The lung is the gas-exchange organ that provides oxygen and removes carbon dioxide from the blood. The environment in which animals live and their metabolic needs determine the evolved design of their gas exchange system. Gills are the primordial respiratory organs that evolved for water ‘breathing’, while other adaptive solutions evolved for bimodal breathing, that is, the ability to extract oxygen from both water and air. The transition to fully terrestrial life was accompanied by significant changes in dimensions of respiratory units (alveoli) which decreased in size, whereas the number of units and total lung volume increased, leading to more efficient gas exchange and oxygen supply. While the shape and make-up of lungs in humans suggest adaptations to long-distance running and possibly to the exposition of smoke caused by fire, the exposure of the human respiratory system to novel environments has brought about a diverse array of disease patterns, including lung cancer, autoimmune diseases, and infectious diseases.