New York: University of Utah scientists have shown that bird-like breathing also developed in green iguanas -- reptiles not known for high-capacity fitness.
The finding bolsters the case that uni-directional bird-like flow evolved long before the first birds, arising nearly 300 million years ago in a common ancestor of lizards, snakes, crocodiles and dinosaurs.
"We thought we understood how these lungs work but, in fact, most of us were completely wrong," said lead researcher Colleen Farmer, an associate professor of biology.
In humans and other mammals, lungs have airways with a tree-like branching structure.
Air flows in and out in a tidal fashion. Oxygen and carbon dioxide pass to and from blood respectively in tiny air sacs, called alveoli, at the tips of the smallest airway branches.
In bird lungs, air loops in one direction through a series of tubes lined with blood vessels for gas exchange.
Aerodynamic forces act like valves to sustain the one-way flow through cycles of inhalation and exhalation.
"For years, people thought that the design evolved to meet the energetic demands of flight. That is all wrong. Iguanas do not fly," Farmer added.
The revelations make clear that scientists have much to learn about the physiology of lungs in species other than mammals.
The study was published in the journal Proceedings of the National Academy of Sciences.