National Geographic

Living optic fibres bypass the retina’s incompetent design

This is the first of eight posts on evolutionary research to celebrate Darwin’s bicentennial.

Revisitedbanner.jpg

Blogging on Peer-Reviewed ResearchIf you were a designer tasked with creating a machine for collecting and processing light, the last thing you would come up with is the human eye. Darwin marvelled at the eye as an “organ of extreme perfection”, but in this, he was wrong. Aside from the many illusions that can fool them, our eyes have a major structural flaw. In humans and other back-boned animals, the light-sensing cells of the eye – the “photoreceptors” lie at the back of the retina.

fibreoptic.jpgIn front of these sensors lie several layers of nerve cells that carry their signals, and blood vessels that supply them with nutrients. The nerves join to the main optic nerve which passes through a hole in the centre of the retina and connects to the brain. It’s a back-to-front design. Light has to pass through several layers of nerves, not to mention blood vessels, before it hits the retina itself. It’s a bit like designing a camera, and sticking the wiring in front of the lens.

Octopuses and squid have a very similar eye to ours, but theirs’ are much more sensibly structured. Their nerves and blood vessels connect to the light sensors from behind so that light can hit the photoreceptors without having to negotiate an obstacle course. And because their retina doesn’t need a hole to accommodate the optic nerve, they have no blind spot.

In our own retinas, nerves and vessels are random in their spacing and irregular in their shape. The light that shines past them is reflected, scattered and refracted. It’s amazing that our eye can see at all. But even though there is clearly no designer, evolution has done a pretty good job instead, with its remarkable capacity for making the best of a bad lot. In the case of our eye, some of the obscuring cells act as living optic fibres, to funnel light onto the sensors they cover.

Kristian Franke and colleagues from the Paul Flechsig Institute for Brain Research first noticed these fibres by shining light onto the retinas of guinea pigs. They looked at a cross-section near where the photoreceptor lie and saw a very regular pattern of bright spots. Clearly, some parts of the retina were transmitting light far better than others.

As they looked at further cross-sections throughout the retina, they realised that the bright spots were the endpoints of long tubes that stretched throughout the retina. Near the top, the tubes widened into funnels. Franze identified these tubes as Muller cells. These brain cells aren’t nerves themselves, but are part of their supporting cast. They are long cylinders arranged in columns across the entire retina, and provide a route for light to pass through the tangled morass of nerves and blood vessels.

The Muller cells gather light at the top of the retina and channel it to the light sensors as a tight beam. Along the way, the light is barely reflected or scattered and little is lost when it finally reaches the photoreceptors, just like modern optic fibres.

Light enters the Muller cells at a shallow angle and is slowed down considerably by the cells’ high refractive index. When it hits the cells’ boundaries, it is almost completely reflected back along the tube. Their funnel shape allows the Muller cells to gather and transmit as much light as possible. But as they narrow in the middle, they take up a very small amount of space and leave plenty of room for the blood vessels and nerves that the retina needs.

On average, each Muller cell serves a single cone cell and several rod cells. This one-to-one system ensures that the images that eventually hit the light sensors keep strong contrast, and are not distorted.

Evolution has given the vertebrate eye a remarkably ingenious solution to its ludicrous inverted retina. The eye may not be the perfect organ that Darwin thought, but new insights into its evolution still provide us with awe-inspiring surprises.

Reference: Franze, Grosche, Skatchkov, Schinkinger, Foja, Schild, Uckermann, Travis, Reichenbach & Guck. 2007. Muller cells are living optical fibers in the vertebrate retina. PNAS 104: 8287-8292.

Subscribe to the feed

Bookbanner.jpg

There are 16 Comments. Add Yours.

  1. Lassi Hippeläinen
    February 9, 2009

    “It’s a bit like designing a camera, and sticking the wiring in front of the lens.”
    It’s worse than that. You can put all kinds of little things in front of the lens without affecting the image, because the obstruction is so far out of focus. There even is an old photographer’s trick to repair a scratch on the lens: paint it black, so it won’t scatter light.
    The problem with the eye is that the wiring is in front of the film.

  2. Ian
    February 9, 2009

    Keep ‘em coming!

  3. Paul Murray
    February 9, 2009

    Har! All you evilutionists claiming that the eye is imperfect, when all along God had invented optic fibers when he created the earth 6000 years ago drool gibber.

  4. Paulino
    February 11, 2009

    Exactly… modeling creationist/IDists behavior has become very easy.

  5. Barry Moebius
    April 29, 2009

    Ok, I get it. The eye is incompetent. But wait, no it’s not, we’ve just discovered (2007) that it’s actually got an incredibly ingenious design. It’s a masterpiece of miniaturised optics…grafted onto something ludicrous…kind of like when we repaired that other incompetent eye, the first Hubble Space Telescope.
    It must have been awful bumping around in the dark for all those millions of years with the incompetent design waiting for the brilliantly conceived workaround to be retrofitted.
    Given that it’s taken our not-incompetent-at-all biologists until 2007 to find these optical fibres in the eye which work so brilliantly, is it remotely possible I wonder that we might not yet quite still know everything there is to know about the design of the eye?
    Who knows, next year we might just find that it is even more incredible than we think now. What a concept.
    And one day, we might even discover that there is actually an extremely good reason why it’s designed the way it is, and that the only reason it looks incompetent today is because we still are.
    This ludicrous eye. By the way, can you see this? With your ludicrous, incompetent eye? I mean, if there was a place to take this pair of mine back and exchange them for something, I don’t know, more scientifically competent, I sure would. But there doesn’t seem to be a manufacturer’s name and address anywhere on them that I can find. More incompetence. What a junk body this is. I can’t wait until the biologists build us a better one. That’s gotta be available soon, right?

  6. Ed Yong
    April 30, 2009

    As should be painfully obvious every time you look in a mirror, it’s entirely possible to be fully functional yet utterly incompetent…

  7. Barry Meobius
    May 1, 2009

    Not according to the actual meaning of the words. If something is competent, then it is sufficiently capable for the task. Obviously if something is fully functional, it is sufficiently capable for the task, and therefore, it is, by definition, competent.
    Meanwhile the NY Times reports in your sidebar that bone is an absolute miracle of engineering, beyond what our best and brightest engineers can do.
    Seems like evolution is sometimes brilliantly ingenious, and sometimes incompetent. Or maybe it just has bad days.
    The crucial point is that the fibre optics described in the above article have been integral to the functioning of the eye from the beginning. There never was a time when our retinas were obscured by the nerves but lacked the fibre optics. There never was a time when the eye was a ludicrous, incompetent, non-working lump of jelly. It’s always worked like it does now: brilliantly. In heat or cold, for many decades, with ability to focus from very close to very far, with full 3d colour and real time image processing, requiring no maintenance, all packed into a 1 inch diameter sphere.
    Obviously, the design decision to pack the circuitry inside the eye was made for other considerations, and the reduced light falling on the retinas as a result was simply compensated for by the brilliant fibre optics design.
    I would also add that this absurd argument about the eye being incompetent has been around for years, and certainly dates from before 2007. One might imagine that the discovery of these ingenious fibre optic channels would result in a re-think and re-evaluation, but apparently not. Instead we get articles like the above which now can’t decide which it is: is the eye incompetent, or is it a piece of optical genius?

  8. MattK
    May 1, 2009

    Barry, regardless of the merits of your argument (there aren’t any) I would not hitch my cart to the idea that everything is optimally ‘designed’. There are lots of examples of non-optimal design, the eye being only one (the vertebrate eye still has a blind spot). Humans are conspicuously badly designed, for example the direction of bend in our knee is inefficient and induces wear and tear (consider other cursorial animals who’s analagous (but not homologous) joint turns the other way.
    It is a transparently ludicrous to nitpick at the metaphorical language in articles written for more general consumption instead of actually dealing with science. Being ludicrous, the tactic is naturally adopted by creationists and such low-watt intellects as Denyse O’Leary. In this case, you have not even executed this lame maneuver without resorting to a straw man. You are free to fantasize about a designer who designs things for a purpose so obscure and convoluted (designed to fool rational people?) that it is completely untestable (how convenient). (Ah god, somewhat less competent than a NASA engineer, and apparently rather dickish to boot.) The rest of us can look at a functional, but not optimal eye, whose evolution has been shaped by historical constraints, and appreciate its history and imperfect elegance.

  9. Barry M
    May 2, 2009

    Thanks Matt. Imperfect elegance I can appreciate. What I don’t really fathom is how a construction (I know how you folks hate to call it a “design”) can be both incompetent and breathtakingly ingenious at the same time, especially when the ingenious bit is clearly addressing the limitation imposed by the incompetent bit. The whole thing of the fibre optics seems like an extremely nice workaround to a design problem (woops, sorry) created by a constraint. That constraint may well have arisen from historical development, ie evolution. I don’t have a problem with that.
    What I don’t understand is how the situation can be described as incompetent or ludicrous, when the design problem has obviously been solved with extraordinary ingenuity. Like a car model that is built on a wheelbase developped for another model, which requires some retrofit to make it more suitable for a different size.
    I’m not even trying to establish whether or not a literal “designer” was behind these things. I just wonder why evolutionary biologists feel the need to denigrate the object of their study, even in situations where they cannot help but marvel at how extraordinary it is. As in the above article. If there’s no designer, why the need to trash her? Call it imperfect. But ludicrous and incompetent are loaded terms which seem intended to insult this non-designer. It’s a way of getting a little stick back at the opposotion. The language betrays a certain mindset which seems to be clouding the issue. To me.
    And again: the key point is that there was never a time when we had the problem with the wiring in front of the film but we didnt have the fibre optics to act as a workaround. There never was such a time. There are no eyes that have the problem but not the solution. So how, I ask, can this situation be deemed incompetent or ludicrous? It’s not just this article, this is a whole meme out there, from Dawkins book and elsewhere.
    With all of these “imperfect” design elements, it may ust be that we haven’t figured out why they are there in the first place. Until these fibres were found, we were superconfident that the eye was incompetent. Now its ingenious. So one day we’ll discover why the knee doesn’t do reverse 360 double pikes. Sure that would be cool. I don’t know. Anyway, have a nice day.

  10. Dr Ismail Musharraf
    May 2, 2009

    Actually, as a PhD in molecular biology, I think Barry makes a pretty good point, even making allowance for his obviously amateurish understanding of the topic.
    The question of “competence” can only arise in relation to a specific set of criteria or specifications. But if there is no designer, then clearly we cannot think in terms of any design specifications. In this case, then labelling the eye “incompetent” is really having a dollar bet both ways: we deny that there are any design criteria which the eye ought to meet, but then we want to judge it as falling short of these same criteria.
    To say that putting the nerves in front of the retina is incompetent is to demand of the eye that it ought to maximise the amount of light falling on the rods and cones. We can require this of an optical designer building us a camera, but not of the blind processes of natural selection.
    Competency, is, then, as Barry points out, a loaded term which surreptitiously imports into the discussion the notion of a designer even as we would want to deny any such necessity. It would be much better to stick to words like “imperfect” than “incompetent” or “ludicrous”.
    Meanwhile, Barry, you need to do some more homework, but thanks anyway for your contribution.

  11. MattK
    May 2, 2009

    that function exists without conscious design is the key insight of evolution by natural selection.

  12. Albert Einstein
    May 23, 2009

    look at the design of the orange. talk about incompetent. if you were designing the orange today, of course you wouldnt make it spherical *snort*. you would, obviously, make it cubical so that it would pack more efficiently for transport. how ludicrous would it be to make fruit anything but cubical in shape? what an appalling waste of transport efficiency.
    any first-year engineering student could do this, in their first semester, even if they were a complete doofus. last year i had a student, a complete idiot, who nevertheless assembled a fully functional horse in her spare time from junk DNA we had laying around the lab. see what i mean?

  13. Ed Yong
    May 23, 2009

    Loud and clear. But you could have used far fewer words to tell us you’re a moron.

  14. Professor Lord Sir Carnegie of Bristlethwaite
    May 23, 2009

    thanks Ed. clever comeback. you sure have a way with words. guess it’s that “stick of good writing” you use. memorable phrase.
    i see now what you were trying to get at with your first response, a sly dig, but it was too subtle huh. this one much better. a moron. nice. scientifically inaccurate, and offensive to intellectually challenged people, but hey, you dont mind throwing around the pejoratives do you, so no problem
    my point, and im going to make it again, is that you don’t seem to have absorbed the point of your own article. you can stop calling the eye ludicrous and incompetent now because, as this very research shows, it is not at all. it incorporates an ingenious construction which means that the light does not have to pass through the “wiring” at all. so while you and dawkins and the rest have been banging on for years about the appalling design of the eye, you;ve actually been completely oblivious to what was only discovered in 2008, namely that the photons dont need to travel through the wiring at all.
    this isn’t science journalism, it’s propaganda. a much more interesting article would have used this research to show how shortsighted (haha) the idea that the eye was incompetent turned out to be. why would that be such a terrible thing to admit? your whole precious Darwinian edifice wouldn’t collapse. you would just be admitting that, wow, the eye didnt turn out to be a complete optics disaster after all.

  15. Whitecoat Tales
    May 25, 2009

    @Professor Lord Sir carnegie of Idiocy

    you don’t seem to have absorbed the point of your own article. you can stop calling the eye ludicrous and incompetent now because, as this very research shows, it is not at all. it incorporates an ingenious construction which means that the light does not have to pass through the “wiring” at all.

    I’m confused, how is a rube-goldberg type design that requires placing the electronics in FRONT of the retina, and then ad-hoc placing optics inbetween so that light has a better chance of getting through anything other than an incompetant design? Yes, it’s cool. it’s hardly Ideal design.

    so while you and dawkins and the rest have been banging on for years about the appalling design of the eye, you;ve actually been completely oblivious to what was only discovered in 2008, namely that the photons dont need to travel through the wiring at all.

    Actually, it’s still passing through wiring, wiring that is more light transparent, but not lossless. Just less bad. Again, how is this ingenius? Having the electronics behind the retina still makes more sense. This is just a workaround.

    this isn’t science journalism, it’s propaganda. a much more interesting article would have used this research to show how shortsighted (haha) the idea that the eye was incompetent turned out to be. why would that be such a terrible thing to admit?

    Nope, propaganda is what you’re doing. See Ed reported the facts. You’ve given the facts an implication that just doesn’t follow from the facts.

    your whole precious Darwinian edifice

    Aha, you do know that use of the word Darwinian labels you immediately as an idiot yes? We don’t refer to physics as Einsteinin, we don’t refer to biology as darwinian. Only idiots do that.

    wouldn’t collapse. you would just be admitting that, wow, the eye didnt turn out to be a complete optics disaster after all.

    Still an optics disaster. Just less of one. We can admit that it’s got a workaround and lo and behold, the edifice of evolution stands strong. It’s just an example of historical contingency. Not an example of excellent design.
    But of course, you can’t percieve any interpretation except the one you already had.

  16. ben
    June 24, 2009

    Sorry to comment rather late but the vertabrate retina actually can’t be seen as poorly designed. Not to say that there is a designer (credulous bullshit), but the act of regenerating the active form of the vertabrate pigment, 11-cis retinal, from the all trans or inactive form takes place in the pigmented epithelial cells (extremly opaque). To use the rods as an example here, phototransduction occurs on the disc closest to the pigmented epithelial cells to allow for rapid regeneration of 11-cis retinal. to place the photo receptors at the front of the retina would require that light penetrates an essentially black layer.
    This is not to implicate a designer. The retina is an outgrowth of brain tissue and the photoreceptors and pigmented epithelial cells have a common progenitor cells (in fact transplanted PE progenitor cells can form into photoreceptors allowing blind rats to regain vision). Evolution has allowed a functioning vertabrate visual system only by several “work-arounds” including cells and systems of neurons which increase gain, signal to noise ratio, and use of muller cells as “fibre optic cables”, a result open to interpretation (i.e. all retinal cells fall into mosaic patterns).
    Two final points:
    1) I quite enjoy the complexity of the mechanisms evolved to allow functioning visual signals to be evoked in a system of cells essentially created backwards (figuring out how this works keeps me employed) and it hurts me when people call the retina “incompetent” so please fuck off with that shit.
    2) To say that such a system was “designed” is laughable (I shit you not, every retinal physiologist or neuroanatomist I know would laugh their ass off if you told them this.) It is a demostration of evolutionary forces selecting the most adapted members of a species for procreation, i.e. you see, you survive.

Add Your Comments

All fields required.

Related Posts