Air flows mostly in a one-way loop（环，圈） through the lungs of monitor lizards -- a breathing method shared by birds, alligators and presumably（大概，推测） dinosaurs, according to a new University of Utah study. The findings -- published online Dec. 11 in the journal Nature -- raise the possibility this breathing pattern originated 270 million years ago, about 20 million years earlier than previously believed and 100 million years before the first birds. Why remains a mystery.

"It appears to be much more common and ancient than anyone thought," says C.G. Farmer, the study's senior author and an associate professor of biology at the University of Utah. "It has been thought to be important for enabling birds to support strenuous（费力的） 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."

But Farmer cautions that because lizard lungs have a different structure than bird and alligator lungs, it is also possible that one-way airflow evolved independently about 30 million years ago in the ancestors of monitor lizards and about 250 million years ago in the archosaurs, the group that gave rise to alligators, dinosaurs and birds. More lizard species, such as geckos（壁虎） and iguanas（鬣蜥）, must be studied to learn the answer, she says.

Farmer conducted the study with two University of Utah biologists -- first author and postdoctoral fellow Emma Schachner and doctoral student Robert Cieri -- and with James Butler, a Harvard University physiologist.

The research was funded by the American Association of Anatomists, the American Philosophical Society, the National Science Foundation and private donor Sharon Meyer.