Take a deep breath in. Slowly let it out. You have just participated in one of the most profound evolutionary revolutions on Earth-breathing air on land! It’s unclear how the first vertebrates thrived after crawling out of the sea nearly 400 million years ago, but the lungs hold an important clue. Birds, reptiles, mammals and amphibians have evolved diverse lung structures through which air flows in complicated ways.
A savannah monitor lizard, with one of its relatively large nostrils visible.
Birds and mammals are on extreme ends of the airflow spectrum. Mammals inhale oxygen-rich air that funnels into smaller branches, ending in tiny sacs where oxygen enters and carbon dioxide leaves the bloodstream. When mammals exhale, the depleted air follows the same route out of the body, exhibiting a so-called tidal flow pattern.