Evolution of the lung

Lung of Protopterus dolloi, a lungfish

Evolution of lug, describes the evolutionary process by which bony fish living in oxygen-poor waters developed lungs. Air breathing is a critical event for the transition from water to land and the evolution of tetrapods. The vertebrate lung originated from a progenitor structure found in primitive bony fish. The origin of the neural substrates that are sensitive to metabolically produced CO² and rhythmically activate respiratory muscles to match lung ventilation with metabolic demand is not yet known. A specific periodic centrally generated rhythm described as “cough” occurring in Lamprey in vivo and in vitro was found to be modulated by central sensitivity to CO². This suggests that elements critical for the evolution of breathing in tetrapods were present in the most basal vertebrate ancestors before the evolution of the lung. Scientists think that the evolution of breathing in all vertebrates occurred through exaptations derived from these critical core elements.

Tetrapods, a group of pioneer lungfish and 4-limbed land vertebrates, still have the structures of their ancestors today. They lived in water and had scales and fleshy fins. However, they also had lungs that they used to breathe oxygen. Between 360 and 390 million years ago, descendants of these organisms began living in shallower waters and eventually moved to land. As they did, they experienced natural selection that shaped many adaptations for a terrestrial lifestyle. Because fish appear in the fossil record earlier than the branch called tetrapods, it is important to assume that modern fish share the same characteristics that they and humans had in their common ancestors. This line of reasoning is intuitive, but not correct. While it is true that both modern ray-finned fish and the common ancestors of tetrapods were finned and aquatic, this is not the case when it comes to lungs. The evolution of the lungs is basically a process that begins with the formation of a skin sac around the gills. While there is normally water pressure in the gills, vertebrate life on land was possible because the air, which is not as dense as water, could provide the same pressure with these sacs.

Gills and lung

Current evidence suggests that gills were present in the earliest bony fishes – the common ancestor of Sarcopterygii and Actinopterygii. However, lungs, gas-filled organs that serve the respiratory function, also evolved very early. The common ancestor of fish and ray-finned fish had lungs as well as gills. Sarcopterygii had lungs attached to them, which tetrapods, coelacanths, and lungfish inherited and used to obtain oxygen. Coelacanths and lungfish have also retained their gills until this time. There is evidence that modern tetrapods once had gills, but these were lost during early evolution. Some ray-finned fish have retained their lungs for a long time. But in the lineage that gave rise to most ray-finned fins (and at least one other lineage), lungs evolved into swim bladders. This is a gas-filled organ that helps the fish control its buoyancy.

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