Origin and Evolution of the Lungs

The lungs are one of the most important respiratory organs in mammals, including humans. But how did these organs evolve in animals? This question has interested scientists for a long time, and today we know that the lungs have a long evolutionary history, starting in fish.

We find the first hints of lungs in some groups of fish. Some primitive fossil fish developed a protrusion at the anterior end of the digestive tract, and in the branch of fish that later gave rise to terrestrial vertebrates, the lung developed from this protrusion. In other fish, this outgrowth has turned into a swim bladder, which serves mainly to facilitate swimming, although sometimes it also has a respiratory function. The swim bladder is found today in fish of various species and has different sizes and shapes.

But not only fish have swim bladders. Some lungfish, which are close relatives of land vertebrates, also have this organ. They live in periodically drying up reservoirs and during the dry season they remain in the silt of a dry riverbed, where they breathe using swim bladders. They also have a pulmonary artery, a precursor to the lungs of other vertebrates.

The lungs of most primitive amphibians, such as newts and amblystomas, are two simple long bags covered on the outside with capillaries. Frogs and toads have folds inside the pulmonary sac that increase the respiratory surface. Their breathing mechanism is completely different from that of humans. It is based on the action of the valves in the nostrils and the muscles in the throat area. When the nasal valves are open, the floor of the mouth lowers and air enters. Then the nasal valves close, and the throat muscles contract, reducing the size of the oral cavity and displacing air into the lungs.

Further evolution of the respiratory organs occurred in the direction of gradual division of the lung into smaller and smaller cavities, so that the structure of the lungs in reptiles, birds and mammals gradually became more complex. In birds, air sacs arise from the lungs in several places and spread throughout the body. Thanks to them, air can pass through the entire lung and be completely renewed with each breath. In mammals, the lungs are composed of many small chambers called alveoli, which provide a large breathing surface for gas exchange.

Thus, the evolution of the lungs occurred gradually, with the appearance of simple respiratory organs in fish and amphibians and their subsequent complication in reptiles, birds and mammals. These changes occurred under the influence of various factors, such as environmental changes, adaptation to new living conditions and competition with other species. As a result of these changes, the lungs became one of the most important respiratory organs in mammals, providing them with the necessary amount of oxygen to live.