Traditional wisdom of the evolution of lungs in fishes is that lungs arose when gill ventilation was hindered by an aquatic habitat that was low in oxygen. This scenario has been buttressed primarily by a proposed correlation between extant air-breathing fishes and hypoxic habitats, as well as by the fact that early vertebrate fossils were found in sediments believed to indicate a semi-arid environment. There are problems with this scenario, yet it retains a dominant influence on how the evolution of aerial respiration is viewed. This paper presents a new hypothesis for lung evolution that is more consistent with the fossil record and physiology of extant animals than the traditional scenario; I propose that lungs evolved to supply the heart with oxygen. The primitive vertebrate heart was spongy in architecture and devoid of coronary support, obtaining oxygen from luminal blood. By supplying oxygen to this tissue, lungs may have been important in ancient fishes for sustaining activity, regardless of environment. Furthermore, this function for lungs may have influenced cardiovascular adaptations of tetrapods because their divided cardiovascular system isolates the right side of the heart from pulmonary oxygen. I propose that three innovations compensate for this isolation: In extant amphibians oxygen-rich blood from cutaneous and buccal respiration enters the right side of the heart; in chelonians and lepidosaurs the intracardiac shunt washes oxygen-rich blood into the right side of the heart; in mammals, birds, and perhaps in crocodilians, support of the heart by coronary vasculature eliminates this problem.
