Subject: The biology of the vertebrate eye

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Subject: The biology of the vertebrate eye

#1  Postby DavidMcC » Jul 16, 2018 3:48 pm

... largely because Richard Dawkins got it wrong when he said we would have been better of with a squi-type eye.
I'm an atheist, but I sometimes make my self unpopular here, and one of the reasons may be my disagreement with Richard Dawkins over the pros and cons of the vertebrate eye. He is famous for describing it as "badly designed", and suggesting that we would have been better of with a squid-like eye, with its nice, elegant-looking "right-way-out" retina.
I want to make the point that this is a naïve view, because the inverted retina is the only way of achieving a retina that can be maintained against photo-oxidative damage by the light that falls on the retina, in the photopic environment (ie, surface waters and the land surface). This maintenance is one of the two most important functions performed by the retinal pigment epithelium (RPE). The RPE layer lies directly behind the photoreceptors, so that it can carry out its maintenanace. It is in turn, backed by the blood-rich choroid layer, which takes the waste products away, and into the blood stream.
All long-lived, sighted animals have to cope with the issue of photo-oxisdative damge to their visual opsins, and some invertebrates have their own solutions, such as the American lobster, which has cheap, throw-away, stalk-eyes,with very poor resolution, and that get replaced when either damaged in a fight, or damaged cumulatively by light.
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Re: Subject: The biology of the vertebrate eye

#2  Postby DavidMcC » Jul 16, 2018 3:53 pm

I'm pretty sure I have posted something like this before, but it was about 8 years ago, and I can't find it. :(
It is likely to cause quite a stir, with some people even concluding (incorrectly) that I am some kind of crypto-theist! :roll:
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Re: Subject: The biology of the vertebrate eye

#3  Postby Animavore » Jul 16, 2018 4:37 pm

I don't think Dawkin's objection wasn't about the retina facing the wrong way. It was, I recall, about the optic nerves coming from in front then through the retina causing a blind spot. Your eye has to constantly be moving around to compensate for the mesh of nerves which the light must pass through.

I'm not sure how Dawkins is being naive. I doubt he really believes there is a "right-way" of doing this. I think he was only making a point of how evolution has to deal with whichever comes before it.

And lastly, I doubt this -

DavidMcC wrote:because the inverted retina is the only way of achieving a retina that can be maintained against photo-oxidative damage by the light that falls on the retina


[Emphasis mine] Only way you can think of is not the same as the only way. We don't know what other ways could've been selected for. And I don't think the evolution of the vertebrate eye was a matter of design either, it's not like it "knew" that this was the most optimal design and went for it.
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Re: Subject: The biology of the vertebrate eye

#4  Postby DavidMcC » Jul 16, 2018 6:05 pm

Animavore wrote:I don't think Dawkin's objection wasn't about the retina facing the wrong way. It was, I recall, about the optic nerves coming from in front then through the retina causing a blind spot. Your eye has to constantly be moving around to compensate for the mesh of nerves which the light must pass through.

Incorrect. The reason that the optic nerves come out from the front is precisely that the photoreceptors are facing the "wrong way" (in his terms).
I'm not sure how Dawkins is being naive. I doubt he really believes there is a "right-way" of doing this. I think he was only making a point of how evolution has to deal with whichever comes before it.

Nonsense. He argued that the vertebrate retina was a "bad design" precisely because of the retina "facving the wrong way".
And lastly, I doubt this -

DavidMcC wrote:because the inverted retina is the only way of achieving a retina that can be maintained against photo-oxidative damage by the light that falls on the retina


[Emphasis mine] Only way you can think of is not the same as the only way. We don't know what other ways could've been selected for. And I don't think the evolution of the vertebrate eye was a matter of design either, it's not like it "knew" that this was the most optimal design and went for it.
A. It's not a matter of what I can think of, it's a matter of the Dawkins actually claiming that having an inverted retina (with the attendant problem of neurons coming out over the retina was "suboptimal". He regretted that we didn't have retinae that were like that of a squid, with the neurons coming out at the back, not the front.
PS, please stop playing word-games with "design", especially when Dawkins recognized that NS was a "designer" of organs and organisms. (He put it this way: that they are "designoid" structures.)
PPS: I'm surprised that I have to spell all this out for you.
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Re: Subject: The biology of the vertebrate eye

#5  Postby Sendraks » Jul 16, 2018 6:22 pm

DavidMcC wrote:
PS, please stop playing word-games with "design", especially when Dawkins recognized that NS was a "designer" of organs and organisms. (He put it this way: that they are "designoid" structures.)
PPS: I'm surprised that I have to spell all this out for you.


This could be a fascinating discussion, if you didn't carry on in such a patronising matter.
But, as long as you insist on you being "right" and anyone who disagrees with you being "wrong" there's not much productive discussion to be had.
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Re: Subject: The biology of the vertebrate eye

#6  Postby Thommo » Jul 16, 2018 6:32 pm

DavidMcC wrote:I'm pretty sure I have posted something like this before, but it was about 8 years ago, and I can't find it.


Advanced search - keyword "squid" - user "davidmcc":

http://www.rationalskepticism.org/post2 ... d#p2360665
http://www.rationalskepticism.org/post2 ... d#p2241028
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Re: Subject: The biology of the vertebrate eye

#7  Postby Animavore » Jul 16, 2018 6:41 pm

No wonder I had deja vu.
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Re: Subject: The biology of the vertebrate eye

#8  Postby LucidFlight » Jul 16, 2018 7:31 pm

Thommo wrote:
DavidMcC wrote:I'm pretty sure I have posted something like this before, but it was about 8 years ago, and I can't find it.


Advanced search - keyword "squid" - user "davidmcc":

http://www.rationalskepticism.org/post2 ... d#p2360665
http://www.rationalskepticism.org/post2 ... d#p2241028

That second link is a wild ride. Recommended reading for those with popcorn to spare.
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Re: Subject: The biology of the vertebrate eye

#9  Postby zoon » Jul 16, 2018 8:00 pm

Wikipedia (here) puts forward DavidMcC's suggestion, but only as a possible argument rather than a fully supported theory:
Wikipedia wrote:The cephalopods have a non-inverted retina which is comparable in resolving power to the eyes of many vertebrates. Squid eyes do not have an analog of the vertebrate retinal pigment epithelium (RPE). Although their photoreceptors contain a protein, retinochrome, that recycles retinal and replicates one of the functions of the vertebrate RPE, one could argue that cephalopod photoreceptors are not maintained as well as in vertebrates and that, as a result, the useful lifetime of photoreceptors in invertebrates is much shorter than in vertebrates.[6] Having easily replaced stalk-eyes (some lobsters) or retinae (some spiders, such as Deinopis[7]) rarely occurs.

A 2015 paper (brief write up here) by astrophysicists suggests that the glial cells in the layer above the photoreceptors are channeling light of different colours to the correct cone cells.

I think Dawkins was putting forward the consensus opinion, rather than an idea of his own? The Berkeley website here is saying more or less what Dawkins said, that the vertebrate eye has the receptors at the back, and a blind spot, because of evolutionary history; I don't suppose they are merely quoting Dawkins.
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Re: Subject: The biology of the vertebrate eye

#10  Postby DavidMcC » Jul 17, 2018 11:44 am

Thanks, zoon!
The following is what I was going to post before your intervention:
Dawkins - The Greatest Show on Earth wrote:
Chapter 11 - History writtten all over us
...
But now, suppose I tell you that the eye's 'photocells' are pointing backwards, away from the scene being looked at. The 'wires' connecting the photocells to the brain run all over the surface of the retina, so the light rays have to pass through a carpet of massed wires before they hit the photocells. That doesn't make sense - and it gets worse. ... blind spot...

Of course, what he didn't mention (perhaps because he did not know about it) is that what he describes is the down-side of an enormous plus - very long-lived vision (that woud have faded away within a year or two with a "sensible" squid or octopus eye).
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Re: Subject: The biology of the vertebrate eye

#11  Postby LucidFlight » Jul 17, 2018 12:09 pm

This does appear to be a fortuitous development. Now, I'm not a scientist, but... could evolution, say, find a way to get around the not-very-long-lived vision that is perhaps the limitation of the squid or octopus eye? That is to say, if humans happened to share an ancestor with the aforementioned cephalopods and inherited their "sensible" eye, would that have necessarily modified our evolutionary path to meet the constraints of said "sensible" eye, or could humans have evolved an even more suitable "sensible" eye to overcome the problem of not-very-long-lived vision?

ETA

Would an implication of humans having the "sensible" eye be that they would die earlier, or that they would go blind earlier? Or, perhaps, going blind earlier means they won't live as long... I guess one is tied to the other.

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Re: Subject: The biology of the vertebrate eye

#12  Postby scott1328 » Jul 17, 2018 12:20 pm

NO. This is the best of all possible worlds. We are the acme of evolution. There can be no improvements.
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Re: Subject: The biology of the vertebrate eye

#13  Postby DavidMcC » Jul 17, 2018 12:25 pm

LucidFlight wrote:This does appear to be a fortuitous development. Now, I'm not a scientist, but... could evolution, say, find a way to get around the not-very-long-lived vision that is perhaps the limitation of the squid or octopus eye? That is to say, if humans happened to share an ancestor with the aforementioned cephalopods and inherited their "sensible" eye, would that have necessarily modified our evolutionary path to meet the constraints of said "sensible" eye, or could humans have evolved an even more suitable "sensible" eye to overcome the problem of not-very-long-lived vision?

ETA

Would an implication of humans having the "sensible" eye be that they would die earlier, or that they would go blind earlier? Or, perhaps, going blind earlier means they won't live as long... I guess one is tied to the other.

/stream of consciousness

Ancient animals (which would be fish) that went blind early would definitely get eaten early, LF. Sorry I didn't spell that out before.
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Re: Subject: The biology of the vertebrate eye

#14  Postby DavidMcC » Jul 17, 2018 12:33 pm

scott1328 wrote:NO. This is the best of all possible worlds. We are the acme of evolution. There can be no improvements.

Tongue-in-cheek as usual, Scott?

Of course, there are problems associated with our inverted retinae (such as the risk of blindness due to a detached retina) but I for one am happy tpo take that chance, because it avoids the certainty of blindness at a very young age, due to cumulative photo-oxidative damage to visual opsins that cannot be recycled without the RPE and choroid layers behind the retina.
EDIT: Actually, I doubt that there will be any improvements on the vertebrate eye - it has lasted almost unchanged in respect of the layer structure for hundreds of millions of years.
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Re: Subject: The biology of the vertebrate eye

#15  Postby Thomas Eshuis » Jul 17, 2018 12:53 pm

DavidMcC wrote:
Ancient animals (which would be fish)

Based on what definition of ancient animals exactly?
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Re: Subject: The biology of the vertebrate eye

#16  Postby Animavore » Jul 17, 2018 1:49 pm

LucidFlight wrote:This does appear to be a fortuitous development. Now, I'm not a scientist, but... could evolution, say, find a way to get around the not-very-long-lived vision that is perhaps the limitation of the squid or octopus eye? That is to say, if humans happened to share an ancestor with the aforementioned cephalopods and inherited their "sensible" eye, would that have necessarily modified our evolutionary path to meet the constraints of said "sensible" eye, or could humans have evolved an even more suitable "sensible" eye to overcome the problem of not-very-long-lived vision?


I'm sure there would have to be other ways. like developing pigment in that jelly stuff inside your eye (I'm science me :teef:) or over the cornea such that we have natural sunglasses.

It's not hard to imagine other ways it would be possible.
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Re: Subject: The biology of the vertebrate eye

#17  Postby DavidMcC » Jul 17, 2018 1:51 pm

Thomas Eshuis wrote:
DavidMcC wrote:
Ancient animals (which would be fish)

Based on what definition of ancient animals exactly?

Fish, living 100's of millons of years ago.
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Re: Subject: The biology of the vertebrate eye

#18  Postby Animavore » Jul 17, 2018 1:53 pm

DavidMcC wrote:
Thomas Eshuis wrote:
DavidMcC wrote:
Ancient animals (which would be fish)

Based on what definition of ancient animals exactly?

Fish, living 100's of millons of years ago.

Starfish?
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Re: Subject: The biology of the vertebrate eye

#19  Postby DavidMcC » Jul 17, 2018 1:55 pm

Animavore wrote:
DavidMcC wrote:
Thomas Eshuis wrote:
DavidMcC wrote:
Ancient animals (which would be fish)

Based on what definition of ancient animals exactly?

Fish, living 100's of millons of years ago.

Starfish?

No, I was thinking of potential ancestors of the vertebrates, such as ancient hagfish, or hagfish-like fish.
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Re: Subject: The biology of the vertebrate eye

#20  Postby Animavore » Jul 17, 2018 2:00 pm

DavidMcC wrote:
Animavore wrote:
DavidMcC wrote:
Thomas Eshuis wrote:
Based on what definition of ancient animals exactly?

Fish, living 100's of millons of years ago.

Starfish?

No, I was thinking of potential ancestors of the vertebrates, such as ancient hagfish, or hagfish-like fish.

These fish are deep-sea fish, though. Why would they be selected for inward facing retina? It seems to me the inward facing retina would have to have already been in place and this is what evolution would have to have worked with going forward as they ascended out of the deep (or more likely tectonic shift caused the deep to become shallow in areas), rather than the retina being selected for in shallower, more light-filled waters, if the likes of hagfish and lamprey were our ancestors. Contrary to -

DavidMcC wrote:Ancient animals (which would be fish) that went blind early would definitely get eaten early, LF. Sorry I didn't spell that out before.
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