The Evolution Of The Rectangular Eye
    By Ashley Cox | October 21st 2008 04:00 AM | 17 comments | Print | E-mail | Track Comments
    About Ashley

    I'm an undergrad in chemistry on my way to getting a Ph.D.

    I have many different interests including genetics, fire fighting, rock


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    Eyes are the interface between the outside world and our brain's perceptions.   Being the instrument of sight, and therefore a lot of our information, we rely on the functions of our eyes quite obsessively, and why wouldn't we?  They make it possible to observe our beautiful world and really appreciate different species and the unique characteristics they hold - like their eyes. 

    Photo by David Reece
    Evolution of the Eye

    Since eyes do not fossilize very well, we cannot exactly depict the changes in eyes as we can with the evolution of, let's say, legs and what not. What scientists can do, and what they have done, is re-create the most likely plan of the evolution of eyes.  This includes the development of light sensitive cells which, over thousands of generations and under the guidelines of natural selection, allowed the development of the eye. Sounds simple?  Not quite. There are no 100% accurate models for the eye, though scientists believe they have come close.

    One of the best explanations derive from the first animals that have what resembles an eye. These creatures lived about 600 million years ago, with not so much an eye as light-sensitive genes called "opsins".   The Hydra is a freshwater cnidarian similar to corals and jellyfish. It's body is coated with opsins, which scientists believe gave it an advantage in capturing prey, and keeping its distance from predators likewise. With the discovery of this animal, latest researches are naming it the possible origin of vision.
     David Plachetzki, UCSB
    Biologist Todd Oakley at UC Santa Barbara says that this new development, "show[s] very clearly that specific mutational changes in a particular duplicated gene (opsin) allowed the new genes to interact with different proteins in new ways. Today, these different interactions underlie the genetic machinery of vision, which is different in various animal groups.”

    Hydras have no eyes or light-receptive organs, but they have the genetic pathways to be able to sense light, allowing scientists to more accurately model the evolution of the eye. 

    Basic Structure and Function of the Eye

    The basic eye is anything but basic.    Instead, it is very complex, constantly adjusting to surroundings and to the amount of light it lets in.  Though the intricate workings of an eye contain several components, the ones we'll discuss today are the pupil and the iris.

    The pupil is the point in which light enters the eye and results in the mind conjuring an image. The larger the pupil, the more sunlight shines through.    The evolution of the pupil lies in the idea of "survival of the fitter", better known as Darwin's Theory. Differences in circumstances and in surroundings determine not only mundane physical characteristics, but how we use our eyes as well. The pupil is involuntarily controlled by the iris, which constricts and dilates in order to regulate the amount of light that enters the cornea.

    The iris is the control center for light, controlling how much is let into the eye. When there is an abundance of light the iris adjusts by shrinking, allowing just enough light to be absorbed. Under darker circumstances, however, the iris expands to allow maximum light to be absorbed. The function of the iris is great proof of evolution by clearly defining which animals would survive better.  Animals whose eyes could adapt to lighting variations could easily and under any circumstances avoid predators, and thus pass those genes on.

    Variations in the structure of the eye have happened throughout evolutionary history. What causes these Differences?  Why, Natural Selection, of course!

    Pupils also come in types, depending on the purpose it may hold for the animal that contains it. For example, a species of animals who spends a majority of time in the sun usually have eyes with spherical pupils. Those who need be active during all times of day and night are better equipped with pupils which can easily vary or adjust, such as rectangular eyes.   Common variations include:

    Spherical: This is the typical pupil seen in the human species as well as in primates, canines, and the just of the animal kingdom. This type of pupil adapts well to the intensity of light during the day, though isn't necessarily proficient during nighttime outings. Useful in broad observations of carnivores and like predators.

    Credit: University of Florida

    Vertical Slit:
    Cats, many species of snakes, alligators and crocodiles possess these eyes. Although these types of pupils are a useful adaptation for nocturnal animals, the animals who usually have them are both active day AND night. Because of their exposure to daytime lights, this pupil allows protection of the retina in daylight glare.

    Photo By Fiona J. Lowrie

    Rectangular:  Sheep, Goats, Octopuses and Toads have these rectangular shaped pupils. Typically classified as prey, these animals need to have a defense both day and night. But they don't have vertical slits due to their need to survey their surroundings more accurately. The narrower the pupil in relation to the horizon, the greater the accuracy of depth perception is in the peripheral vision of the animal. The perception of depth must be considered with these animals who spend their time evading predators in a rugged terrain.

     Photo By Ken Ching

    "If eyes were made for seeing, then beauty is its own excuse for being."  - Ralph Waldo Emerson


    On Cubozoans ..............

    Reading this article immediately sent me to Life's Solution by Simon Conway Morris (ISBN 0521603250).  Stumbling over dinoflagellates (single-celled protista, as we are taught to call them these days) with "bulbous lens and underlying cup" (genera Erythropsidinium and Warnowia), I find what my memory has prompted me about, namely the Cubozoa or "box jellyfish".

    I will not "rabbit on" as we say this side of the Pond, but recommend the section Vision in the Wikipedia article on Box jellyfish.
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    "But the don't have vertical slits due to their need to survey their surroundings more accurately. The narrower the pupil, the greater the accuracy of depth perception is in the peripheral vision of the animal." -- So you've just contradicted yourself: if they needed greater acuity, then a slit would suit them better.

    And, as the proverbial bright schoolchild (and every photographer) knows, a smaller aperture gives you a *greater* depth of field, meaning that, if it has any direct effect on depth perception at all, it makes it less acute, not more acute. (There is an indirect effect: the image on the retina is sharper in the direction normal to the orientation of the slit, boosting the effect of binocular vision - but it seems safe to assume that that's not what you meant to say.)

    "The narrower the pupil, the greater the accuracy of depth perception is in the peripheral vision of the animal." Narrow in respect to the horizontal, not vertical planes... I'm sorry I should have clarified more.
    I thought the earth was estimated to be around 4.5 billion years old - It's interesting that the hydra is older than the earth (evidence of extra-terrestrial life perhaps). I think that the author ment 600 milllion years rather than billion. Even the universe isn't considered to be that old, estimated to be about 15 billion years.

    .....(evidence of extra-terrestrial life perhaps). I think that the author ment 600 milllion years rather than billion.
    haha yes a typo, can't believe no one caught that (including myself) thank you for pointing it out.
    The article on the hydra (opsin) states that 'the authors speculate that they use light sensitivity in order to find prey' and your article states 'scientists believe gave it an advantage in capturing prey.

    I'm not so sure some researchers 'speculation' could ever be termed a 'belief'.

    Speculation: guess: a message expressing an opinion based on incomplete evidence
    Belief: any cognitive content held as true

    Why would you state that what is merely a 'guess' in the original article be held up as 'true' in your article?

    Something's wrong here.....

    while i find your article fascinating, i'm a little concerned about your use of copyrighted images. i appreciate that you gave me credit, but i do not recall giving permission to use my photo. this is especially important if you are paid for this publication! i wouldn't necessarily ask you to remove it, but in future either ask for permission or use images which are not copyrighted (eg. creative commons).

    i would have emailed you privately, but could not find your contact details.

    please be careful!

    Hi Fiona, Yahoo is the API/engine of the text editor and they provided the Flickr search feature inside it. Their documentation says they only return images that are usable under creative commons. Since they own Flickr, we assume they know what they are doing but no software works perfectly, even that made by multi-billion companies. I went to your site and you have a big ole © on everything else so I don't know how they missed it. Apologies for the confusion - it's a terrific picture! We'll take it down if you'd like.
    thanks for the prompt reply, hank! and thank you for identifying the culprit. perhaps yahoo needs a little reminder that things aren't running so smoothly. i'd do it myself, but unfortunately i'm on a hectic schedule before heading out of town on vacation.

    understanding the confusion, i'm going to say leave my photo up. there is my permission. (can i also say, prints will be available in the near future at ? or is that pushing the limits? lol!)

    apologies to ashley for hijacking the thread!


    Never assume that a big company knows what it's doing.

    The image named "The Human Eye" is being used without permission. Please email me a billing address for licensing the use of my image, or remove it immediately.

    By opening a Flickr account you agreed to the Terms of Service, which included consenting to a common use clause. Flickr can distribute your pics with the only obligation being to provide credit to you. Credit is provided. If you float your mouse pointer over the image there's a popup note which says that you are the photographer. So, if Flickr has a usage agreement with Scientific Blogging, they're within rights to post your pic.

    Flickr has no such clause in their Terms of Service. They leave the licensing issue to the owner of the image, by letting him/her choose the kind of licensing they want for each individual image, such as putting an image into the Creative Commons or making the image available through Getty under some other form of licensing. My image clearly states "All rights reserved", and as such, the use of the image on this site is clearly copyright infringement. Whether I'm credited or not is completely irrelevant.

    Ok, if you say so. But if that's the case, it's unproductive to complain about it here. I just happened to see your post by accident, and I'm nobody in particular. You need to contact the Site's administrators directly if you want anyone responsible to know about it.

    If you'd cared to read some of the previous posts, this seems to be the right place to bring this up.

    That discourse about Flickr pics was way back in Spring of last year. Hank Campbell hasn't said anything to you directly on this thread in the last two days, so what makes you think he's even read your complaint? There's a lot of stuff posted on this site every day. He may not even have time to read all of it. It's a matter of record that he doesn't comment on every blog post. If you want to make sure he reads your complaint, then contact him. There's a "Contact Us" link at the bottom of this page that takes you to a message template.

    He did send me an email so we changed it, of course.   As I mentioned above, the Yahoo API they provide, including Flickr, does not work as well as they think it does; our graphic search feature built in here is only supposed to retrieve images available for use with attribution.

    Honestly, why anyone puts an image online is beyond me.    With all of the companies that use just the Yahoo API it must be practically a fulltime job tracking down the people who use a graphic by mistake.    But Ashley had nothing to do with that - there are 10 million good images of the human eye and I found a public domain one in 5 seconds on Google - unfortunately we can't use flickr without using the Yahoo API so we are stuck cleaning up their mistakes.
    This article is interesting, but not very well written; also, perhaps the title should be "The Evolution of the Rectangular Pupil." A rectangle, to my knowledge, is a two-dimensional geometric shape, and can't be used to describe a three-dimensional object...not to mention there isn't a species on the planet with a "rectangular" eye.