecological specialization are presented. Theories of the origin and evolution of digestive mechanisms, their possible new interpretations based on the achievements of modern biology are discussed.