_{As discussed last week, tetrapods are the four legged creatures that evolved once bones developed and were strong enough to push us around the ocean floors and river banks. All life evolving from this point on this particular branch of this tree are tetrapods, including birds, whales and humans, none of which have four legs, but you get the idea; they all evolved from such and have at least vestigial limbs.}

Becoming a tetrapod was not sufficient, however, to get from water to land, despite the new plentiful source of air-oxygen. No, we needed... nostrils, lungs and a whole mess of complicated systems put in place.

Making Land

The Nostrils

Being a fish out of water comes with a lot of obstacles, primarily breathing, but also gravity, food and so on.

_{Public Domain}

The first thing to tackle was the old conk. In fish, you may typically find four external nostrils, or nares, two on either side of the face. As you might imagine, noses were not historically used for breathing, lest you get a gulp of salty brine in your gut.

Tetrapods, on the other hand, have two external nostrils, and two internal nostrils, called choanae. Yes, we have an inner nose. These choanae created a passageway from the outer nose to the throat, allowing us to breathe with our mouths shut, repurposing the old-fashioned nose.

How they got inside was not precisely known for quite some time, because in order to migrate nostrils inwards, at some point it would have to go through the teeth (picture your own face nose). This seems pretty unlikely because it means the bones holding the teeth (maxilla and premaxilla) would have had to split, let the nostril through, and then fuse together again. Which seems... pretty unlikely. So other theories such as nostrils forming independently were batted around.

HOWEVER, an intermediate fossil was actually 'caught in the act' doing just this whacky migration. Once again, we take a look at the Kenichthys which I mentioned briefly last week which also developed fins more designed for shuffling around than swimming; walking.

The Kenichthys actually had a pair of nostrils in a gap between the teeth of the upper jaw, a perfect transitional phase fossil. This actually backs up analysis of human embryos, in which you can observe a gap in the same place of the jaw which, later in development, fuses back together.

Should this fail for some genetic reason in development, we get a cleft palate. So if you have a weird upper lip, blame the Kenichthys fish. For everyone else, you may get a small vestigial philtrum:

_{CC BY-SA 3.0}

This internal nasal maneuver was one of the earliest steps towards becoming a tetrapod, long before that of the four limbs. So what's next? Well, the air has to go somewhere right?

The Lungs

The journey to lungs is a complicated one. In fact, lungs are arguably not the primary requirement for breathing, since there needs to be a bunch of incredibly vital systems put in place before they can even be of any use.

You see, as with all evolution, nothing comes out of... thin air... but builds on earlier designs, much like our nostrils. To get proper land-lubbing lungs, some mechanisms were already good to go with a few tweaks, which is strange because Osteichthyes - just the fancy word for bony fish - are distinctly non-gassy creatures, being fish and all*. So why are they developing a gaseous system?

_{*I'm aware fish swim bladders fill up with air like a lung, but their involvement is not what you might expect. More on that later.}

Osteichthyes... ok fine, let's just go with bony fish... actually gave us a couple of vital mechanisms:

The 'CO₂/H+' detection system.

This system detects the amount of gases dissolved in the bloodstream. This is how we know when to breath. When there is a buildup of CO₂ in the blood, it triggers the impulse for us to gulp in more oxygen and thus prevent death.

The 'Surfactant System'

Bony fish also seem to be the origin of the surfactant system. This is a complex and not entirely understood system with the primary goal to:

...decrease surface tension in alveolar air spaces to a degree that facilitates adequate ventilation of the peripheral lung._Source

Again, somewhat strange for a completely submarine fish to evolve such a thing, but regardless, we wouldn't have been able to breathe without it. But it does serve more roles that would have pre-dated breathing. The neural circuitry involved also allows the oxygen to feed cells that depend on the gas to convert food to energy and expel the Carbon Dioxide. It's pretty important.

The Cough

Another invention for breathing is the actual mechanism of the motion of breath. This came even before bony fish, being found in Lampreys, which I described previously so there's no need to show their horrifying, jawless suction-faces here again.

_{OK, I lied. - Pixnio}

Lampreys lack lungs, and instead get oxygen by pumping water through their bodies. They have been observed doing a kind of 'coughing' mechanism in response to debris getting trapped in their airways which is an important thing to note.

Researchers in 2012 suspected this coughing action could have been related to the origin of breathing mechanisms. when the CO₂/H+' detection system kicks in for the lamprey, the 'cough' triggers, and when the researchers had a look at lamprey brains, they found the nerve activity associated with breathing to be strikingly similar to that of the moment of cough.

This demonstrates yet another example where a system is not created specifically for its modern day purpose. In fact, this system sensitive to Carbon Dioxide was a repurposing of an already important mechanism.

Now all that's out the way, we can actually start developing proper lungs. Yay!

_{Not a lung - CC BY-SA 3.0}

I mentioned above that swim bladders and lungs were somewhat similar, but contrary to 'popular' belief, lungs did not evolve from swim bladders, nor vice versa; both are an example of convergent evolution.

Both lungs and bladders evolved from a common, ancestral, primitive lung system that actually served multiple purposes for a lineage that was building up the courage to consider land retail. As a result, the ancient lungs served, again, multiple purposes:

• Stability. Like the swim bladder, a gassy chamber would have helped previous fish stay the right way up in water
• Storage. The lungs could have acted as a storage place so when in low oxygen environments, a fish could go to its reserves to prevent hypoxia and drowning.
• Resonance. A gassy chamber also has the added benefit of boosting sound production for communication.

In all likelihood, modern fish would have lost these types of multi-purpose lungs when evolving in environments that no longer needed them, like the vestigial legs of a whale, or the Boneless sharks.

Land ahoy!

Around 340mya, lungs evolved an efficient way to expel Carbon Dioxide; the aspiration pump, which finally allowed animals to move permanently to land in the form of amniotes, a kind of small, reptile-y tetrapod.

But once again we have a problem. For a long time there was no transitional phase between the fishy buccal pump - The way they use their cheeks to push air out, like a saxophonist attempting circular breathing - and the modern system of costal ventilation. This has been the source of a decades-long argument among physiologists, dating back to the early 1970's, and no real consensus was landed upon.

Until now!

Well, until the late 1990's, anyway. Amphibians were analysed and it was noted that they do indeed have an intermediate mechanism in which muscles were used to exhale (like our diaphragm), and the buccal pump was used to inhale! Another golden transitional fossil highlighting the messy innovations evolution comes up with.

But, counter to what you might think, modern lungs are not necessarily a more 'complex' form of the ancient lungs. Some may actually be a simplification. A study in 2015 showed that the lungs of amniotes were complex, multi-chambered systems, but in amphibians and some lizards including snakes, a notable simplification made them single-chambered. Other reptiles went through miniaturisation.

So although the ancestral lungs were crucial for the success of migration to land, the extra details were no longer necessary once up there, and different tetrapods ditched various aspects depending on their direction of evolution - Just like the fish that ditched them for swim bladders.

One Small Step for Amniotes

I am sad to say that from this point on, we depart from amphibians. They take their own path as anamniotes, which consists of both amphibians and fish.

We continue on with birds and reptiles in the amniote group, with hopes to one day form into mammals. But hey:

WE MADE IT!

It took billions of years, and we still have 340 million years to go, but we're finally out of the water... and into the...frying pan?

References:

_{New Scientist | Lungs of the first amniotes: why simple if they can be complex? | Evolution of Tetrapods | Origin of Tetrapoda, Berkley | Evolution of lung breathing from a lungless primitive vertebrate | The origin and evolution of the surfactant system in fish: insights into the evolution of lungs and swim bladders. | The surfactant system of the adult lung: physiology and clinical perspectives. | Wandering nostrils (paywall) | Buccal Pumping | New perspectives on the evolution of lung ventilation mechanisms in vertebrates | Amniote |}