Air flows mostly in a one-way loop through the lungs of monitor lizards -- a breathing method shared by birds, alligators and presumably dinosaurs, according to a new study that may push the evolution of this trait back to 270 million years ago.
Humans and most other animals have a "tidal" breathing pattern: 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.
Birds, on the other hand, have some tidal airflow into and out of air sacs, but their breathing is dominated by one-way airflow in the lung itself. The air flows through the lung in one direction, making a loop before exiting the lung.
A new study conducted by C.G. Farmer, the study's senior author and an associate professor of biology at the University of Utah, found a mostly one-way, looping air flow in African savannah monitor lizards, Varanus exanthematicus -- one of roughly 73 species of monitor lizards -- although there was some tidal airflow in regions of the lungs. That means one-way airflow may have arisen not among the early archosaurs about 250 million years ago, but as early as 270 million years ago among cold-blooded diapsids, which were the common, cold-blooded ancestors of the archosaurs and Lepidosauromorpha, a group of reptiles that today includes lizards, snakes and lizard-like creatures known as tuataras.
One-way airflow may help birds to fly without passing out at high altitudes, where oxygen levels are low. Before the new study, Farmer and others had speculated that the one-way airflow may have helped dinosaurs' ancestors dominate the Earth when atmospheric oxygen levels were low after the Permian-Triassic mass extinction -- the worst in Earth's history -- 251 million years ago.
"But if it evolved in a common ancestor 20 million years earlier, this unidirectional flow would have evolved under very high oxygen levels," Farmer says. "And so were are left with a deeper mystery on the evolutionary origin of one-way airflow."