[Origin and evolution of the respiratory tract in vertebrates]

The description of the evolution of Vertebrate lung is based on comparative anatomy and physiology, combined with phylogenetics. An air-breathing organ (ABO), in addition to gill respiration, appeared in fishes about 400 M years ago. It consisted of a single primitive lung ventilated by a buccal pump. This breathing system was adopted by the first terrestrial tetrapods, and is largely preserved in current amphibians. In the Amniotes, the buccal pulsing pump has been replaced with a costal aspiration pump. In mammals, this ABO evolved into a bronchoalveolar lung. In "Reptiles", the ABO is a partitioned lung with one or several cavities. It serves as a gas exchanger but also as an O(2) reserve, which allows for long periods of apnoea. In birds, the ABO is a tubular structure ventilated by unidirectional airflow. Such a breathing system allows for high rates of O(2) consumption. The diversity of structure and function of the Vertebrate lung should not be analysed as a progression of increasing complexity towards mammal lungs, but rather as diverse responses to varying environmental conditions and phylogenic constraints, and to the challenge of acquiring oxygen necessary for aerobic metabolism.