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Author Topic:   Evolution of bird lungs from reptile lungs impossible?
Dr Jack
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Posts: 3514
From: Immigrant in the land of Deutsch
Joined: 07-14-2003
Member Rating: 8.7


Message 3 of 33 (218657)
06-22-2005 11:21 AM
Reply to: Message 1 by Andya Primanda
06-22-2005 5:11 AM


Reptile lung's (scroll down a bit)
Bird lungs
As you can see there are clear similarities and clear differences - the important one for birds is the direct and valved connection to the air sacs (incidently air sacs are present in many reptiles, for example, in snakes). I don't think it's actually that hard to see a evolutionary path between the two:
Initially birds evolve to breath by expanding/contracting the air sacs rather than the lungs: this is beneficial because it frees the cycle of breathing from the beat pattern of the winds.
Valves evolve at the front of the lung allowing air out but not in, meaning that air now follows a more circular path with less mixing with used air. This is directly beneficial in terms of oxygen requirements.
More valves evolve at the back of the lung to keep the air more efficently in the lungs during their contraction phase.
This system resembles the modern one, we have respiration drawing air into the posterior air sacs and then pushing them through the lungs. However the air is not yet being cleared from the lungs so some efficency is lost in mixing.
The development of anterior air sacs helps by pulling the used air out as the fresh air comes in, their placement naturally allows the used air to be blown out through the forward valve as the sacs contract but mixing will still occur as some will go back the way it came albeit at a reduced level
We're now almost there; the final stage is to add more valves to the system to prevent the re-entry of used air into the posterior sacs from the lung and the re-entry of used air from the anterior sacs to the lung. Both these adaptations have immediate benefits in terms of reducing mixing. And the system now naturally switches to the two-stage, unidirectional breathing pattern of modern birds.
This is of course pure speculation, but so is the claim that it can't evolve. Anyone see any glaring errors in the pathway I describe above?

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Replies to this message:
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Dr Jack
Member
Posts: 3514
From: Immigrant in the land of Deutsch
Joined: 07-14-2003
Member Rating: 8.7


Message 4 of 33 (218954)
06-23-2005 11:47 AM
Reply to: Message 1 by Andya Primanda
06-22-2005 5:11 AM


The earliest stages in the derivation of the avian abdominal air sac system from a diaphragm-ventilating ancestor would have necessitated selection for a diaphragmatic hernia in taxa transitional between theropods and birds.
Just noticed this in the quote. Reptiles don't have diaphragms, that's mammals - reptiles breath purely by motion of the ribs.

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Dr Jack
Member
Posts: 3514
From: Immigrant in the land of Deutsch
Joined: 07-14-2003
Member Rating: 8.7


Message 7 of 33 (219235)
06-24-2005 4:42 AM
Reply to: Message 5 by deerbreh
06-23-2005 2:29 PM


Re: Reptiles are not a monophylogenic Group
I believe the usual claim is that birds evolved from Dinosaurs (specifically theropods) who's lungs, of course, we cannot identify. However we do know that the lungs of the extant groups that diverged from the common line shared with the dinosaurs all use the same pattern of lung design so unless all these groups happened to seperately evolve the same lung design dinosaurs must have started with the same lung design - at which point it becomes moot whether the "bird" lung evolved in the bird lineage or the dinosaur lineage.

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Dr Jack
Member
Posts: 3514
From: Immigrant in the land of Deutsch
Joined: 07-14-2003
Member Rating: 8.7


Message 13 of 33 (220633)
06-29-2005 8:38 AM
Reply to: Message 12 by Andya Primanda
06-29-2005 7:14 AM


Any chance of a link to your discussion, Andya?

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Dr Jack
Member
Posts: 3514
From: Immigrant in the land of Deutsch
Joined: 07-14-2003
Member Rating: 8.7


Message 16 of 33 (220905)
06-30-2005 8:26 AM
Reply to: Message 15 by Andya Primanda
06-30-2005 8:08 AM


Ah... I see.

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Dr Jack
Member
Posts: 3514
From: Immigrant in the land of Deutsch
Joined: 07-14-2003
Member Rating: 8.7


Message 19 of 33 (250648)
10-11-2005 4:55 AM
Reply to: Message 18 by Springer
10-10-2005 9:33 PM


Re: problems with avian lung
Why would it result in less oxygen being utilised? The lung capacity is not being reduced in any way. Incidently, some snakes breathe by expanding/contracting the air sacs and drawing air into the lungs that way so we know the method is, in principle, functional.

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Dr Jack
Member
Posts: 3514
From: Immigrant in the land of Deutsch
Joined: 07-14-2003
Member Rating: 8.7


Message 20 of 33 (250649)
10-11-2005 4:56 AM
Reply to: Message 17 by Rahonavis
09-14-2005 7:29 PM


Re: Hi there! Found this and thought you might be interested:
Interesting, thanks.

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Dr Jack
Member
Posts: 3514
From: Immigrant in the land of Deutsch
Joined: 07-14-2003
Member Rating: 8.7


Message 22 of 33 (251038)
10-12-2005 5:04 AM
Reply to: Message 17 by Rahonavis
09-14-2005 7:29 PM


Re: Hi there! Found this and thought you might be interested:
Although, thinking about it, this would debunk part of my suggested evolutionary path. If the theropods are air sac breathers then that adaptation can't have evolved to allow breathing cycles to be seperated from wing beats. Any other ideas for possible adaptive advantages?

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