How one lung became two

Look inside the chest cavity of a land-living vertebrate, like one of us, and you find a pair of lungs. But life on land owes its origins to animals that evolved first in the water and then, about 400 million years ago, came ashore. The animals that did that were fish, and the fossil record, understandably patchy as it is from this era, suggests that these species developed primitive lungs to enable them to breathe air at the surface so that they cold survive in oxygen-poor water. But based on species that still survive today, these lungs were all unpaired single structures, not the paired entities in you and I. So, how, and why did one turn into two? Speaking with Chris Smith, Camila Cupello, at the University of the State of Rio De Janeiro, has been scanning embryos of different groups of lunged fish, and salamanders that spend part of their lives in the water yet also have two lungs. What the developmental patterns reveal is that the switch to a pair of lungs seems to have coincided with the exit from the water and appears to be a critical part of the adaptation to life on land, probably by boosting the efficiency of the respiratory system…

Camila - Fish are the first group that have a lung, that have the same origin as our lung. So we can trace the evolution of this organ.

Chris - Are we talking here about primitive fish that develop lungs so they could breathe air from the surface? So they would live in water, but they would breathe air from the surface. And they did that by having some kind of lung structure?

Camila - Yes, we are talking about this kind of fish that can breathe air.

Chris - And so which fish have you studied to get at this question then?

Camila - We studied very primitive fish. We can say like that. All the groups of fish that have a lung.

Chris - And are their lungs like our lungs? If you look at those fish and look at their descendants today, do they have lungs that if I saw them I would recognise as the same as mine? Or are they different?

Camila - They are a little bit different because they are paired and they have less compartments. So they are a little bit different, but they have the same origin, the same evolutionary origin. The lung.

Chris - When you say unpaired, I've got a right lung and a left lung. So, hopefully have you! But these fish would've had a single lung, is that what you're saying?

Camila - Yes. They have only one lung.

Chris - So does that mean the question becomes how that one lung turned into two lungs?

Camila - Exactly. We try to understand how this single lung, this one lung turn into a paired lung.

Chris - And so how did you study that? How did you attack the problem?

Camila - When we scan embryos, we can understand the evolutionary pattern of the development of the lungs. So we made some tomographies to understand this.

Chris - And what did that tell you? Did it reveal how you got from a single lung in these fish to two lungs in animals like us?

Camila - Yes. This was very important to understand and to reveal. How was the lung during evolution, during the water to land transition. Because the paired lung appeared in groups that live out of water.

Chris - How do you know that? What was it that told you that the lungs only became a pair once the animals were out of the water?

Camila - When we studied the groups, we could see that single lungs were present in groups that were in in the water and paired lungs were present in groups that are out of the water. So we can trace back this.

Chris - Do you have any way of narrowing the gap between the water and the land? Do you think that the change happened into a pair of lungs only after they were on land, or do you think having paired lungs helped animals get onto land out of the water.

Camila - Groups that had a paired lung when they were inside the water so they could go out of the water because they had already a lung, a paired lung.

Chris - How do you know that though? Because you, you just told us that, that the animals that were on land had these paired lungs and the animals in the water had a single lung. So how do you know that the paired lungs evolved in those fish first?

Camila - I'm not saying that paired lungs evolved in fish first because fish, all the fish, all the groups of fish that have unpaired lungs - have a single lung - but salamanders are amphibious, and this group live sometimes inside the water yet. So they make this transition during their life. And when we see that paired lungs were present in groups that make this transition for water to land when they are during their life, we can understand this.

Chris - And, presumably because paired lungs have become the way to do it because all these animals around us do it this way pretty much these days. Why does it give them an advantage? There must have been some kind of advantage of having lungs like that or animals wouldn't have done it. What's that advantage, do you think?

Camila - They have that advantage to have a paired lung is that they have more surface to make gas exchange. So it's better, it's more efficient to breathe air.

Chris - And you think that was the main driving force? It was a more efficient respiratory system?

Camila - Yes, certainly it was more efficient to have a paired lung out of the water.