Researchers found that monitor lizards have something in common with alligators, birds, and possibly even dinosaurs.
Air flows through the monitor lizard's lungs in a "one-way loop," a feature the reptile shares with the unlikely creatures, a University of Utah news release reported.
"It appears to be much more common and ancient than anyone thought," C.G. Farmer, the study's senior author and an associate professor of biology at the University of Utah, said in the news release. "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."
The lizard's lungs do have a different structure from bird's and alligator's, which suggests the organs could have evolved independently as far back as 30 million years ago for the monitor lizards and 250 million years ago for alligators and birds.
Humans and other animals with similar lung structures have "tidal" breathing patterns. "Air flows into the lungs' branching, progressively smaller airways or bronchi until dead-ending at small chambers called alveoli, where oxygen enters the blood and carbon dioxide leaves the blood and enters the lungs. Then the air flows back out the same way," the news release explained.
Birds also experience tidal airflow, but instead of moving into alveoli it goes in and out of air sacs. The air flows into the lung in one direction before making a "loop" and moving out the same way.
Recent research suggests alligators also employ one way or "unidirectional" air flow. The feature most likely developed in crocodilians (an alligator and crocodile ancestor) when they split from the archosaur (bird ancestors) family tree about 250 million years ago.
The presence of one-way airflow in the monitor lizards suggests the feature may have actually developed 270 million years ago among cold-blooded diapsids, a group of creatures that are believed to have developed into modern-day lizards and snakes.
The study suggests one-way airflow could also help birds fly at high altitudes where the air is extremely thin. The feature also may have helped dinosaurs survive during the Permian-Triassic mass extinction, when oxygen levels were extremely low.
"But if it evolved in a common ancestor 20 million years earlier, this unidirectional flow would have evolved under very high oxygen levels," the researcher said. "And so were are left with a deeper mystery on the evolutionary origin of one-way airflow."
