It took an awful lot of clever adaptations to the produce common bird. It’s got to be light, so you give it hollow bones. It’s got to be strong, so you give it less a breastbone than a keel, to which powerful wing muscles can be attached. And it’s got to have a prodigious respiratory system—flying ain’t easy—so you give it a peculiar system of multiple air sacs serve as lungs, with the air rushing through in one direction, making a tour of each of the chambers and then exiting, rather than rushing in and out, as in our lungs.
Now, a new study in Nature suggests that something else is at work in those lungs: avian respiratory architecture may be a feature left over from the long-ago days when birds were dinosaurs. That conclusion was announced by a group of scientists from the University of Utah, who did their work not by studying birds, but their distant cousins, monitor lizards. Earlier evidence had already shown that alligators have a four-chambered respiratory system, so it was no news that monitor lizards do too. But the researchers were less interested in what the architecture of the animals’ lungs is than in how the whole system works.
The Utah team tracked the flow of air through five living sets of monitor lizard lungs (that is, still embedded in the animals) and nine that had been removed from their owners. With living animals, the researchers surgically implanted air flow monitors. With the disembodied lungs, they filled them with water and watched the movement of tiny floating particles through the lungs, as well as using air flow monitors.
The organs are complex and full of crannies, like a system of caves. But overall, the flow of air through the lungs remained one-way. And the valves that open and close between the chambers and prevent any backwash of air in monitor lizards look a lot like those of alligators and birds. Monitor lizards and alligators, it goes without saying, don’t fly, so why should their lung architecture be so similar to birds’?
One likely answer is that the birds did not evolve their respiratory system to cope with the demands of flight, but rather simply inherited it from an ancestor they share with alligators and lizards. That ancestor, the researchers write, would have been a small, cold-blooded creature that walked the Earth more than 100 million years before birds began to evolve during the Jurassic period, and was ancestor to both them and the dinosaurs.
More measurements in a greater variety of creatures must be undertaken before it can be said for sure that the one-way flow in all three modern types of animals didn’t arise independently. But evolution does tend to be parsimonious—producing an innovation once and distributing it to any critter that can use it, rather than starting anew each time with each species. If that’s so in this case, it’s just one more sign that your tiny finch or caged canary may have had some very powerful kin a long, long time ago.
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