One-way airflow through the gas-exchanging parts of the lungs is unique to birds.

We breathe in and out through much the same airway system. Birds have a different arrangement: air passes through many of their gas-exchanging lung passages in one direction during both phases of a breath. Their air sacs help move it through.

For decades, that arrangement was associated with the high energy demands of flight. Other reptiles were generally assumed to move lung air back and forth, more like mammals.

In 2010, measurements of American alligators overturned the exclusivity claim. These animals do not fly and do not have bird-style air sacs, yet their lungs maintained one-way flow through key passages. Later work found related patterns in other crocodilians and in some lizards.

The finding changed how researchers reconstruct the history of breathing. Shared airflow in birds and crocodilians supports an origin before modern birds appeared. The lizard findings raise a further question: was the pattern older still, or did similar flows evolve independently?

One-way flow inside the lungs does not mean the animal never exhales through its mouth or nose. Nor does every part of these lungs have one-way flow. The correction concerns particular internal pathways, and the exact evolutionary history remains under investigation.