Avian Lung Design
A unidirectional airflow system radically different from mammalian lungs.
The Evidence
Mammals (and reptiles) have 'bellows' lungs: air goes in and out the same way, mixing oxygen-rich and oxygen-poor air. Birds, however, have a unique flow-through system where air moves in one direction, allowing for continuous oxygen absorption. This is critical for the high-energy demands of flight.
Historical Context
The transition from a reptilian bellows lung to an avian flow-through lung is a major problem for evolution. Any intermediate stage (a 'half-bellows, half-flow' lung) would likely be non-functional, suffocating the animal. The system requires a complex arrangement of air sacs and parabronchi to work at all—it is an 'all-or-nothing' design. It had to function instantly and fully formed; a partially evolved lung would be fatal.