← In Our Time
In Our Time
The Evolution of Lungs
00:48:24
From the mechanics of the first breaths on Earth to the reason we hiccup.
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- [00:00]
Melvin Bragg
This BBC podcast is supported by ads outside the uk.
- [00:06]
Steve Brusatte
It had never been done before.
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Jonathan Codd
Gonna launch like a rocket and land like an airplane.
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Steve Brusatte
And we got people on it. Roger.
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Emily Rayfield
13 Minutes presents the Space Shuttle. Coming soon.
- [00:25]
Jonathan Codd
BBC Sounds Music Radio podcasts.
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Steve Brusatte
This is in our time from BBC Radio 4 and this is one of more than a thousand episodes you can find on BBC Sounds and on our website. If you scroll down the page for this edition, you find a reading list to go with it. I hope you enjoy the program. Hello. The evolution of lungs and of the first breath can be traced back 400 million years to when life spread from rock pools and swamps onto land, as some fish found evolutionary advantage in getting their oxygen from. From air rather than water. Breathing with lungs may have started with fish filling their mouths with air and forcing it down into their chests like frogs do now. But slowly, their swimming muscles adapted to work their lungs like bellows. And while lungs developed in different ways, there are astonishing continuities. For example, the distinct breathing system that helps tiny birds fly thousands of miles now is the one that once allowed some dinosaurs to grow so huge. With me to discuss the evolution of lungs are C. Brusati, professor of paleontology and Evolution at the University of Edinburgh, Jonathan Codd, professor of Integrative Zoology at the University of Manchester, and Emily Rayfield, professor of Paleobiology at the University of Bristol. Emily, can you remind us what the point of lungs is? What's their main function?
- [01:51]
Chris
Okay, so the point of lungs is to essentially allow gaseous exchange. So we use. We bring our air into the bodies and we bring. With that, we bring oxygen in and we then expel air from the body and we expel carbon dioxide. And so within our lungs, gaseous exchange happens, which is the uptake of oxygen into the blood, that vessels that surround the lungs, and then the removal of carbon dioxide. And that happens across a very thin membrane where essentially high concentrations of oxygen that we breathe in diffuse into the blood, and then high concentrations of carbon dioxide that have built up in the blood then diffuse out and are breathed out.
- [02:39]
Steve Brusatte
Can you elaborate that even a bit further? What is this oxygen doing?
- [02:43]
Chris
So oxygen, as we all know, is really essential for life. It's essential for us in order to make energy. As any good student of GCSE biology will know, oxygen is needed for respiration. So this is the process of a reaction where we have glucose from food, which combines or reacts with oxygen to produce carbon dioxide and water and also energy in the form of ATP. So that's adenadine triphosphate and this process happens within the cells of our tissues. So the oxygen that diffuses into our bloodstream is picked up by our red blood cells, and then it's carried around the body via the heart to the tissues where it's needed for this reaction to take place.
- [03:29]
Steve Brusatte
And you have to get rid of the carbon dioxide, and then you need.
- [03:32]
Chris
To get rid of the carbon dioxide, because if carbon dioxide is allowed to build up, it's toxic to tissues.
- [03:37]
Steve Brusatte
Without lungs or even gills, how would the first organisms get the oxygen they needed?
- [03:44]
Chris
Okay, the first organisms, first animals, for example, that we would see would be small. And with small organisms, you have a very. What's called a large surface area to volume ratio, and that enables diffusion to occur simply across the boundary of these organisms. So the process that's actually happening in our tiny membranes, in our lungs can happen in a tiny organism just across the exterior of that organism.
- [04:08]
Steve Brusatte
I see. Thank you, Steve. Life began in water, we understand, and gills were a way to get dissolved oxygen out of the water. Yet some fish had air sacs. Why is that?
- [04:22]
Jonathan Codd
We have lungs. We use our lungs to breathe oxygen. It's very normal for us. We breathe air. And when you think of fish, you think fish live in the water, they have to get oxygen somehow. How do they do it? They do it with gills, and that is what's normal for fish. But many fish also have lungs, which is a very strange thing to think about. But some fish use lungs as a secondary way to get oxygen. They can actually gulp air. Now, those lungs that some. That a few fish have are basically equivalent to another structure that a lot of fish have that's called a swim bladder. And this is something that probably, we don't know for sure, but probably first evolved many hundreds of millions of years ago in fish to help control their buoyancy. They could gulp some air or diffuse some air from their blood in. Into this swim bladder, which is basically a balloon inside of them. And that air, if there's more or less, could help them go up or down as they were swimming. And then at some point, that swim bladder, at least in some fishes, including in the fishes that were our ancestors, that swim bladder developed the ability to exchange gases, to actually take in oxygen to release carbon dioxide. And then you had a proper lung. But the first lungs would have evolved, not in animals that were living on land. They would have evolved in fishes to help those fishes breathe better in the water.
- [05:42]
Steve Brusatte
Do we know exactly how they did that? So they're in the water. Do you want to say a Little bit more about that.
- [05:47]
Jonathan Codd
There's so much we don't know, of course, because these were things that were happening in evolution many hundreds of millions of years ago, probably more than 400 million years ago. But what we do know about evolution is that it's something that doesn't plan ahead. It's something that acts in its time and place as organisms have to become fit to their environment or not survive. That's the basics of natural selection. So these swim bladders and these fishes, and many fishes today have these swim bladders, use them to control buoyancy. But at some point, some fishes develop the ability to also get oxygen through those swim bladders. And you can imagine those fishes, there would have been benefits to that. They would have still had gills, they would have got oxygen from their gills, but they could get extra oxygen through these swim bladders. And that's where lungs came from. And then some of those fishes moved on to land, changed their fins into arms and legs, and those lungs helped them colonize the land.
- [06:42]
Steve Brusatte
And that's fascinating. Do we know more specifically how that happened?
- [06:46]
Jonathan Codd
We don't really, because lungs do not normally fossilize. Lungs are very soft, very pliable. They'll break down almost immediately when an animal dies. So it's not like we have a fossil record of the lungs. Occasionally we can tell from the bones that maybe a certain type of lung was there. But we do know certain things about the genetics of lung tissue today. And we know that the swim bladders that many fishes use to control their buoyancy is the developmental equivalent, the evolutionary equivalent to our lung.
- [07:15]
Steve Brusatte
Is it an extraordinary thing that this happened or do other things like that happen in the past with other objects, animals and so on?
- [07:22]
Jonathan Codd
All kinds of things happen in evolution. The Earth's four and a half billion years old. Life's been evolving for at least four billion years. And a lot of these major transitions, we've talked about some of these before. You know, you and I have talked about the origin of birds on this show, Emily. And you and I have talked about those fish moving on to land on this show. And so many times when there's a major transition into a new environment, there are certain features that had already evolved for another reason that are co opted in order to be used in that new environment. And that unlocks the potential for organisms to make an evolutionary jump.
- [07:55]
Steve Brusatte
Thank you very much, Jonathan. To push us a bit further, do we know how and when breathing developed?
- [08:01]
Melvin Bragg
Yeah, we have some idea. As Steve said, it's very hard to reconstruct the Evolution of soft tissues like the lung. But what we think is that sometime around 400 million years ago in the Silurian period, we know from the geological record that it was a period of great fluctuations in rainfall and water availability. And so what that does is if you've got lower water content, where all the fish are living, that water column is going to get warmer, that's going to reduce the amount of dissolved oxygen in the water. So what you've got there is you've got a selection pressure. So any way of supplementing the oxygen you can get into your body is going to be advantageous. And so some of these fish that were pre adapted for breathing oxygen, whether that was swallowing air into their stomachs or exchanging across the lips, these guys had an advantage. And so that's basically the beginnings of what we see with terrestrial lungs in animals. Was that kind of selection pressure occurring.
- [08:50]
Steve Brusatte
Can we be even more specific about the hinge moment of this?
- [08:54]
Melvin Bragg
I don't think there's ever a specific time point. It's always a very gradual change with evolution. So what we know is at some point these animals were getting an advantage from being able to breathe air that the other fish didn't have. So any of these animals, do we.
- [09:06]
Steve Brusatte
Still recognize these fishes?
- [09:07]
Melvin Bragg
There are still armored fish around. So yeah, we do see some of these fish around. I mean, most fish still have swim bladders. And we of course have examples of living fish, things like lungfish that actually have lungs as well. So basically what we see in modern animals that are still living today is any fish that lives in an environment where it experiences fluctuations in water level, these fish tend to have adaptations for breathing air, like the lungfish of South America or Africa.
- [09:30]
Steve Brusatte
Why did that happen then?
- [09:31]
Melvin Bragg
Like I said, it's all linked to the, to these changing environmental conditions. So basically when these animals were experiencing these low levels of water, as soon as water levels drop, temperature increases in the water column. And what that does is reduce the amount of dissolved oxygen in it. So that gives you an advantage if you can breathe air over the other fish that are in that environment. So these animals that could breathe air with their kind of putative proto lungs, if you like, would have been, have an advantage over these other fish and they would have been able to take, survive better, basically, and reproduce better. So that's why they would have been selected.
- [10:00]
Steve Brusatte
What happened to the gills?
- [10:03]
Melvin Bragg
So even us as embryos, we still have evidence of gills when we're embryos. So most vertebrates, you waved your hand.
- [10:10]
Steve Brusatte
At your ears is that they're in.
- [10:12]
Melvin Bragg
This kind of area, I suppose you.
- [10:14]
Jonathan Codd
Don'T have gills right now?
- [10:15]
Melvin Bragg
I don't have gills, no, I don't have gills. Despite my last name, I don't have gills, but yeah. So we still see embryological evidence of our kind of evolutionary history. Basically in the embryos, even of humans, you'll see at very early stages, you'll see gill slits appearing in the embryos.
- [10:28]
Steve Brusatte
Can you give us some idea what muscles and mechanics had to change when you move from water to land?
- [10:35]
Melvin Bragg
So the big one is that all these muscles along the thoracic cavity, so along your rib cage, these are involved in locomotion. And essentially what they have to do is become co opted to being involved in respiration. So that's one of the biggest changes that occurred, along with kind of the similar time when these animals were evolving the ability to breathe air. They had to be undergoing these changes where the muscle function and the muscle activity patterns could change as well, so that they could be controlled by a different centre of the brain that was responding to changes in carbon dioxide level, which is what the trigger is for us breathing. So that was one of the big shifts that happened, is we had to have these muscles that have evolved for moving when you're swimming around the place, to actually being separate from the locomotor system, to just being involved in the movements associated with breathing.
- [11:18]
Steve Brusatte
Something that all lungs have in common, I understand, is surfactants. What are they?
- [11:24]
Melvin Bragg
So surfactants is one of those systems that was basically got right very early on. So what surfactant is, it's this complex mixture of kind of lipids, proteins and some neutral lipids. And we find surfactant everywhere. We have an air liquid interface in any respiratory surface. So across any lung, no matter what the animal is, you find a surfactant system. And you also find it in the swim bladder of fish. Some studies were done a few years ago where they looked at the origin of the different proteins that are involved in surfactants and they found that all surfactants share a common single origin. So the surfactant system is one of those systems that predates the evolution of the lung. And all air breathing animals have a surfactant system. What surfactant does is it lines the air liquid interface, like I said, and, and it helps the animal overcome surface tension and so surface tensions. You're probably all familiar. If you've ever floated a needle on the surface of a glass of water, that's surface tension. And when you have something like a lung, which is incredibly Folded has a huge surface area to volume ratio. The tiny force of that surface tension actually becomes a massive force that the animal has to overcome to get the oxygen in and the carbon dioxide out. So overcoming surface tension is one of the key functions of lung surfactants. And as I said, we find different mixtures and different constitution types of surfactant in all the different air breathing animals that we see today.
- [12:35]
Steve Brusatte
Emily, can you tell us about what's called buccal pumping and who did it and why it was important?
- [12:41]
Chris
Yes, absolutely. Yeah. So buccal pumping relates to the depression of the mouth cavity. So if you imagine like a video, the footage of a frog, and you can imagine the floor of the mouth is going up and down and essentially what buccal pumping is is kind of depression of the floor of the mouth which enables at the same time that air is being brought in your frog, for example, into the mouth cavity through the nostrils. And then as the floor of the mouth is raised, then that forces air into the lungs. And at the same time as the mouth is depressed, air is moved out of the lungs again. So it's sort of this two way system of bringing air into the body by creating a negative pressure and then forcing that air back into the lungs. Frogs do it and other types of amphibians as well too. There are a number of fish that do it as well too. It's one way that you can actually get water into the mouth as well and over the gills and also as well, from what we think as well is that probably the earliest animals with limbs and digits to move onto land were also likely buccal pomping as well too. So using that as a mechanism to bring air into their lungs through kind of depression of the mouth and then forcing it into the lungs themselves.
- [13:53]
Steve Brusatte
You wanted to come in, Jonathan?
- [13:54]
Melvin Bragg
Yeah, I was just going to say one of the reasons we still see buccal pumping in frogs today is that most frog species don't have ribs. And the reason for that is quite simply that if you're jumping around the place your rib cage, you're going to break your ribs if you land and hit things. So frogs have no system like we do to sort of change volume with expanding and contracting our rib cage. So the way they have to do it is, as Emily was explaining, by enlarging this buccal cavity and then closing off the nares and then forcing that air into the lungs. So they have to have some system of creating the negative and positive pressures that they need to breathe. And in frogs, they do that with their buccal cavity in the absence of a rib cage.
- [14:27]
Chris
Yeah. And picking up on that as well, too. If we go back to some of these early fossils, animals that were the first to come onto land, we've already established that these animals had lungs by virtue of their deep ancestry within the Barney fish, and so they had the capability to breathe that way. They also, these very earliest animals that were at the cusp of coming onto land also still had gills as well, too. And we know that because we see some of the skeletal supports of gills actually being preserved. But what seems to not be the case for these very early animals coming onto land was that they were not capable of being able to expand and contract their ribcage in the same way that some of the later, more terrestrial animals were able to do.
- [15:08]
Steve Brusatte
Steve, can you tell us a bit more about the function of the rib cage before we move on?
- [15:13]
Jonathan Codd
So when we breathe, I'm breathing now, we're all breathing. Now, mammals like us have a particular way of breathing. We breathe in, we breathe out, and our ribs and the muscles around our ribs power a lot of that. So that's why we're not breathing like a frog in the same way. And I think what really interests me about this conversation, and we're getting there, we're leading up to more of it, is just the variety of different types of lungs and different ways of breathing that animals have today. So we breathe very differently than a frog, very differently than a bird.
- [15:44]
Steve Brusatte
We have fossil records of dinosaurs appearing many millions of years after the movement onto land. Can we say something about dia lungs, how they operated?
- [15:54]
Jonathan Codd
Believe it or not, we can. And I still find this remarkable because as far as I know, and Emily can correct me if I'm wrong, Emily studies dinosaurs as well, as does John occasionally. I don't think there's any. Has anybody. I think in some fossil birds, maybe somebody's claimed to find a little film of fossilized soft lung. But I mean, do you know, have you ever heard of anybody claiming a.
- [16:14]
Chris
Fossil lung, not a dinosaur?
- [16:16]
Jonathan Codd
Yeah. So you would. And it makes sense, right, because lungs are very, very, very soft. They decay very quickly. It's bones, shells, teeth, the hard stuff that normally turns to fossils. So you would think it's impossible to know about the lungs of these long extinct species, but. But different types of lungs occasionally can leave marks on the bones or there's aspects of the bones that tell you what kind of lungs were there. And so in some dinosaurs, we can actually tell that they had the very sophisticated and Very peculiar lungs that birds have today, which are very different from our lungs. And that's because bird lungs, and we can talk about why this is. But bird lungs have air sacs that extend off of them, basically a bunch of soft little balloons that expand and contract off of the bird lungs. We do not have that. And in birds those air sacs often invade the bones. They go into the bones, they create a hollow cavity in the bones. We see those same hollows in the exact same places, the exact same bones on many dinosaurs, including T. Rex. So that tells us that these dinosaurs had bird like lungs, even though we don't have the actual fossil lung to look at.
- [17:22]
Steve Brusatte
But the thing that is one of them, but the many things that is amazing is that very small birds flowers thousands and thousands of miles on the same principles that the dinosaurs trundle along.
- [17:33]
Jonathan Codd
It's incredible, isn't it? And not, you know, some birds can migrate tremendous distances from the Arctic to the Antarctic. Other birds fly over the Himalayas. I mean that's more than 30,000ft if we're in an airplane and the cabin depressurizes. It's never happened to me. Fingers crossed. But you know, the oxygen mass comes down because we, our mammal lungs cannot breathe at that altitude, but birds can. And the reason is that bird lungs can take in more oxygen than our lungs, than mammal lungs. And the reason for that is that the lungs are constructed completely differently. And so our lungs, our mammal lungs, are basically a bag, a sack. We breathe in, we breathe out, inflates, deflates. But a bird lung is more like a set of pipes or a set of straws where air only can go through in one direction. And birds can take in oxygen when they breathe in, when they breathe out. It sounds impossible, I know you have to kind of think through it, but it's those air sacs that store extra air that act as bellows to make sure there's always oxygen rich air going over the bird lungs. That's why birds can fly at such high altitudes today.
- [18:41]
Steve Brusatte
Jonathan?
- [18:42]
Melvin Bragg
Yeah, so the other interesting thing about bird lungs is that for each breath a bird takes, it breathes twice.
- [18:47]
Steve Brusatte
What does that mean?
- [18:47]
Melvin Bragg
So for the unique construction, as Steve was saying about the bird lung, they have this rigidly fixed lung and it's surrounded by this series of air sacs. Some birds have six air sacs, some birds have 14. No one really knows why, but what happens is when birds breathe for the first time, air goes in, it actually bypasses the lung and goes straight to the posterior air sac. And that's the first inspiration. And then you get the first expiration, which is when that posterior air sac will change. And then the air actually goes through the lung, as Steve said, in a unidirectional way. You then get the second expiration where that air gets drawn out into one of the thoracic air sacs. Inspiration. Sorry. And then you get the second expiration where they actually comes out to the outside. So birds are breathing twice for each cycle of air moving in and out of their bodies.
- [19:25]
Steve Brusatte
You've compared them, or somebody did, around this table to dinosaurs?
- [19:28]
Melvin Bragg
Yeah, well, I mean, dinosaurs, I mean we now, I think the commonly accepted status of science is that birds are dinosaurs. They're just the dinosaurs that are still with us. And so, yeah, what we know, and as Steve was suggesting, we have some good evidence from the fossil record. But the other thing we know is that there are still crocodilians around. So we know that birds and crocodiles, that living members of birds and crocodiles are the closest living relative of each other. And we know that all the dinosaurs are somewhere in the middle. So all these theropod dinosaurs are somewhere in between these two things. So we can use this technique where we can look at the lungs of crocodiles and we can look at the lungs of birds and we can look at similarities that these two things share. And then what we can do is we can reason that any common ancestor of these animals must have had the same, the same features. And so we know because the theropod dinosaurs, bracketed by crocodilians and birds, that they shared some of the characteristics of both their lungs.
- [20:14]
Steve Brusatte
What were they?
- [20:15]
Melvin Bragg
So both birds and dinosaurs have multi camera lungs. So that means they're incredibly folded internally. And that's a kind of common trait.
- [20:22]
Steve Brusatte
You see what folded internally mean, it.
- [20:25]
Melvin Bragg
Basically, it literally means that the tissues are kind of convoluted and instead of being like a simple bag, they're kind of, you know, they've got lots of folding in them. Kind of like a concertina. Yeah, like a concertina, yeah. And that's basically a mechanism, as Emily touched on earlier, to maximise the rate of diffusion that occurs. And so, yeah, so what you see in an animal that needs a high aerobic capacity, so an animal that's doing a lot moving around, they have a real need for oxygen. And so what you see in these animals are complex lungs. And we see examples of that in both living alligators and in modern birds.
- [20:54]
Steve Brusatte
Emily, do you want to tell us a bit more why they hide this air inside their Bodies or where they put it there seem to be a lot of places, don't they?
- [21:02]
Chris
Yes. So, Steve, Stephen, are we talking about dinosaurs? Yeah. Yes. Essentially these tissues grow out from the, the lung cavity. And some of the evidence that we can see in fossils are these holes in some of the bones, so particularly in regions of the backbone and also in some animals as well, in the limb bones as well, too. So these are tiny holes called foramina, and they're like 2 to 3 millimeters in size. But they, you know, they're not, they don't like a break. They're a bit too big to be something like nerves, for example, and they're kind of nice and rounded around the edge. So they're the same kind of shape. And in the same places as that, we see some of these kind of what we call pneumatic features in what.
- [21:46]
Steve Brusatte
Is a pneumonic feature.
- [21:47]
Chris
Yeah, so these are basically, they're pneumatized, so they're filled with air. And we see these features in not all, but many living birds. And we also see these same kind of tiny marks and holes in the bones of fossils as well, too, within dinosaurs, but also within pterosaurs as well too. So extinct large flying reptiles that are quite closely related to dinosaurs. So that's quite good evidence to suggest at least the earliest ancestor of those two groups potentially had these kind of features where air sacs were growing into the bones and providing this extra space and potentially enabling this special unidirectional flow that we see in living birds.
- [22:27]
Steve Brusatte
Steve, when you consider bird lungs now, how do they compare with some of those of the dinosaurs?
- [22:34]
Jonathan Codd
Some dinosaurs had the same lungs as birds for the reasons Emily was just talking about. We can see those holes in the bones that then lead into hollow cavities inside the bones that are dead ringers, exact perfect matches for those places in birds today where those air sacs that stick out of the lungs go into the bones. So we can be sure that a lot of dinosaurs had similar lungs. And the pterodactyls, the pterosaurs that Emily mentioned, the flying reptiles, but not all dinosaurs might have had those lungs. There might have been some variability. Some dinosaurs don't have those holes in their bones that correspond to the air sacs.
- [23:08]
Steve Brusatte
Just to make things complicated, just the.
- [23:10]
Jonathan Codd
Dinosaurs lived for over 150 million years. They still live now as birds. But the canonical dinosaurs, they lived for such a vast time, they had such incredible diversity. They did lots of things. Some of them had very bird like lungs and included among Those dinosaurs are the meat eating dinosaurs, the actual ancestors of birds, the Velociraptors and so on, but also the giant long necked dinosaurs, Brontosaurus diplodocus. They had those air sac holes in their bones. That means they had bird like lungs. And many paleontologists think, and I agree with this, that that is one of the essential ingredients that allowed those dinosaurs to become so giant. Because those lungs, the bird like lungs, which we know from today, are very efficient, as Jonathan was talking about, they take in basically twice the oxygen, oxygen when you breathe in, oxygen when you breathe out. And if you can take in a lot of oxygen, that speeds up your metabolism, speed up your growth, that can help you get bigger. And I think that was a fundamental feature that allowed some of those dinosaurs to be the very biggest animals that have ever lived on land, and much bigger than any mammal that's ever lived on land. Because mammals like us have those simpler lungs that are just bags that breathe in and out.
- [24:22]
Steve Brusatte
You want to come in, Emily?
- [24:23]
Chris
Yeah. As Steve was saying, some of these dinosaurs that had these air sacs were actually extremely large. And another benefit of being able to literally scrape out hollows within the tissues of your bones is actually lightening those tissues. And we think of this as a feature associated perhaps with birds and with flight. But actually when you're dealing with animals that were, you know, 6 tons, 20 tons, for example, actually being able to remove as much unnecessary bone as possible is actually really helpful in terms of the energy it requires to literally carry your skeleton around. And so I think there's also an issue there with tissue being lost within the bones themselves, perhaps as the air sacs are invading, but also because it's simply not needed. There's bone tissue where it needs to be to structurally support the skeleton, but the skeleton's removing excess tissue that it doesn't need to carry around, but it's not needed for structural purposes. So air sacs serve this useful kind of tool as well, of helping to lighten the skeleton in these really big animals as well as obviously the skeleton of birds eventually becoming extremely light as well too.
- [25:27]
Steve Brusatte
Jonathan, the act of breathing calls for the body to lower the pressure in lungs in order to draw in air. Can you tell us how that developed in what had to change?
- [25:38]
Melvin Bragg
So in mammals, we see the evolution of the diaphragm, which is basically the principal breathing muscle. And so what happens in mammals is as the diaphragm contracts, it reduces down and this expands the volume of the ribcage. And as the diaphragm contracts down, you have the expansion of the rib cage outwards from the intercostal muscles. And so breathing is all about the changes and differences between pressure and volume. So you increase volume, you decrease pressure. That creates an inward flow of air from the outside and the animal will breathe in. And when you want to breathe out, the diaphragm relaxes and it then contracts up into that space, which reduces the volume, which increases the pressure, and the animal will breathe out.
- [26:13]
Steve Brusatte
Is this as efficient or less efficient than the other ways of breathing?
- [26:17]
Melvin Bragg
So efficiency is a really interesting concept to discuss. Bird lungs are more efficient than other lungs in the sense that they can extract more oxygen for a given breath. But efficiency is one of those measures in biology that's really hard to quantify because it depends how you define it. So if your aim is to breathe effectively, then all lungs are as efficient as each other. But if you want to extract the maximum oxygen out for each breath, then bird lungs are more efficient. It just really depends how you want to define it and what the animal is interested in doing.
- [26:43]
Steve Brusatte
If you like, what about our lungs? How do they rate?
- [26:46]
Melvin Bragg
Our lungs are pretty good. Yeah. So mammal lungs are pretty efficient. One of the interesting things is that if bats didn't exist, people would be writing papers about how bats can't fly because they have a mammal lung. And so there's a sort of idea that at some point you're going to hit a ceiling in the efficiency of how a mammal lung can work at extracting oxygen. But we have bats that fly around the place, and bats have what we in the sort of respiratory biology field term, the sort of the Ferrari of mammal lungs. So they've really pushed these lungs to like their absolute maximum. The lungs are huge. They have incredibly thin blood gas barriers, which is the distance the oxygen has to be transported. And they have incredibly efficient lungs that enable them to fly. And some bats can fly very long distances, not quite as far as some of the birds fly, but some bats can migrate over thousands of kilometers.
- [27:30]
Steve Brusatte
Where do animals like lizards fit in, Emily? And why are they so different?
- [27:35]
Chris
So lizards are interesting because they were thought for quite a long time not to have air sacs. And also there was thought to be. If you imagine a lizard moving, it moves from side to side. And it was self suggested that maybe this place to construct strain on them being able to move and breathe at the same time. Because if the body was sort of constricting on one side and then the other, this might actually force air from one lung to the other rather than enabling air to be taken in and out as is needed for respiration. But actually we know from at least a number of lizards that they're able to also buccal pump as well too, to be able to force air from the mouth and throat into the lungs so that they, they're not necessarily constrained by either moving or breathing at the same time. And also some lizards as well that people have looked at in more detail, like large monitor lizards, so that's the group that things like Komodo dragon belong to and other pretty large for modern day terms, lizards as well, they have air sacs too, or they have a type of air sac. It's not like as extensive as what we see in birds, but they do have some projections that come off the lungs as well too, that supplement breathing.
- [28:46]
Melvin Bragg
Jonathan yeah, so it's really interesting idea that Emily's talking about and it's this concept of breathing in locomotion. So as Steve said earlier, we all evolved from fish. And so when a fish is moving, it's using all the muscles of its ribcage to move its body from side to side as it's swimming. And the problem that all kind of terrestrial vertebrates have is that we're now using these muscles for breathing. So you have this constraint, which is known as Carrier's constraint, because Dave Carrier in the University of Utah is a fan, first person to describe it. And this basically means that for a lot of animals, breathing and moving at the same time is not as straightforward as it seems. And there is actually a species of iguana that lives today that can't do both things. So it can move and then it has to stop to take breaths and then it can move again. And what we see in all the animals, in things like birds, in things like crocodilians, in things like some lizards, as Emily was suggesting, they've evolved these accessory breathing mechanisms to enable them to circumvent this mechanical constraint of breathing during locomotion.
- [29:34]
Steve Brusatte
Steve, can we look at, are there different forms of lungs in different animals and how does that affect them?
- [29:39]
Jonathan Codd
If there are, there's a great diversity of lungs and we've been touching on some of them and really all lungs go back to this ancestral swim bladder in fishes. They came from this quite simple ancestral state many hundreds of millions of years ago. And since then, fishes in the water and then when fishes moved on to land, have diversified into all different types of environments, all different types of lifestyles. Of course, the Earth has changed, climate's changed, the atmosphere has changed. And over these hundreds of millions of years, lungs have adapted, just like eyes and skin and arms and legs and so on have adapted. And that's why you see this diversity today. We're talking about how frogs breathe, we're talking about how mammals breathe, how birds breathe. It's an incredible diversity. They're all lungs. They all share a common heritage in an evolutionary sense. They all do the same thing. They all got to take in oxygen so we can metabolize. But the variety is astounding. And even Darwin himself wrote about this back in the 1850s and 1860s, the variety of lungs and how incredible it is.
- [30:42]
Steve Brusatte
Is there an ideal form, Jonathan, to turn to you, the ideal lung?
- [30:48]
Melvin Bragg
I'd say no. I'd say no, there isn't an ideal form of the lung. I think what you see is there's just different solutions to the same problem. So all the animals are not sitting there. So frogs are not sitting there wishing they had a bird lung so they could fly over Everest. Frogs are perfectly happy in their own world, in their own niche, and they're very, very successful at it. One of the analogies I use when I'm talking to the students is it's a bit like if you wanted to go from London to John o', Groats. If you take your Ferrari, you get there quicker. If you take your Morris Minor, you still get there. You just get there slower. So if the goal is to get to John o', Groats, there's no difference between a Ferrari and a Morris Minor. If you want to get there faster, you take the Ferrari. So that's what we see in terms of the bat lung. They've got the Ferrari lung. Other animals have a Morris Minor of a lung, but at the end of the day, they're still efficient at exchanging the oxygen the animal needs for its metabolism and for it to live its life in the niche that it occupies.
- [31:33]
Steve Brusatte
You want to come in, Emily?
- [31:35]
Chris
Yeah, I want to. Thing that we. That we haven't touched on yet is much is what's called cutaneous respiration. So that's essentially breathing through your skin. And this is something that we see in modern amphibians are able to do that frogs and some salamanders, and that's another way in which they supplement the oxygen into their tissues. You know, they're still breathing and they're still using lungs, but they're also using this cutaneous respiration as well to.
- [32:05]
Melvin Bragg
Yeah, and that's actually something that almost all animals can do, including us humans. We can exchange gas, carbon, Dioxide across our skins. Bats do it across their skin membrane in their wings. And so it is a really important part of understanding the sort of evolution of breathing structures is that lots of animals have different ways of breathing and exchanging gases. And skin breathing is something that lots of animals make use of.
- [32:25]
Jonathan Codd
Can I ask, so we can do that, in what situation? Which skin is it? My bulb, the skin on top of my head. Like that's news to me.
- [32:35]
Melvin Bragg
It's just happening all the time. So anyway, you've got blood vessels on the surface of your skin, you're going to have gas exchange occurring in there. It's, it's in us, it's going to come nowhere close to meeting any of our metabolic needs. It's just something that happens. But interestingly, there are animals like these salamanders that don't have lungs at all. So everyone thinks that, everyone has this idea that evolution is heading in a particular direction or something, which is completely sort of the wrong way to think about it. But you would imagine once you can breathe air, why would you want to stop breathing air? And yet there's a species of salamander that lives in Thailand that is lungless. It doesn't have lungs at all. And this animal, it doesn't move around a lot. You wouldn't be surprised. But it lives in very cold, very fast flowing rivers. And in a similar way that when water is very warm, it has less oxygen in it. When water is very cold, it is densely packed with oxygen. So these animals live in an oxygen rich environment and they've actually experienced a selection pressure where their lungs are no longer useful for, for them and they've evolved to live without them.
- [33:27]
Steve Brusatte
Steve, is there any way in which the lungs are still developing that you can note and pass on to the rest of us?
- [33:32]
Jonathan Codd
I'm sure they are. It's always tricky to ask how are structures evolving in the present because we don't have the benefit of hindsight really. When we study evolution, we're looking at things that have happened in the past and we can reconstruct the process. But certainly the world is changing very quickly. I mean, our atmosphere is changing very quickly. Lungs have been adapting for a long time. So I could maybe speculate, I'm sure we could all speculate on maybe how we think lungs are going. But if we are going into a much warmer planet, a planet with a lot more carbon dioxide in the atmosphere, that's probably going to have an effect on the lungs.
- [34:08]
Steve Brusatte
What effect did you predict?
- [34:10]
Jonathan Codd
If I had to predict, and this, I mean, paleontologists like Emily and me, we look into the past. We're much more comfortable looking back than predicting forward usually. But my, I suppose my gut feeling would be that it usually takes longer for the gross structure of something to change. But the biochemistry of it, the details of the gas exchange interface, those smaller things that can maybe change on a quicker time scale under natural selection, might change in an atmosphere where there's going to be and is right now a lot more carbon dioxide.
- [34:42]
Steve Brusatte
Okay, Jonathan, back to you there. What would you most like to discover about long evolution in the context of your work?
- [34:49]
Melvin Bragg
So in my work, it would be great if some of these palaeontologists like Emily and Steve could find some fossilized lungs. That would be amazing. That would give us some of the tough call. Okay, that would give us some of the concrete answers. But yeah, I think in the absence of that, which is incredibly unlikely, it's still fascinating to find some of these, the evidence for these respiratory systems in the absence of the lungs themselves. So looking for structures like pneumatization of the vertebrae, so these little air holes that we find in the different bones that are analogous to what we see in birds, that's fascinating. Looking at the occurrence of accessory breathing structures in the fossil record. So one of the neat things we find is that things like velociraptors and birds actually have uncinate processes on their ribs and they look almost identical. And so this tells us that the mechanics of breathing in these two animals are very similar. And that's something that we can get from looking at the fossil animal and looking at an extant living animal that's around today to help us understand how it breathes.
- [35:40]
Steve Brusatte
Do you want to come in on that for me?
- [35:42]
Jonathan Codd
There's so many things that I'd love to know about extinct animals. And I guess as we say in America, maybe this is a little bit like inside baseball. But for me as somebody who studies dinosaurs, you know, we have a lot of information that meat eating dinosaurs, the long necked dinosaurs and the pterodactyl cousins of dinosaurs, had these bird like lungs because they have those holes in the bones that were caused by air sacs. But the other dinosaurs, ones that haven't come up in conversation yet, Triceratops with the horns on his head, Stegosaurus with the plates on its back, the duck billed dinosaurs, they don't have those marks on those bones. So what kind of lung did they have? Did they have that bird style lung but without so many air sacs, did their ancestors have a bird style lung? But they lost it, they simplified it. Did they have a completely different lung altogether? We don't really know that. And that's a huge amount of dinosaur diversity. Some of the most familiar dinosaurs of all and, and we really don't know how they breathed.
- [36:40]
Steve Brusatte
Do you want to take that up?
- [36:42]
Chris
Yeah. In terms of the evolution of lungs, I think one thing that's really interesting is when did animals in the fossil record switch from using buccal pumping to rib based breathing? Because that actually has a whole series of kind of related consequences as well too, to do with things like the size of the head and perhaps even the ability of these animals to feed as well too. So if you're, imagine if you're creating a cavity with your, with your mouth and with your throat maybe as well too, it is beneficial probably to have quite a wide head. But the downside, if you imagine of having a wide head is that all the muscles that close your jaws are kind of angled outwards a little bit and they're not as efficient as they would be if they were angled straight up if you had a narrower head. And so there's been some really intriguing suggestions that actually if you were able to evolve rib based breathing, then you're kind of freed from this constraint of having a big wide head. And that means you can have a narrower head, your muscles are more aligned. And that might have been one of the things that then facilitated things like evolution of plant eating, herbivory. Because to do that you need to be able to, you know, cut through plant matter rather than just kind of grab something and eat it. So there's all these kind of knock on related consequences. And there's been some intriguing suggestions from looking at living animals about how the ribs start moving against the backbone that might give us some clues to what to look for.
- [38:05]
Melvin Bragg
Yeah, I was just picking up on Emily's point. One of the interesting things that a diaphragm lets you do is it lets you create different pressures in your thoracic cavity and your abdominal cavity. And there's a suggestion that in humans one of the things that evolving a diaphragm did was actually allow the birthing of preferentially large headed babies. So our big thing is our brain. So we have big brains, we have big heads. But actually giving birth to a large headed baby session is not straightforward. But one of the things the diaphragm actually does is it allows you to create high pressure and low pressure in your thoracic cavity and different pressures in your abdominal cavity. So you can make extremely High pressures in the abdominal cavity for birthing that you wouldn't be able to do if you didn't have a diaphragm.
- [38:42]
Steve Brusatte
Is a diaphragm getting in the way of things now, Steve?
- [38:45]
Jonathan Codd
Well, diaphragms are things that we have, of course, as we've been talking about, and other mammals have them and you can more or less. And I'd be curious to hear you guys opinion on that. But more or less, I think tell in the fossil record where it's coming in based on the shape of the backbone and the ribs and which backbones have ribs and so on. So it seems like, at least from my understanding, that the diaphragm came in fairly early in mammal evolution as some of the ancestors of mammals. Emily's studied these animals more than I have, but that's essential to the way that we breathe. But that's very different from a bird, very different from a crocodile. And I think it just illustrates the point that lungs, we think of lungs, we think of our lungs, but it's really just one of many types.
- [39:26]
Steve Brusatte
We're coming to the end now. Is there anything that you would like to add to the development of lungs? Do you see lungs developing? And now over the next, people like you can even talk about 200 million years as if it was yesterday afternoon. So maybe you can give us an idea here.
- [39:40]
Chris
Yeah, I think one of the interesting points building on from what Steve was saying about increasing levels of CO2 in the atmosphere is that potentially raises a challenge for actually acquiring sufficient amounts of oxygen. So maybe, you know, that creates a challenge for creating enough surface area in order to.
- [39:59]
Steve Brusatte
What does that mean?
- [40:00]
Chris
So if you imagine, so imagine your small animal like we talked about, you're able. It's, it's surface area to volume ratio is such that the volume is proportionally not that big compared to the surface area. But as, as animals, objects increase in size, your surface area increases at a slower rate than your volume does. And so your volume gets proportionally bigger. Which means that if you're trying to, you can't diffuse stuff across that surface anymore. And you need to kind of develop these sort of intricate surfaces with a bigger surface area. So they're things like lungs, but they're also things like, you know, feathery gills, for example, or, you know, insects have a kind of special lung system as well too that help them breathe. So it's expanding that amount of surface area to which you can actually absorb and diffuse gases across the final word.
- [40:46]
Steve Brusatte
Dorothy?
- [40:47]
Melvin Bragg
Well, I think it's fascinating. One of the. One of the most interesting facts that I always talk to the students about is we've all hiccuped, right? You've all had the hiccups at some point. And there was some interesting work done a few years ago that actually traced the origin of the hiccup. And basically you can trace it all the way back to these early amphibians. And so what these guys were doing is it was a kind of mechanism where they would take a huge gulp of water, close the glottis off, and then rapidly force the water across their gills. So this was a mechanism basically of getting a little boost when they needed to perhaps escape a predator or something. And what's interesting is that the same neural control mechanisms still occur in us today, and that's why we get hiccups in us. It manifests as, like, a weird signal to our diaphragm, so you get that kind of awkward, painful breathing. And it's quite. You always feel slightly frightened when you have hiccups. And it's because essentially our brains have harked back to our earliest ancestors, and that's why we're hiccuping. And what's interesting, the other thing about it is that the way we get over it, the way we stop hiccuping, is we essentially think about something else and we trick our brain, kind of like resetting your computer. We basically reset it back to the breathing mechanism that we use normally to stop us hiccuping. Like our sort of amphibian ancestors.
- [41:51]
Jonathan Codd
We all learn things in our time.
- [41:54]
Steve Brusatte
Yeah, I know. To stop hiccups, think about something else. Well, and more than that. Thank you. That was terrific. Thank you. Emily Rayfield, Steve Brusetti, and Jonathan Codd. Next week, it's the earliest surviving poem in Old Scots, John Barber's the Bruce, his epic medieval tale of Robert the Br and the Battle of Bannockburn. Thank you for listening. And the In Our Time podcast gets.
- [42:19]
Chris
Some extra time now with a few.
- [42:20]
Steve Brusatte
Minutes of bonus material from Melvin and his guests, really asking each of you what you didn't have time to say that you'd like to have a time to say. Can we start with you, Jonathan?
- [42:31]
Melvin Bragg
Can't remember what I said.
- [42:34]
Steve Brusatte
Well, Melmen, did you want to start?
- [42:36]
Chris
Yeah, sure, I'll start. I can think of it. There's a couple of things, so I'll start with. One is that in terms of trying to see where air sacs might have appeared in fossils, there's also some other intriguing evidence that we might be able to see in the bones That a new tissue type that people have identified recently that's been termed pneumosteum. So basically like breath bone or lung bone as we think of it, imagine it's a bony tissue, but it's got dense tiny fibres and it's been identified in bones that sit adjacent to where the air sacs are. And so people have suggested that if we look and find that bone structure in fossils, it might give us a clue that it was sitting near an air sac. So as well as these foramina, these openings in the bones to which air sacs were invading and then excavating these big cavities, there's also this kind of tissue type which might be just something to do with where we find pneumatic air sacs as well in fossils. So that's something that will be really interesting to look at more.
- [43:42]
Jonathan Codd
Steve I've been thinking a lot about birds recently. I study birds. I mean a lot of my work has been done on the origin of birds and how birds evolved from dinosaurs. But the next book I'm doing is on birds, the next pop science book. And a lot of what I've been trying to articulate there is how these hyper adapted flying machines that we see around and say how they evolved from dinosaurs. And the lungs I think were a very important part of that. And the lungs that birds have, these very efficient lungs that take in oxygen when they breathe in, when they breathe out, take in more oxygen than our mammal lungs. We can tell that they evolved long before birds. So birds today use those lungs to fly. It helps them fly. Flying is very energetically excited, expensive. So being able to take in that extra oxygen is very beneficial. Now as you know Jonathan mentioned earlier, bats, which are mammals, don't have those kind of lungs. So it's not that you need lungs like that to fly, but boy are they helpful in birds. But they did not evolve for flight. The fact that we see these air sacs invading the bones of the dinosaur ancestors of birds tell us that those lungs first evolved in land living dinosaurs. The same as feathers, the same as wings. Many of these things we think of when we think of birds, birds flying around, they must have evolved for flying. No, they evolved for other reasons. They were co opted for flight. The same way the Wright brothers didn't invent the wheel, didn't invent the propeller, they put them together. So I would just love to know more about where that bird style lung really came in and why. What was the evolutionary pressure? Was it something about the environment? There have been times of Very low oxygen environments after mass extinctions and so on, maybe that facilitated the evolution of these more efficient lungs, but I think we just don't know. But I've been thinking about that just literally the last few months as I've been writing and it's really fascinating. And what Emily mentioned, I totally forgot that there's this, you know, this new approach of looking at tissue type. And I don't know much about it, I'm not a bone histologist, but that to me sounds like that would be a way to check maybe in some deeper fossil ancestors whether they might have had some of these early air sacs, even if you don't see those holes in the bone. So that to me is really fascinating.
- [45:53]
Steve Brusatte
Jonathan?
- [45:54]
Melvin Bragg
Yeah, so we were talking before about sauropod dinosaurs and so one of the mysteries with those was their very long necks. And one of the problems you have is you have what's called dead space. So dead space basically is the distance between your lung itself and your mouth where the air comes out of your body. If you have an extraordinary long neck, all the air that's in your trachea is not exchanging gas, it's just moving in and out of the lungs. So one of the problems with these big sauropod dinosaurs was how are they actually able to overcome, I mean, some of These necks were 10, 11 meters long. So how are they able to overcome this dead space between their lung and the outside world? And one of the ways you can do that is if you have an air sac system. So an air sac system gives you huge volumes of gas with inside your body. And that would be one of the reasons why these animals were able to evolve their really long necks. The other thing I was going to say was that.
- [46:37]
Steve Brusatte
And you need to do more research on that. Is that what you're saying?
- [46:40]
Melvin Bragg
Well, we have good understanding that they had some sort of an air sac system. But what it was, it was one of essentially the findings. Once we started to understand that these sauropod dinosaurs and lots of dinosaurs had more bird like lungs, it basically explained a lot about their biology. I think people used to think sauropod dinosaurs were like 100 plus tons in weight. And as Emily was saying, when you take out a very tissue heavy lizard like lung and you replace it with a very light air sac type bird lung, the weight of these animals would come down to something like 20 or 30 tons. And that is physiologically believable in terms of what we see in animals that are existing in the world today. 100 ton animal living on land is a little hard to believe.
- [47:17]
Steve Brusatte
Well, what a roundup. Thank you very much indeed. That was terrific. Absolutely terrific. That's a five star. Thank you.
- [47:26]
Jonathan Codd
Tea or coffee or a breather? Breather. I'm going to diffuse some oxygen across in skin for a moment.
- [47:36]
Melvin Bragg
Mosquitoes.
- [47:37]
Jonathan Codd
That's crazy.
- [47:37]
Melvin Bragg
Mosquitoes are.
- [47:38]
Jonathan Codd
It rings a bell of like something I read in a textbook in the student or something.
- [47:42]
Steve Brusatte
Thank you very much. That was smashing.
- [47:44]
Jonathan Codd
Can we get a. Can we do a photo? Yeah, can we all do a photo? Of course you can. Would you like to. Tea is just black tea.
- [47:51]
Steve Brusatte
I'll have a black coffee, please.
- [47:54]
Melvin Bragg
In our Time with Melvin Bragg is produced by Simon Tillotson and it's a BBC Studios audio production.
- [48:01]
F
This is Dr. Chris and Dr. Zand here and we are dropping in to let you know about our new BBC Radio 4 podcast in what's up Docs. We are going to be diving into the messy, complicated world of health and well being. Because it can be confusing, can't it, Zant? That's right, Chris. The massive information out there can be contradictory, it can be overwhelming. And Chris and I get confused too. That's right. We get seduced by the marketing, the hype, the trends. So we want to be your guides through it. And I think it's fair to say, Xand, we are going to be getting personal. We're absolutely going to be getting personal, Chris. What I want to do is bring in my own health dilemmas in the hope that we can help you with yours. Listen and subscribe to what's up docs on BBC Sounds.
- [48:52]
Emily Rayfield
First, they told the story of the moon landing.
- [48:55]
Melvin Bragg
60 seconds.
- [48:57]
Jonathan Codd
We choose to go to the moon.
- [48:58]
Steve Brusatte
30Ft down two and a half.
- [49:00]
Jonathan Codd
I thought, wow, what have I gotten myself into?
- [49:03]
Emily Rayfield
Then came the dramatic rescue of the Apollo 13 mission.
- [49:08]
Melvin Bragg
We've had a problem here.
- [49:09]
Steve Brusatte
I literally got on my knees and prayed.
- [49:12]
Emily Rayfield
Now from the BBC World Service, 13 Minutes presents the Space Shuttle. The inside story of a dream to revolutionize spaceflight.
- [49:21]
Steve Brusatte
We had so much riding on something we'd never done before, unlike anything that had ever flown in space.
- [49:27]
Chris
This is the space shuttle.
- [49:28]
Jonathan Codd
Roger roll.
- [49:29]
Steve Brusatte
We're just hooting and hollering and screaming and yelling for the sheer joy of what you were taking in.
- [49:35]
Emily Rayfield
13 minutes presents the space Shuttle. Coming soon.