Final answer:
Lungs in fish are an adaptation for terrestriality, aiding in the transition of vertebrates from aquatic to terrestrial habitats by allowing gas exchange with air.
Step-by-step explanation:
The new theory by Farmer states that lungs in fish are an adaptation for terrestriality. This adaptation would have allowed fish to exploit terrestrial habitats by facilitating gas exchange in the air, which is a significant departure from obtaining oxygen via gills in an aquatic environment. Over evolutionary time, the development of lungs in some fish is thought to be an ancestral trait that supported the transition of vertebrates from water to land. It should be noted that while gills are highly efficient in extracting dissolved oxygen from water, they are not suitable for extracting oxygen from the air, hence the development of lungs was a key evolutionary step towards terrestriality.