Lizard Lungs' Origins May Date Back 270 Million Years

Monitor lizards mostly breathe in a one-way loop, a new study found, adding the animal to a list previously dominated by birds, alligators and presumably dinosaurs.

Published in the journal Nature, the discovery suggests the breathing pattern possibly arose some 270 million years ago - about 20 million years earlier than previous estimates.

"It appears to be much more common and ancient than anyone thought," said C.G. Farmer, the study's senior author and an associate professor of biology at the University of Utah.

Previously, unidirectional breathing was believed to have evolved in birds in order to sustain the physically demanding task of flight. In 2010, Farmer published a study revealing that alligators boasted a similar system, placing the origins of the breathing pattern back roughly 250 million years when the shared ancestor of alligators and crocodiles split from the branch of the family tree that eventually grew to include birds.

Based on this, the researchers theorized that one-way airflow may have allowed dinosaurs' ancestors to thrive during a time when Earth's atmospheric oxygen levels were lower than they are today.

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

It's possible that, given the structural differences between lizard lungs and those belonging to birds and alligators, that one-way airflow sprung up independently 250 million years ago in the archosaurs - a group from which alligators, dinosaurs and birds all originate - and then again 30 million years ago in the ancestors of monitor lizards.

The monitor lizards' lungs contain more than a dozen chambers, or bonchi, in each lung. The system resembles a feather, with one long primary airway with lateral bronchi branching off of its sides.

Using CT scans, 3-D imaging and surgically implanting flow meters into the bronchi, the scientists found that air enters the lizard's windpipe before flooding the two primary airways, which enter the lung.

"But then, instead of flowing tidally back out the same way, the air instead loops back in a tail-to-head direction moving from one lateral airway to the next through small perforations between them," the University of Utah wrote.

Farmer called the perforated walls that enabled air to flow from one chamber to the next "lace curtains."

In the end, Farmer said more research into other lizard species such as geckos and iguanas is needed in order to get a clearer sense of how and when this system may have developed.