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When we breathe out, we empty our lungs. But an ancient reptile figured out a more efficient way to breathe, which ultimately helped birds dominate the skies.

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Go to to check out their Physics of the Everyday course. [♩INTRO]. Okay everybody, breathe in.

Now let it out. Wow, that was that inefficient. We mammals empty our lungs when we breathe out, and that’s kind of a waste during that time, the body doesn’t have fresh air to pull oxygen from.

Dinosaurs breathed very differently. We’re pretty sure they had unidirectional airflow so, T. rex and its dinosaur buddies probably had cooler lungs than you. And their feathered cousins alive today still breathe this way an adaptation which helps them dominate the skies.

Unidirectional airflow works a little something like this:. When a bird breathes in, it doesn’t fill up its lungs it fills up air sacs around its lungs instead. This air then moves into its lungs when it breathes out.

So all the old air is replaced by this fresh, external stash. That means the lungs are never waiting for the next breath of oxygen-rich air. If we could get our lungs to breathe like this, we would probably upgrade our athletic performance considerably.

After all, we’d eliminate that oxygen-lacking exhalation step, and getting more oxygen is the whole point of doing physiological hacks like blood doping. And at first, that’s why biologists thought birds evolved this neat trick because the constant supply of oxygen gave them a wing up for flight. But there’s a problem with this idea:.

Alligators. Also crocodiles. And certain iguanas, and monitor lizards….

Turns out, lots of animals breathe unidirectionally. And many of them do not fly. As biologists have figured out these species breathe this way, they’ve grown less confident about why it evolved in the first place or even when.

Since it’s found in birds, crocodilians, and some lizards, it’s possible that unidirectional flow evolved over 310 million years ago in the common ancestor of all those groups maybe as a way of helping this early reptilian clear out its lungs. When we breathe, some old air tends to be left behind. And when we sit still or aren’t breathing deeply, even more air tends to linger.

Which isn’t awesome, because it can take up valuable space, leading to less oxygen per breath. If early reptiles were pretty sedentary, like many are today, this lingering air issue could have been a problem for them…. Maybe even enough of one that they evolved a way to shove it all out.

But, unidirectional breathing has only been found in a couple of lizards. So, it’s possible lizards evolved it independently of birds and crocs an example of convergent evolution. That could bump the origin of unidirectional breathing to when just those groups last shared an ancestor: about 250 million years ago.

And that’s right around the Permian Extinction, the largest mass extinction in Earth’s history. It was bad enough that scientists often just call it The Great Dying. And it just so happens that, at this time, the amount of oxygen in the air dropped dramatically.

So, some experts think unidirectional breathing helped keep this lineage of reptiles from suffocating, which could either explain how they made it through or why the trait evolved. Whether it was 250 or 310 million years ago, though, it’s pretty clear this one-way breathing appeared long before birds took to the skies. Still, it helps them dominate the air now.

Sure, there are flying mammals—namely, bats. But on the whole, they can’t fly as high or as far as birds. While breath-for-breath birds and bats get about the same amount of oxygen, birds' super-smooth breathing keeps getting them new air no matter what.

That makes them much better at flying in low-oxygen environments, like at really high altitudes, where bats would suffocate. So even if unidirectional breathing didn’t evolve for or because of flight, it sure helps birds excel at it. If all this talk of bird breathing has you craving to learn more about flight,.

I have some great news! Today’s sponsor, Brilliant can help. For instance, their Physics of the Everyday course can help you brush up on things like lift and how it allows planes to fly.

And if you’re one of the first 200 people to sign up at, you’ll get 20% off an annual Premium subscription. It’s the same learning, just for less money. So if you’re interested, head on over to and check out what Brilliant has to offer! [♩OUTRO].