Evolution of respiratory organs

The first organisms lived without oxygen and obtained little energy from glycolysis and fermentation; the rise of oxygen produced aerobic respiration, which gives much more energy and speeded up evolution. Unicellular organisms and simple multicellular ones take in oxygen by diffusion through the whole body surface. Aquatic animals evolved gills (molluscs, crustaceans, fish): in fish gills a capillary network increases the respiratory surface. Insects breathe through air tubes, the tracheae, while spiders have lungs and tracheae. Amphibian larvae have gills and adults breathe with lungs and skin; their lungs are sac-like, so much gas exchange is through the skin. Reptile lungs are divided into chambers, they have a rib cage, and the skin does not take part in breathing. Birds’ lungs are connected to air sacs, so the blood takes up oxygen on both inhalation and exhalation and the body is lightened for flight. In mammals the bronchi branch many times and end in alveoli, and the diaphragm powers breathing. Some fish (lungfish) have true lungs. According to modern data, lungs and the swim bladder are homologous organs: lungs probably appeared first in ancient fish, and the swim bladder of bony fish arose from them (the textbook’s idea that «the swim bladder turned into a lung» is outdated).