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if pharyngeal slits turn into gills, why don't humans have gills?

It seems that Intelligent Design advocates have challenged PZ Myers to answer '10+1' questions about evolutionary biology, while he's on an upcoming visit to Glasgow. PZ's published the list here. One in particular caught my eye, because it mentioned lungs (the subject of a recent post of my own), so I thought I'd have a crack at answering it myself.

9) If, as is often claimed by Darwinists, the pharyngeal pouches and ridges are indeed accurately thought of as vestigial gill slits (thus demonstrating our shared ancestry with fish), then why is it that the ‘gill-slit’ region in humans does not contain even partly developing slits or gills, and has no respiratory function? In fish, these structures are, quite literally, slits that form openings to allow water in and out of the internal gills that remove oxygen from the water. In human embryos, however, the pharyngeal pouches do not appear to be ‘old structures’ which have been reworked into ‘new structures’ (they do not develop into homologous structures such as lungs). Instead, the developmental fate of these locations includes a wide variety of structures which become part of the face, bones associated with the ear, facial expression muscles, the thymus, thyroid, and parathyroid glands (e.g. Manley and Capecchi, 1998).

Let's get the really glaring error out of the way first. Our questioner says that pharyngeal pouches "do not develop into homologous structures such as lungs." Bzzzzzzzt! Wrong! Lungs are not homologous to gill slits - they do not have anything approaching a common evolutionary history. I guess you could describe lungs & gills as analogous, because they are both involved in gas exchange, but that's as far as it goes. Lungs formed as outpocketings from the gut, not from the pharynx.

Down to the meatier stuff. The questioner is happily conflating pharyngeal pouches with gill slits, presumably so that s/he can set up that 'gills -> lungs' strawman. Pharyngeal pouches are found in some form in all chordates, even if only during embryonic development. As the name suggests, they are outpocketings from the pharynx. But not all of them pierce the body wall, or function in gas exchange.  In cephalochordates like Branchiostoma (personally I find the old name Amphioxus more euphonious), the pouches become slits that pass through the pharynx wall & open into the surrounding body cavity (the coelom).  These structures do not function in gas exchange - this takes place across the thin epidermis that covers the animal's body & lines its coelom (which is open to the sea). Instead, the pharyngeal slits act as a filter-feeding mechanism, trapping food particles from the current of water flowing over them. The same is true for sea squirts.

Yes, in fish the pharyngeal pouches go on to push through to the outside of the body, and the tissues between the 'slits' develop into the gill bars of bone or cartilage that support the delicate filaments of the gils themselves. Those gill bars are important, because we can trace their developmental fate, not just in fish but in frogs, reptiles, birds and mammals. And what do we find?

Jawless fish, aka agnathans - like lampreys - have mouths but (rather counterintuitively) no jaws. Juveniles are filter-feeders, just like Branchiostoma, but adult lampreys show vampiric tendences, latching onto passing fish with their toothy lips and rasping at the prey with a tooth-bearing tongue until they can suck at its blood. But embryological studies show that the first two sets of gill-bars in agnathans are homologous to the jaws of all other fish - they were co-opted for another function, a very long time ago indeed. The first set formed the 'mandibular' bones of the jaw, while the second is called the hyo-mandibular arch & helps to link a fish's jaw to its skull. One explanation for this co-option is that it involved a change in Hox gene switching, opening up new ways of getting food. (A mutation, in other words. Which pretty much negates the tired old contention, revived in another of those 10+1 questions, that mutations are always harmful.)

And we can trace the fate of other sets of gill bars - and the associated circulatory loops - in frogs and reptiles, birds and mammals. The pharyngeal pouches are homologous to the various small glands that you find in the neck (thymus, thyroid, and parathyroid glands). The cartilage of trachea and larynx are ultimately derived from the gill bar tissues. And remember the hyomandibular arch in fish? It's ended up as the stapes in your middle ear.

Oh yes, I mentioned the plumbing. In fish, each gill slit has an accompanying set of blood vessels (5 in most sharks - at Massey we used to dissect them out as part of first-year bio labs). Lewis Held (2009) comments that

Human embryos go to the trouble of making five pairs of aortic arches (which once sent blood to five pairs of gills) but then destroy two of them completely**. This Sisyphean stupidity only makes sense as a historical constraint: it must have been genetically easier to reconfigure the existing plumbing than to scrap it altogether and start afresh.

(** The remaining aortic arches become the vessels that take oxygenated blood to the lungs, head, and body.)

Lewis Held (2009) Quirks of human anatomy: an evo-devo look at the human body. Cambridge University Press. ISBN 978-0-511-59384-0 (e-book, Kindle edition)

 

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8 Comments

This is really Creationism rather than ID, isn't it? They've moved on from claiming that the early stages of life were nudged along by interventions from another dimension -- intelligent entities reaching into this universe to diddle with the hair on a bacterium's bum -- to claiming that all evolution was driven by a constant stream of interventions.

Anyway, the main problem facing them is that the various life-forms on Earth do fall naturally into a tree-like structure of family relationships. Cats look more like dogs than centipedes, and have more of their biochemical machinery in common. The creationists don't seem to deny that, even if they talk about 'baramin' rather than genera. So evidently it was easier for the Biblical Creator (or the intelligent entities intervening from their non-material dimension) to make life-forms that share large amounts of design in common, at all levels... skeletal structure, biochemistry, embryonic development.

The result of it all is that the Designer has designed all forms of life in such a way that they look as if they evolved (natural evolution, I mean; the IDists do seem to accept that life evolved, but they think it was guided evolution). Whether by a malicious desire to trick us, or because it was easier.

Which means in turn that all 10 criticisms of Darwinian theory apply with equal force to ID / creationism.

I mean, pharyngeal arches do appear in non-fish vertebrate embryos. Myers' antagonists are asking him, "Why don't these develop into gill slits in humans?" -- but you can ask them the same feckin' question. The answer, within both theories, is WHY SHOULD THEY?

This is really Creationism rather than ID, isn't it?
Well, yes, but most observers would say that ID i creationism anyway. (Cue ID infestation in 3... 2... 1... [she said hopefully!])

BECAUSE EVOLUTION IS COMPLETELY INACCURATE, DARWINISTS ARE HERE LOOKING LIKE FOOLS TRYING TO EXPLAIN NON EXISTENT/UNPROVEN THEORIES OF THEIRS.

Usually when people shout (write in all-caps) it's a sign they don't really have much of an argument. What, exactly, do you disagree with in that particular post? And what evidence do you have that supports your point of view?

Alison Campbell wrote: "Let's get the really glaring error out of the way first. Our questioner says that pharyngeal pouches "do not develop into homologous structures such as lungs." Bzzzzzzzt! Wrong! Lungs are not homologous to gill slits - they do not have anything approaching a common evolutionary history. I guess you could describe lungs & gills as analogous, because they are both involved in gas exchange, but that's as far as it goes. Lungs formed as outpocketings from the gut, not from the pharynx."

JOHN KING: I'm not sure what type of patented fallacy you "higher educated" types would call this (seems you got a tag for every kind of argument you don't like these days -- all of which seem to fall under confirmation bias), but your argument does not follow the facts (non sequitur mabye? ;-) ) The writers of the paper you cited ( Manley and Capecchi, 1998) were saying the SAME thing, so your "Bzzzzzzzzt! Wong!" nonsense was just that -- NONSENSE. Also you can't even seem to keep straight on who is who. It was NOT the questioner who made the statement about the pharyngeal pouches not developing into homologous structures such as lungs, but Manley and Capecchi.

ALISON CAMPBELL:"Yes, in fish the pharyngeal pouches go on to push through to the outside of the body, and the tissues between the 'slits' develop into the gill bars of bone or cartilage that support the delicate filaments of the gils themselves. Those gill bars are important, because we can trace their developmental fate, not just in fish but in frogs, reptiles, birds and mammals. And what do we find? '''

[then you go on about lampreys]

....But embryological studies show that the first two sets of gill-bars in agnathans are homologous to the jaws of all other fish - they were co-opted for another function, a very long time ago indeed. The first set formed the 'mandibular' bones of the jaw, while the second is called the hyo-mandibular arch & helps to link a fish's jaw to its skull."

JOHN KING: This reads like another "common materials equal common descent" fallacies. HOW (exactly) do you know that these structures were not ALWAYS set up like this in lampreys? Because they may look similar to gill structures DOES NOT mean that they were vestigial "gills" and no amount of your incessant references to them as "gills" will change that.

This reminds me of those people who posit that the little bumps seen on SOME (not all) of the fossil Velociraptor forearms are in fact "Quill knobs" thus presenting "evidence" that birds evolved from dinosaurs. Well not so fast. One who participated in the STUDY of these fossils reports:

"Calling these features quill knobs generates new mysteries. For example, other Velociraptor forearms are quite smooth—no knobs. Assuming all members of this group had feathers that required such strong anchoring, shouldn't at least the well-preserved ones show quill knobs?

Next, the bumps identified as quill knobs on all these dinosaur bones have peculiar differences from the real ones we see today on certain bird bones. Some are much smaller in proportion. Some have different or less regular spacing. Other dinosaur bone knobs run along the outside of their arms, instead of along the back of the bone as in modern birds....

tiny bone bumps have more uses than just anchoring feathers. They sometimes mark attachment points for connective tissue...

Why must tiny bumps on dinosaur bones signify feather attachments if they don't on mammal bones?" [1]

JOHN KING: So you may just be seeing 'gills' where none really exist (my conclusion) the same as these fellows are seeing 'quill knobs' where they do not exist.

Oh, and some of us use all caps to add EMPHASIS to certain words because we do not have the option of italics or underline here -- so you have no argument there either.

[1]. Brian Thomas M.S. "Do 'Quill Knobs' Show Dino-to-Bird Evolution?" Institution of Creation Research http://www.icr.org/article/do-quill-knobs-show-dino-bird-evolution/

This is a science blog. Referencing the Institute of Creation Research doesn't gain you any brownie points, I'm afraid.

Maybe ppl are saying things just to learn, and they sound unlearned because they are in the middle of learning, ponder that without bitching and bickering for an hour. You're possibly insulting potential "students" for inquiring. Stupid people are just smart people who haven't asked yet.

The people doing the asking - the ones I'm responding to in this post - are not students. They are scientists/educators who are making particular claims about evolutionary biology. Those claims are very very wrong. If that makes the questioners sound unlearned, that is their problem.

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  • Millie: Maybe ppl are saying things just to learn, and they read more
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  • Ciara Macdonald: BECAUSE EVOLUTION IS COMPLETELY INACCURATE, DARWINISTS ARE HERE LOOKING LIKE read more
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