The Evolution of Lungs

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Hello, the evolution of lungs and of the 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 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.

Brussatti, 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?

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.

Can you elaborate that even a bit further?

What is this oxygen doing?

So, oxygen, as we all know, is really 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 adenidine 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.

And you have to get rid of the carbon dioxide.

And then you need to get rid of the carbon dioxide because if carbon dioxide is allowed to build up, it's toxic to tissues.

Without lungs or even gills, how would the first organisms get the oxygen they needed?

Okay, so 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.

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?

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 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.

Do we know exactly how they did that?

So they're in the water.

Do you want to tell a little bit more about that?

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 developed 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.

And that's fascinating.

Do we know more specifically how that happened?

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.

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?

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.

Jonathan, to push us a bit further, do we know how and when breathing developed?

Yeah, we have some ideas.

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 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: that kind of selection pressure occurring.

Can we be even more specific about the hinge moment of this?

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...

Do we still recognise these fishes?

There are still armoured 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.

Why did that happen then?

Like I said, it's all linked 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.

What happened to the gills?

So, even us as embryos, we still have evidence of gills when we're embryos.

So, most

vertebrates.

You wave your hand at your ears, is that you mean?

They're in this kind of area, I suppose.

You don't have gills right now.

I don't have gills, no, I 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.

Can you give us some idea of what muscles and mechanics had to change when you moved from water to land?

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 locomotive system to just being involved in the movements associated with breathing.

Something that all lungs have in common, I understand, is surfactants.

What are they?

So, surfactants is one of those systems that was basically got right very early on.

So, what surfactant is, 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.

And 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 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 is 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 constitutions and types of surfactant in all the different air-breathing animals that we see today.

Emily, can you tell us about what's called bucal pumping and who did it and why it was important?

Yes, absolutely, yeah.

So, bucal 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 bucle pumping is, is kind of depression of the floor of the mouth, which enables at the same time that air is being brought in 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 bucal pumping 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.

You wanted to come in, John.

Yeah, I was just going to say one of the reasons we still see bucal 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 exchange volume with expanding and contracting our rib cage.

So the way they have to do it is, as Emily was explaining, by enlarging this bucal cavity and and then closing off the narries and then forcing that air into the lung.

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 the bucal cavity in the absence of a rib cage.

Yeah, and I think picking up on that as well, too, if we go back to some of these early fossils, the 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 bony 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 rib cage in the same way that some of the later more terrestrial animals were able to do.

Steve, can you tell us a bit more about the function of the rib cage before we move on?

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.

We have fossil records of dinosaurs appearing many millions of years after the movement onto land.

Can we say something about their lungs, how they operated?

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...

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 if you ever heard of anybody claiming a fossil lung?

No, in dinosaurs.

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 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.

But the thing that is one of the many things that is amazing is that very small birds fly thousands and thousands of miles on the same principles that the dinosaurs trundle along.

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 30,000 feet.

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 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.

Jonathan, sorry, Jonathan.

Yeah, so the other interesting thing about bird lungs is that for each breath a bird takes, it breathes twice.

What does that mean?

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 of the thoracic air sacs,

inspiration, sorry, and then you get the second expiration where that actually comes out to the outside.

So birds are breathing twice for each cycle of air moving in and out of their bodies.

You've compared them, or somebody did around this table, to dinosaurs.

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 features.

And so we know because the theropod dinosaurs are bracketed by crocodilians and birds, that they shared some of the characteristics of both their lungs.

What were they?

So both birds birds and dinosaurs have multicameral lungs so that means they're incredibly folded internally and that's a kind of common trait you see where...

What does folded internally mean?

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 a zero.

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.

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We demand to be home.

Winner, best score.

We demand to be seen.

Winner, best book.

We demand to be quality.

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Suffs!

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Tickets at BroadwaySF.com

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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?

Yes, so Steve, Steve and...

Are we talking about dinosaurs, Steve?

Yeah, yeah.

So if we, yeah, if we...

Essentially, these tissues grow out from 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 two to three 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 is a pneumatic feature.

Yeah, so this is 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, providing this extra space and potentially enabling this special unidirectional flow that we see in living birds.

Steve, when you consider birds' lungs now,

how do they compare with some of those of the dinosaurs?

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.

Just to make the extra case.

Just the 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, it can speed up your growth.

It 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.

You want to come in, Emily?

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, six tonnes, 20 tonnes, 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 is 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.

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 mammals?

What had to change?

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 rib cage.

And as the diaphragm contracts down, you have the expansion of the rib cage outward 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 increases the pressure, and the animal will breathe out.

Is this as efficient or less efficient than the other ways of breathing?

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, if you like.

What about our lungs?

How do they rate?

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.

Where do animals like lizards fit in, Hamelie?

And why are they so different?

So lizards are interesting because

they were thought for quite a long time not to have air air sacs, and also there was thought to be, if you imagine a lizard moving, it moves from side to side.

And it was suggested that maybe this placed a constraint 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 were able to also bucle pump as well, too, to be able to force air from the mouth and throat into the lungs so that 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.

Jonathan?

Yeah so it's a really interesting idea that

Emily's talking about and it's this concept of breathing and 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 rib cage 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 was the 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.

Steve, can we look at the different forms of lungs in different animals and how does that affect them if there are?

There's 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 onto 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.

Is there an ideal form John Hunter to turn to

the ideal lung?

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 of 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.

You want to come in, Emily?

Yeah, one other thing 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 too.

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.

Can I assume so we can do that?

We can do that.

In what situation?

Which skin?

Is it my bulb?

Skin on the top of my head.

That's news to me.

It's just happening all the time.

So anywhere you've got blood vessels near the surface of your skin, you're going to have gas exchange occurring in there.

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's very warm, it has less oxygen in it.

When water's 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 them, and they've evolved to live without them.

Steve, is there any way in which the lungs are still developing that you can note and pass on to the rest of us?

I'm sure they are.

It's always tricky to ask how our structures are 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.

What do you think do you predict?

If I had to predict, predict, and this is, 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 takes, 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.

Okay, Jonathan, back to you there.

What would you most like to discover about lung evolution in the context of your work?

So in my work, it would be great if some of these paleontologists like Emily and Steve could find some fossilized lungs.

That would be amazing.

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 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 unsonate 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.

Do you want to come in on that?

For me, there's so many things that I'd love to know about extinct animals.

And it's, 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 his 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 we really don't know how they breathed.

Do you want want to take that up?

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 bucal 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.

So if you're imagine if you're creating a cavity 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 the 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.

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 brains.

So we have big brains, so we have big heads.

But actually, giving birth to a large-headed baby, as Emily's suggesting, 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 a diaphragm.

Is your diaphragm getting in the way of things?

No, it's near.

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 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.

We're coming to the end now.

Is there anything that you would like to add to the development of lungs?

Do you see that lungs developing?

And now over the next,

people like you can talk about 200 million years as if it was yesterday afternoon.

So maybe you can give us an idea here.

Yeah,

I think one of the interesting points and 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...

What does that mean?

So if you imagine, so imagine you're a small animal, like we talked about, you're able, its surface area to volume raiser is such that the volume is proportionally not that big compared to the surface area.

But 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 feathery gills, for example, or insects have a kind of special lung system as well too, that helps them breathe.

So it's expanding that amount of surface area to which you can actually absorb and diffuse gases across.

Final word Donathan?

Well I think it's fascinating.

One of the most interesting facts that I always talk to the students about is we've all hiccupped, 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 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's 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 hiccupping.

And what's interesting, the other thing about it is that the way we get over it, the way we stop hiccupping 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 hiccupping like our sort of amphibian ancestors have.

Well, we all learn things in our time.

Absolutely.

I have no idea.

Yeah, I know how to stop hiccups.

Think about something else.

Well, and more than that.

Thank you.

That was terrific.

Thank you.

Emily Rayfield, Steve Brussatti, and John Hancord.

Next week is the earliest surviving poem in Old Scots, John Barber's The Bruce, his epic medieval tale of Robert the Bruce and the Battle of Bannockburn.

Thank you for listening.

And the In Our Time podcast gets some extra time now with a few 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 time to say.

Can we start with you, Jonathan?

I can't remember what I said.

Well, Mermel, did you want to start?

Yeah, sure, I'll start.

I'll 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 pneumostium, so basically like breath bone or lung bone as we think of it.

And it's a imagine it's a bony tissue, but it's got dense tiny fibers 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.

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 evolve 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 us say how they evolve 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 expensive.

So being able to take in that extra oxygen is very beneficial.

Now, as 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, 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.

Jonathan?

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 extraordinarily long neck, all the air that's in your trachea is not exchanging gas.

It's just moving in and out of the lung.

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...

And you need to do more research on it, is that what you're saying?

Well, we have a good understanding that they they had some sort of an air sac system but what it was was it was one of essentially the 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.

A hundred-ton animal living on land, it's a little hard to believe.

Well, what a roundup!

Thank you very much indeed.

That was terrific.

Absolutely terrific.

That's a five-star.

Thank you.

Tea, your coffee, your

breather.

Breather.

I'm going to diffuse some oxygen across your skin for a moment.

Oh,

that's crazy.

Yeah, mosquitoes are drinking.

Rings the bell of like something I read in that textbook.

in the studio.

Oh, thank you very much.

Oh, we're smashing.

Oh, but

I'm not sure.

Yeah, can we all do a photo?

Because you can.

Who would you like to get?

Oh, yeah.

Tea is.

So just black tea.

I'll have a black coffee, please.

In our time with Melvin Bragg is produced by Simon Tillotson, and it's a BBC Studios audio production.

This is Dr.

Chris and Dr.

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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, Zahn?

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