One-way airflow is a breathing technique observed in the lungs of monitor lizards, birds, alligators and likely dinosaurs. According to a news release from the University of Utah, one-way airflow may be 270 million years old, approximately 20 million years older than previously thought and 100 million years before the first birds. Why continues to puzzle researchers.

“It appears to be much more common and ancient than anyone thought,” noted senior author C.G. Farmer, an associate professor of biology at the University of Utah, in a statement. “It has been thought to be important for enabling birds to support strenuous activity, such as flight. We now know it’s not unique to birds. It shows our previous notions about the function of these one-way patterns of airflow are inadequate. They are found in animals besides those with fast metabolisms.”

21 SUPERFOODS YOU ARE MISSING

However, lizard lungs have a different structure than bird and alligator lungs, meaning that one-way airflow may have developed independently approximately 30 million years ago in the ancestors of monitor lizards and approximately 250 million years ago in the archosaurs, the group that gave rise to alligators, dinosaurs and birds. To solve this mystery, additional lizard species must be examined.

Humans and most other animals have a “tidal” breathing pattern: air moves into the lungs’ branching bronchi until dead-ending at small chambers known as alveoli, where oxygen enters the blood and carbon dioxide exits the blood and enters the lungs. Then the air moves back out the same way.

One-way airflow dominates birds’ breathing. The air moves through the lung in one direction, making a loop before leaving the lung.

In 2010, Farmer published research revealing that a generally one-way airflow managed by aerodynamic valves exists in alligators. This suggests that the breathing pattern probably developed before 250 million years ago, when crocodilians split from the archosaur family tree that led to the development of flying pterosaurs, dinosaurs and eventually birds.

The new research discovered a generally one-way air flow in African savannah monitor lizards. This implies that one-way airflow may have emerged not among the early archosaurs approximately 250 million years ago, but as early as 270 million years ago among cold-blooded diapsids (the cold-blooded ancestors of a group of reptiles that today includes lizards, snakes, etc.).

Prior to this new research, Farmer and other researchers had thought that the one-way airflow may have aided dinosaurs’ ancestors rule the Earth when atmospheric oxygen levels were low after the Permian-Triassic mass extinction.

“But if it evolved in a common ancestor 20 million years earlier, this unidirectional flow would have evolved under very high oxygen levels,” Farmer noted. “And so were are left with a deeper mystery on the evolutionary origin of one-way airflow.”

Farmer and her colleagues revealed mainly one-way airflow in the lungs of monitor lizards in several ways. They conducted CT scans and made 3-D images of lizard lungs to see the anatomy of the lungs. They implanted flow meters in the bronchi of five monitor lizards to determine airflow direction and they utilized lungs taken from 10 dead lizards to determine airflow as they moved air into and out of the lungs.

The research revealed that air enters the lizard’s trachea or windpipe, then moves into the two primary airways, which enter the lung. But then, instead of moving tidally back out the same way, the air instead loops back in a tail-to-head direction flowing from one lateral airway to the next through tiny openings between them.

The study’s results are described in greater detail in the journal Nature.