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Author Topic:   Evolution of Air-breathing Tetrapods
caffeine
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Posts: 1558
From: Prague, Czech Republic
Joined: 10-22-2008
Member Rating: 4.0


Message 1 of 7 (527377)
10-01-2009 7:42 AM


I’ve been reading the Living Fossils Expose Evolution thread and, whilst new posts are accumulating at a rate with which I could never hope to keep up, all sorts of things cropped up that I felt the urge to reply to. Most of them were distantly off-topic though, so I thought beginning a new thread on the one which bothered me most was the best approach.

My ain gripe was Calypsis4’s incredulity at some seemingly impossible (to him) evolutionary transitions – such as that from water-breathing to air-breathing animals. I thought we could then go on to discuss some of the other transitions causing him problems, such as how ungulates could evolve into aquatic mammals, but if the mods think it would keep things tidier to stick to the restricted topic of water-breathing fish evolving into air-breathing tetrapods then fair enough.

[Yes, please, let's keep the focus on the evolution of air-breathing tetrapods. --Admin]

From water-breathers to air-breathers

As stated by Calypsis, the problem is as follows:

(…)I want to know how long those morphing organisms had to hold their breath while changing from breathing water to breahing oxygen. One minute? One hour? One day? a year? Or millions of years?

One important point to mention before we begin is that there are no morphing organisms involved. There are, in fact, organisms which morph from water-breathers to air-breathers over the course of their lives, but we're discussing evolutionary change here. 'How did a population evolve from breathing water to breathing air?' is the question (and the problem is ‘air’ as opposed to ‘oxygen’ – all the creatures involved breathe oxygen, the question is which medium they extract it from).

The key assumption which makes this evolutionary transition a problem is that it’s only possible for the same organism to do one of the two – breathing water or breathing air. There’s no realistic way any species could change from obligate water breathers to obligate air-breathers in one generation. Fortunately, there’s no reason for them to do so.

The dominant lifeforms* in my home aquarium are three-spot gouramis (I think). They look pretty much like this chap from Wikimedia:


Click to enlarge

They can often be observed to swim to the top of the tank and stick their heads out of the water whilst opening and closing their mouths. It looks like they’re taking big gulps of air. I was prompted to do all sorts of reading into what I saw going on in the fish tank, and was pleasantly surprised to discover that they are, in fact, breathing air. They possess what’s called a labyrinth organ, formed through the invasion of blood vessels into the bones of their gill arches and so named because it’s labyrinthine structure leaves a large surface area through which oxygen can be absorbed from the air – much like our lung.

*Or, in fairness, the biggest and most aggressive. The dominant lifeforms are clearly the snails.

These fish are capable of breathing both underwater and in the open air using different organs, and they’re not the only ones. European eels sometimes have to wriggle overland for a while when they’re migrating back to the sea, and they don’t hold their breath while doing so – they can absorb oxygen from the air around them through their skin. Mudskippers use the same tactic, though these are even better adapted for breathing out of water. They increase the available surface area by also absorbing oxygen through the lining of their mouths and throats. Note that the two below on the bare rock are not holding their breath – they’re happily breathing.


Click to enlarge

Our ancestors didn’t go through any breath-holding while they were in the process of transitioning from an aquatic to terrestrial existence, because they already had lungs. We can see this by looking at our nearest relatives amongst fish – the aptly named lungfish. As you can probably guess, these fish have fully formed lungs for breathing air, and some species can survive out of water for up to two years.

The first vertebrates to make a life for themselves out of water never had to hold their breath. They had lungs, and probably also absorbed oxygen through their skin and the lining of their mouths – just as many amphibians still do today. We know that all these methods of respiration evolved amongst fish still living in the water because we can see them in fish that live in water today. As they became more terrestrial and less reliant on water, their air-breathing equipment would become more important and their water-breathing less important. Some lineages might begin to lose their gills at a certain point in life, again as many (most?) amphibians do today, and some lineages would eventually cease developing them altogether, like ours.

The key point is that there’s no real problem posed by the transition from water- to air-breathing. All that’s required is an overlap period of being able to do both. As many species can do this today, there’s no reason our ancestors couldn’t accomplish the same feat.

Edited by Admin, : Add moderator comment.

Edited by Admin, : Change title.

Edited by caffeine, : corrected a couple of typos


Replies to this message:
 Message 3 by Lithodid-Man, posted 10-01-2009 2:05 PM caffeine has responded
 Message 4 by Dr Jack, posted 10-01-2009 4:35 PM caffeine has not yet responded

  
Admin
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Message 2 of 7 (527405)
10-01-2009 8:59 AM


Thread Copied from Proposed New Topics Forum
Thread copied here from the Gradually bridging insurmountable gaps thread in the Proposed New Topics forum.
    
Lithodid-Man
Member (Idle past 909 days)
Posts: 504
From: Juneau, Alaska, USA
Joined: 03-22-2004


(1)
Message 3 of 7 (527492)
10-01-2009 2:05 PM
Reply to: Message 1 by caffeine
10-01-2009 7:42 AM


Like a fish out of water...
Thanks for this topic caffeine, it is a subject of great interest to me, although one I do not keep as currently informed as I would like. For example, it is well established that aquatic barnacles change into air breathing geese.

Click to enlarge

Bad bio jokes aside...

One concept I believe is misunderstood by many people is the relationship between gills, lungs, and bony fish. I have seen many creationist claims that appeal to the misunderstanding by the implication that a fish gill would somehow need to transform into a lung. One could list many difficulties with that transition (although if one remains wet and very small it can be done).

However, the missing piece in fish evolution is that early in their evolutionary history the bony fishes developed lungs. That is, lungs are considered to be primitive. Their early adaptive radiation seems to have been freshwater and air breathing as a supplement to gills was probably advantageous (even today it is in fish found in shallow, warm freshwater systems where we see numerous air breathing species). At the same time other gnathostomes (sharks, placoderms, etc) dominated the oceans.

As bony fish invaded the ocean their was no need for the lung as a breathing apparatus. In the majority of fishes the lung is transformed into a swim bladder. In primitive forms the swim bladder is connected to the mouth and must be filled manually by gulping air. In advanced forms the esophagus is gone and the bladder fills solely by blood carried gases. The most advanced on this continuum are the bony fishes that have lost the swim bladder altogether (flatfishes, Stichaeids, etc)

So to summarize this part we need to understand that what we might think of as a fish today is not the same as a Devonian fish. At that time (in the teleosts) lungs were the norm. It has been said many times by cladists that we might think of ourselves as highly modified bony fishes. I would clarify that we are highly modified primitive bony fishes because we retain that ancient lung thingy.

Bony fishes, and especially the advanced bony fishes, are the dominant aquatic vertebrates today. Throughout this group we find diverse members who have 'found themselves' in situation akin to that of the primitive species long ago. they are living in marginal conditions where oxygen-depleted water and/or waterless conditions exist. The intertidal zone, ephemeral ponds and streams, marshes, etc. are examples. And in these conditions we repeatedly find fishes 're-inventing' the lung. They often have vascularized mouth and throat tissues. Some, like the mudskippers that caffeine showed, have gone beyond his and have invaginations to increase the surface area.

The central point of this is to envision water to land transition as one involving creatures that were already breathing air. It is not the creationist strawman of something like a salmon jumping onto dry land and hoping an accidental miracle gives it lungs.


Doctor Bashir: "Of all the stories you told me, which were true and which weren't?"
Elim Garak: "My dear Doctor, they're all true"
Doctor Bashir: "Even the lies?"
Elim Garak: "Especially the lies"
This message is a reply to:
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Dr Jack
Member (Idle past 83 days)
Posts: 3507
From: Leicester, England
Joined: 07-14-2003


Message 4 of 7 (527518)
10-01-2009 4:35 PM
Reply to: Message 1 by caffeine
10-01-2009 7:42 AM


Paging Calypsis... paging Calypsis
Please come to this thread. Please come to this thread.
This message is a reply to:
 Message 1 by caffeine, posted 10-01-2009 7:42 AM caffeine has not yet responded

  
caffeine
Member
Posts: 1558
From: Prague, Czech Republic
Joined: 10-22-2008
Member Rating: 4.0


Message 5 of 7 (527684)
10-02-2009 7:16 AM
Reply to: Message 3 by Lithodid-Man
10-01-2009 2:05 PM


How far back do lungs go?
Thanks for this topic caffeine, it is a subject of great interest to me, although one I do not keep as currently informed as I would like. For example, it is well established that aquatic barnacles change into air breathing geese.

No worries, and thanks for the link to the medieval bestiary site - well worth a read!

However, the missing piece in fish evolution is that early in their evolutionary history the bony fishes developed lungs. That is, lungs are considered to be primitive. Their early adaptive radiation seems to have been freshwater and air breathing as a supplement to gills was probably advantageous (even today it is in fish found in shallow, warm freshwater systems where we see numerous air breathing species). At the same time other gnathostomes (sharks, placoderms, etc) dominated the oceans.

It might not be right to separate placoderms here. I've been reading around a bit on this subject trying to find out what fossil evidence we have for lungs in ancient fishes (specifically how to distinguish space for a lung from space for a swim bladder), and I keep coming across the claim that placoderms had lungs. This has been interpreted variously to mean that:

Lungs are primitive for jawed vertebrates, and lost in cartilaginous fish.

Placoderms share more recent ancestry with bony fish than cartilaginous fish.

or, in an article that unfortunately I can't access, that the lungs in Placoderms and bony fish are not homologous and both converged on similar designs.

Reading this, I still didn't get much closer to uncovering the fossil evidence, but I'll carry on searching.


This message is a reply to:
 Message 3 by Lithodid-Man, posted 10-01-2009 2:05 PM Lithodid-Man has responded

Replies to this message:
 Message 6 by Lithodid-Man, posted 10-02-2009 10:58 AM caffeine has responded

  
Lithodid-Man
Member (Idle past 909 days)
Posts: 504
From: Juneau, Alaska, USA
Joined: 03-22-2004


Message 6 of 7 (527708)
10-02-2009 10:58 AM
Reply to: Message 5 by caffeine
10-02-2009 7:16 AM


Re: How far back do lungs go?
Hi caffeine,

I have the article. That issue of P&BZ contains a special collection of articles about tetrapod evolution. Your article is mainly about comparing surfactants between extant teleosts and tetrapods. The authors conclude that the pharyngeal surfactants used by teleosts (proposed as antibiotics) without lungs are homologous to those used by lungfish and tetrapods as an antiadhesive (prevents the primitive open-sack style lung from collapsing and not being able to inflate). They conclude that the surfactants evolved early, before lungs/swim bladders. They also cite a ref I don't have access to regarding the placoderm swim bladder as being independant in origin. I will post their diagram:


Click to enlarge


Doctor Bashir: "Of all the stories you told me, which were true and which weren't?"
Elim Garak: "My dear Doctor, they're all true"
Doctor Bashir: "Even the lies?"
Elim Garak: "Especially the lies"
This message is a reply to:
 Message 5 by caffeine, posted 10-02-2009 7:16 AM caffeine has responded

Replies to this message:
 Message 7 by caffeine, posted 10-06-2009 5:24 AM Lithodid-Man has not yet responded

    
caffeine
Member
Posts: 1558
From: Prague, Czech Republic
Joined: 10-22-2008
Member Rating: 4.0


Message 7 of 7 (528432)
10-06-2009 5:24 AM
Reply to: Message 6 by Lithodid-Man
10-02-2009 10:58 AM


Fossil evidence of lungs in ancient fish?
Thanks for that. If I had any money I think I'd buy a subscription to a bunch of online journals. It's so frustrating sometimes flicking through pages of relevant looking Google links that I can't read.

On which note, I'm having great difficulty finding any information on what, exactly, the fossil evidence for lunged fish is. I wanted to continue this thread with this, establishing quite clearly that lungs are primitive for at least some clades of fish. Unfortunately, all I seem to be able to find so far is the assertion, usually as an aside in articles discussing something else, that various taxa had lungs.

Does anyone have more information on why this is treated as being known with such confidence? Do we have convincing evidence of lungs in any fish fossils, and if so what form does it take?

Edited by caffeine, : No reason given.


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 Message 6 by Lithodid-Man, posted 10-02-2009 10:58 AM Lithodid-Man has not yet responded

  
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