Why did some ancient fish have lungs as well a gills?

Some ancient fish developed lungs alongside gills as an adaptation to survive in oxygen-poor environments, such as stagnant waters or shallow areas where oxygen levels fluctuated. This dual respiratory system allowed them to extract oxygen from both water and air, enhancing their ability to thrive in diverse habitats. The evolution of lungs in these fish is seen as a crucial step toward the emergence of terrestrial vertebrates.