How do we learn about extinct things? Can we use evolutionary theory itself to help us? Yes, but first we need to take a heuristic detour into space.
Suppose you’re an alien from another star system, maybe a thousand light-years from ours. Your scientists pick up radio wave transmissions from Earth, but they are garbled. You know there’s a civilization here, and you can figure that the transmissions came from the third planet, but you don’t have much more detail yet. So, you pack up your spaceship and head over our way.
Unfortunately, in the intervening time, us silly humans manage to blow up the Earth. When you arrive in the Sol system and come out of cryosleep, all that’s left here is a shiny new ring of asteroids where our big blue marble used to be. (Sigh …it happens.)
Still, you took all the trouble to come here. Might as well try to do some space-archeology, right? But we need some very basic questions answered. First on the list is: what was Earth like? What kind of climate did it have?
Well, if you look around the solar system, there are some obvious limits to what could have been here. We can look at the planets that were next to Earth. Closer to the sun, Venus is still brooding and seething, with an atmosphere dense and hot enough to boil lead. On the far side of Earth’s old orbit, Mars spins serenely. Its atmosphere is thin and cold, but not so cold that the carbon dioxide can’t exist as a gas. It’s a no-brainer that the Earthlings of old must have enjoyed temperatures somewhere in between these two extremes. Of course, Mars and Venus have undergone some changes over time. Venus’ high temperature is more due to the heat it has accumulated in its atmosphere over eons than because of its proximity to the sun – but you can account for that. You can set brackets – upper and lower bounds – on what Earth’s temperature must have been like.
Change of scene. Now, you’re not an alien; you’re a human paleontologist and you’re trying to understand the lifestyle and basic physiology (the inner workings, physics and chemistry) of an extinct organism here on Earth (which is fortunately not yet destroyed). Here’s the problem: physiology doesn’t fossilize. Like the alien, you are trying to reconstruct something that is long gone.
You can determine that the organism was yay big and maybe weighed so much. You can count its legs (the ones that were preserved, anyway). Sometimes you can even see glimpses of its soft parts, like shadows cast on the stone that your fossils are embedded in. But you can’t put it on a treadmill to see how fast it could run. You can’t measure its blood pressure. You can’t watch it hunt, or mate, or grow.
What you do know is that your fossil – which we will cleverly call Species A – is closely related to two living species, which (with enormous creativity) we will call Species B and Species C. But Species B and C are still extant, alive! We can study them in all their squishy, living-and-dying glory!
In a general sense, we hope that we can say A is something like B and C, but we want to be more precise. How much like B and C? Couldn’t B and C have changed, evolved a quite a lot since the time when A still roamed the Earth? Can we place limits on A?
The problem is that we need to be able to say something more specific than “B and C are related to A”. What kind of relationship do they have?
Let’s learn to talk about evolutionary relationships. Have a look at these two simple evolutionary trees (called “cladograms” in evolutionary parlance):
The way to read these trees is to look at which species arise from which points of intersection (or “nodes”). Each node represents either a common ancestor or at least a suite of characteristics that a hypothetical common ancestor would have had. Each cladogram is one possible history of the evolution of A, B and C.
In the first cladogram, we see that A and B and C are all descended from a common ancestor, but B and C are descended from a more recent common ancestor than the common ancestor of all three. Another way to think of this is that B and C share a common ancestor that A does not have. A is a side branch, as it were. If B and C are siblings, A is a cousin.
In the second cladogram, we have rearranged the branches. Now, A and B are sibling species, and C is the side branch.
Now, here’s the nifty bit: if we know something about the ancestor of a species, we can say something about its descendants. The descendants probably inherited properties from the ancestor. So, how can we know something about the common ancestor?
In the first cladogram, A is the side branch. If B and C share a characteristic, we can posit that they each inherited that characteristic from their common ancestor. But where did the common ancestor get the characteristic? Did it evolve in that common ancestor, or in an earlier one, such as the common ancestor of all three species? We can’t tell. So B and C are not very useful for understanding A.
But look at the second cladogram. Here, A and B share a common ancestor, and C is the side branch. Now, even though we know very little about A, we can see that any similarity between B and C probably come from the common ancestor of all three! Therefore, it seems likely that the ancestor of A and B also had that characteristic. And so we can hope that A had it too! If this cladogram is the correct evolutionary history, we can use it to understand A.
Because we have sort of trapped A between B and C, we are using a logic kind of like the bracketing method we used on our planets at the beginning of this article. In fact, we call this method “phylogenetic bracketing”. With only three species to compare, it is a very weak method. But if we have dozens of species, and one unknown nested in the middle, we can start to draw reasonable conclusions. We just have to choose our species carefully.
Paleontologists use this method all the time without thinking about it. Say, for example, that we have a fossil species that looks kind of like a cat. We only have the animal’s bones, so we don’t know whether it gave milk. But we know that modern cats give milk, and we know that dogs give milk. So, it seems likely that the common ancestor of dogs and cats gave milk. If the the extinct cat-like thing was descended from the common ancestor of cats and dogs, it probably gave milk as well. In fact, all the mammals we look at give milk, so it’s clear that the common ancestor of all mammals gave milk, and we expect all of its descendants to do so as well. So, we can use evolutionary theory to conclude that the fossil species gave milk.
But milk is trivial. What about something harder to understand?
What about (drum roll…) dinosaurs?
Yes, I know we talked about them last week. But they keep popping up their heads. Probably because they had long necks.
The holy grail of Dinosaur research these days is to figure out whether dinos were cold blooded or warm blooded. Phylogenetic bracketing to the rescue? Have a look at this cladogram:
Here we can see that T. rex is closely related to birds. Next most related (of the hundreds of dinosaur species we could examine), we have Triceratops. The common ancestor of T. rex and Triceratops was the first dinosaur (more or less), and so all of its descendants are also dinosaurs – including birds. So, we can use birds as part of our phylogenetic bracket! What shall we use for the other side of the bracket? The side branch that helps us define the common ancestor of all dinosaurs?
As you can see from the cladogram, the closest living relative of the dinosaurs and birds is the group we call Crocodilians. Crocodilians include the modern crocodiles, alligators and caimans, as well as a large number of extinct species. Taken together, the group formed by the crocodilians, birds and dinos is called the archosaurs. The non-dinosaurian, non-crocodilian archosaurs are an interesting digression; they include the flying Pterosaurs and other fascinating beasties. The modern lizards and snakes are in the Lepidosaurs, which branched off before the origin of the Archosaurs.
Unfortunately, when we use our phylogenetic bracket, we get the following problem: birds are warm blooded, but crocodillians and lepidosaurs are cold blooded. Our bracket fails! We can’t tell whether the common ancestor of all archosaurs was cold or warm blooded, so we can’t tell whether the common ancestor of dinosaurs was cold or warm blooded. We can say that warm bloodedness evolved somewhere along the line from crocs to birds, but we can’t say where.
Or can we?
It turns out that the terms “cold blooded” and “warm blooded” are very coarse-grained. They encompass large suites of characters that don’t have to evolve together. Modern warm-blooded animals keep their bodies warm, but they also have an elevated metabolism, efficient hearts, efficient lungs and so forth. But these different characteristics could evolve individually. Just how “cold blooded” is our crocodile?
If you put a crocodile (or an alligator, or a caiman) on a treadmill, it doesn’t use much oxygen and can’t exercise for long. It doesn’t have a very high resting metabolism. It can’t use its metabolism to elevate its body temperature above the temperature of the environment. So, yes it’s cold blooded.
But it does have some interesting features in its heart. Mammals and birds each have four chambered hearts: the blood supply to the lungs is kept separate from the blood supply to the rest of the body. So oxygenated blood from the lungs isn’t diluted by deoxygenataed blood from the body. Blood is fully oxygenated when it goes out to the body, and fully depleted when it goes to the lungs. The exact opposite is true for the carbon dioxide that the body needs to get rid of. It’s an efficient exchange.
Lizards and amphibians have a three chambered heart. This arrangement has advantages of its own, but it means that blood going from the heart to the body is diluted. Not efficient enough for a high metabolism.
It turns out that Crocs have a four chambered heart, like a bird or a mammal. Now, before you get excited, no this does not mean they are warm blooded. Crocs also have a way of mixing the oxygenated and deoxygenated blood after it leaves the heart. As I said before, this can be useful, especially when you’re holding your breath under water. But it leads us to an interesting question. Did modern crocs inherit cold bloodedness, or did they lose warm bloodedness? Were the first archosaurs warm blooded or cold blooded? It’s not as crazy as it sounds. After all, we’ve found other archosaurs – certain flying pterosaurs – that had fur-like structures on their skin.
We are getting closer to an answer, but once again, the answer is likely to be nuanced. It seems that crocodilians share yet another important characteristic with birds, one that is important for warm-bloodedness. They have a unidirectional, flow-through lung.
Birds’ lungs do not work like our mammalian ones. In fact, they are more efficient than ours. Our lungs are like big, multi-chambered gas bags. The chambers, called alveoli, are tiny and form a sort of sponge. Each time we breathe in, the sponge and all its alveloi expand. When we breathe out, they all shrink, like a thousand deflating balloons. The thousands of little chambers give the mammalian lung a huge internal surface area, which means a high rate of gas exchange between lung and blood. In other words, lots of oxygen can get into the blood, and lots of carbon dioxide can get into the lung to be breathed out again. This system is way more effective than the lungs of a lizard, which have much larger chambers and much less surface area, although they work in a similar way.
But a bird’s lungs blow ours away (so to speak). Instead of a thousand little chambers for exchanging gases with the blood, a bird’s lung has a thousand little tubes. In addition it has a set of strategically placed ducts (for transporting air, not exchanging gas with the blood) and air sacs, which just inflate like balloons. There also seem to be valves of some sort, but we haven’t figured out where exactly. The sacs store air without absorbing much oxygen from it. Every time a bird breathes in, the air rushes through the big ducts to fill the rear-most sacs. At the same time, air in the tubules gets sucked into the forward-most air sacs. When the bird breathes out, air from the rear air sacs is forced into the tubules from behind, and air from the forward air sacs is expelled through the mouth.
If that all sounds very complicated, don’t feel bad. It’s confusing, but the upshot is that air only moves through the tubules in one direction! The air follows a big circuit: mouth, ducts, first sacs, tubules, second sacs, and out the mouth again. It takes two breathing cycles for one breath of air to get all the way through.
What this means for the bird is that the oxygen in the tubules is never diluted! In our lung, the air we breathe in is mixed with the carbon-dioxide that we are trying to breathe out. So the concentration of oxygen in the lung is never as high as the concentration of oxygen in our atmosphere. Likewise, the concentration of carbon-dioxide in the lung is always higher than the concentration in the atmosphere. But in order to move oxygen into the blood, we want as high a concentration as possible in the lung. Similarly, to suck the carbon dioxide out of the blood, we want a very low concentration in the lung.
Birds don’t have this problem because the air enters the tubules (the part of the lung that actually does the absorbing) at essentially atmospheric levels of oxygen and carbon dioxide: high oxygen and low CO2. As the air traverses the tubules, its oxygen goes into the blood at the same time that the carbon dioxide comes out, without any dilution.
For a while, it has been recognized that crocodiles have a similar lung anatomy, or perhaps intermediate between a bird’s and a lizard’s. But because they have less total surface area for exchange of gases, and because they are “cold blooded”, it was assumed that they did not have a unidirectional, flow-though lung. Instead, even though they have tubules, it was assumed that air flowed one way through the tubules during inhalation and the other way during exhalation. The lung as a whole was assumed to act like a big bag, not a cycle.
But here’s the exciting part: a series of recent studies (Farmer 2016, Farmer and Sanders 2010, Farmer 2010) did actual measurements of flow inside the lungs of several different kinds of modern crocodilians, and showed that croc lungs are unidirectional. Just like a bird’s, but with less surface area. So, we can now do phylogenetic bracketing!
Bird’s lungs are unidirectional. Croc’s lungs are unidirectional. So, the common ancestor of birds and crocs – the first archosaurs – probably had a unidirectional lung. The similarities are too great to be pure coincidence. And if the first archosaurs had a unidirectional lung, then dinosaurs probably had a unidirectional lung as well, and they passed it on to our modern birds.
But does this mean they were all warm blooded? Did they have an elevated metabolism? Did they use metabolic energy to keep their temperature high? These are huge questions that we are very far from answering completely or conclusively. For one thing, the unidirectional lung isn’t only useful for warm-blooded critters. It’s efficient, and that never hurts. It also means fewer breaths per unit of oxygen absorbed, which helps to save water (we lose a bit of water each time we breathe out). That could be important in the dry environment that some archosaurs and dinosaurs lived in.
So we can’t say that the archosaurs and/or dinsosaurs were warm blooded based on this information. But we can defintely see where birds got the equipment that they use for their high metabolism. The heart and lungs of the bird’s ancestors beat and breathe on (more or less) in the body of an alligator.
Credits and References
- Farmer, CG, 2015. Similarity of Crocodilian and Avian Lungs Indicates Unidirectional Flow Is Ancestral for Archosaurs. Integrative and Comparative Biology 55(6): 962-971
- Farmer, CG 2010. The Provenance of Alveolar and Parabronchial lungs/; INsights from paleoecology and the discovery of cardiogenic, unidirectional airflow in the American Alligator (Alligator mississippiens). Physiol Biochem Zool 83:561-75
- Farmer CG and Sanders K. 2010. Unidirectional Airflow in the lungs of Alligators. Science 327:338-40
- Cladograms in this article were drawn with: