Some 300 million years ago the ancestors of the present reptiles emerged completely from water and committed themselves to air breathing. They were exothermic and incapable of sustained levels of high physical activity. But from them developed the two great classes of vertebrates with high levels of maximal oxygen consumption: the mammals and birds. A remarkable feature of these two groups is that although the physiology of the cardiovascular, renal, gastrointestinal, endocrine and nervous systems show many similarities, the lungs are radically different. The thesis here is that the bird lung is superior to that of the mammal, and that evolution took the wrong path for the latter. A major difference is that the bird lung with its air sacs and gas exchanging parabronchi successfully separated the ventilatory and gas exchange functions. The combination of these two functions in the alveolar tissue of the mammalian lung results in several problems including collapse of part of the lung if an airway is blocked. This disadvantage is heightened by the fact that mammalian alveolar tissue needs to be compressible whereas the parabronchi are remarkably rigid. Another advantage of the bird lung is that ventilation through the parabronchi is unidirectional and continuous, similar in principle to that of a car radiator. By contrast reciprocating ventilation in the mammalian lung poses the potential for uneven ventilation caused by stratification of inspired gas with that already in the lung. Furthermore this mode of ventilation relies on a combination of convection and diffusion to transport gas to the terminal air spaces which therefore must be much larger in the mammal than in the bird. As a result the pulmonary capillaries are strung out along the alveolar wall in the mammal and are poorly supported compared with the bird where they are embedded in the rigid honeycomb-like structure of the air capillaries (West et al. 2006). As a consequence the mammalian alveolar walls need a collagen cable to ensure their integrity and this interferes with gas diffusion across the blood-gas barrier because it must run through one side of each pulmonary capillary. The result is that the barrier is much thinner and more uniform in the bird. The bird lung also utilizes a crosscurrent gas exchange system which is superior to that of mammals as has previously been described (Piiper & Scheid 1972). The net result of all these differences is that some birds have higher mass-specific maximal oxygen consumptions, and also larger aerobic scopes than any mammals. From a structure-function standpoint, the bird lung is clearly superior.