How did the eye evolve?

Feb 1, 2014


Discussion by: peterparker

According to this it started with a light-sensitive spot on the skin probably in order to see light a day which would help you to survive.

http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html

But that would require a huge step of nerves going from the skin to the brain. How could this massive step happen in one child birth? Because the wires without the light-sensitive skin would be pointless and the light-sensitive without the connecting nerves would be pointless.

36 comments on “How did the eye evolve?

  • 1
    David R Allen says:

    There is great survival advantage to being able to distinguish varying levels of light. And if there is a great survival advantage, then natural selection will act with great power to improve this skill. Your example, “in order to see light a day ” is not a very important survival advantage. Light helps an organism determine which way is and down. Great feeding advantage. It also tells you the difference between light areas and dark areas, which, depending on your food preference would provide a survival advantage. It also allows you to react to a sudden change in light, which might signal the approach of a predator. Again, great survival advantage.

    The link you include provides most of the information you need to answer your own question. I also note that this paragraph:-

    “But that would require a huge step of nerves going from the skin to the brain. How could this massive step happen in one child birth? Because the wires without the light-sensitive skin would be pointless and the light-sensitive without the connecting nerves would be pointless.”

    This is straight out of creationist dogma. Do you Peter, believe that the “Eye” evolved or was it created by god. Just asking.

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  • 2
    I-am-not-a-theist says:

    No big steps are required. Just random mutations and natural selection over many generations. Simple photoreceptor proteins that sense light on early lifeforms (our ancestors) must have given them an evolutionary advantage. Just add millions of years to the evolutionary mix and you get something as marvellous as the eyes of the octopus, eagle, krill and bee.

    If you are asking how this could happen in childbirth, it all starts with something called the sperm and an egg but as I do not know your age I will go no further than that.

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  • 3
    Stuart Coyle says:

    These early creatures that evolved light sensitivity probably had nothing much resembling a brain to run nerves to. You could get a beneficial effect from simply connecting the light sensitive cells to whatever motor cells move the creature. I’m not a biologist and don’t know the details, but looking from a systems control point of view, you can easily create light seeking, constant light or light avoiding behaviour in a system without central control. You simply link up a feedback loop between the light sensor (eye spot) and the motor (cillia, fins, whatever).

    I also note that even plants show light sensitive movement and they have neither nerves nor brain.

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  • The question assumes that light-sensitive skin cells would need to connect to the nerve cells in order for the organism to see. However, this is not the case as illustrated in the following extract:

    The simplest organ which can be called an eye consists of an optic nerve, surrounded by pigment-cells, and covered by translucent skin, but without any lens or other refractive body. We may, however, according to M. Jourdain, descend even a step lower and find aggregates of pigment-cells, apparently serving as organs of vision, without any nerves, and resting merely on sarcodic tissue. Eyes of the above simple nature are not capable of distinct vision, and serve only to distinguish light from darkness.
    — Charles Darwin, The Origin of Species 6th Edition, 1872.

    The nerve cells didn’t have to find the light-sensitive skin cells: the nerve cells were surrounded by them. No massive step required. The extract also shows that there is also no requirement for nerve cells and a brain in order to react to light.

    The mention of “one child birth” in the question implies that the eye evolution happened in modern day humans. They were apparently fumbling about blind until their eye evolved and connected up to their brain with a serendipitous childbirth. Presumably, their cochleae were also having trouble connecting to their brains and required another fortuitous event. That is not what happened.

    Ward et al.’s Light-sensitive neurons and channels mediate phototaxis in C. elegans. shows that conversion of light into electrical signals is an evolutionarily ancient biochemical pathway, which may have been present in the urbilateria, the hypothetical last common ancestors of all bilaterally symmetrical organisms which are believed to have lived some 600 million years ago. These organisms would have been 1-2 mm in size and they, along with their light-sensitive cells, evolved over those 600 million years to modern day humans.

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  • 5
    Zeuglodon says:

    In reply to #3 by Stuart Coyle:

    These early creatures that evolved light sensitivity probably had nothing much resembling a brain to run nerves to. You could get a beneficial effect from simply connecting the light sensitive cells to whatever motor cells move the creature.

    A good example is the phylum of animals known as cnidaria, in which the eyes and the nerves are such that the eyes are basically light triggers for muscular contractions, though it varies from species to species. Really, there are plenty of examples of eyes and corresponding nerves being at various degrees of complexity in different species.

    OP:

    But that would require a huge step of nerves going from the skin to the brain. How could this massive step happen in one child birth?

    It didn’t, but then that’s not how evolution works (i.e. in one massive step). The logic that enables a gradual adjustment of pigment detector to complex eye over multiple generations also enables a gradual adjustment of simple cause-and-effect nerves to large-scale visual processing over multiple generations. The benefit of a mutation is how well it works in tandem with the rest of the body, not in isolation, so while an eye variant that had no effect on the optic nerve (or an organism’s closest equivalents) would be pointless, it also wouldn’t be beneficial in the first place.

    Focusing the light, for instance, doesn’t require modification of the optic nerves (though it could enable it) in order to improve visual acuity, though it could enable it; for instance, when photoreceptors were scattered over the retina, the focusing would in turn trigger evolution to concentrate the receptors closer to the focal point, which in turn could trigger the evolution of the lens to improve focusing, which in turn could trigger more sophisticated tweaks of the visual processors in the nervous system, and so on. Pleiotropy (one gene having multiple effects) is also possible, but rarer and harder to demonstrate by comparison.

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  • “But that would require a huge step of nerves going from the skin to the brain”
    No, the nerves were there already for something else – touch. When one became sensitive to light it responded differently to the cells around it. Assuming that the cell was sitting there doing nothing when it suddenly became light sensitive and needed to connect to the brain shows a gross misunderstanding of how Evolution works.

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  • ” Because the wires without the light-sensitive skin would be pointless and the light-sensitive without the connecting nerves would be pointless.”

    As an eye, you are correct. But the proto-eye described in the PBS article shouldn’t be considered an imaging tool. If you have a cell or collection of cells that respond in some way to the stimulus of light you have a beginning. The response could be limited to the cell(s) involved or the response could in turn create stimuli for neighboring cells, including nervous tissue. This proto-eye might assist the organism in detecting movement or night/day cycles. It might even assist the organism in turning away from or toward light sources, like a sunflower.

    To go from this earliest precursor of an eye to a recognizable eye in one step would not be evolution. Gradual changes over generations that tend to confer advantage is the way natural selection works.

    Here’s a fun (because it’s rather old) video of Dawkins demonstrating stages of eye development.
    Dawkins, Growing Up in the Universe, eye evolution

    In terms of the organism perceiving an image then the photoreceptor would need to be tied to the brain. But perceived images are not the only benefit of photoreceptors. And the association of nerve tissue and photoreceptors is almost inevitable. And if the original ” light-sensitive spot on the skin” were nerve tissue we skip a step.

    The PBS article is too vague to be of much use. But for anyone curious enough there is a wealth of information available. There’s also a wealth of misinformation and disinformation. Beware your own biases.

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  • 8
    AllusiveAtheist says:

    ” Because the wires without the light-sensitive skin would be pointless and the light-sensitive without the connecting nerves would be pointless.”

    As an eye, you are correct. But the proto-eye described in the PBS article shouldn’t be considered an imaging tool. If you have a cell or collection of cells that respond in some way to the stimulus of light you have a beginning. The response could be limited to the cell(s) involved or the response could in turn create stimuli for neighboring cells, including nervous tissue. This proto-eye might assist the organism in detecting movement or night/day cycles. It might even assist the organism in turning away from or toward light sources, like a sunflower.

    To go from this earliest precursor of an eye to a recognizable eye in one step would not be evolution. Gradual changes over generations that tend to confer advantage is the way natural selection works.

    Here’s a fun (because it’s rather old) video of Dawkins demonstrating stages of eye development.
    Dawkins, Growing Up in the Universe, eye evolution

    In terms of the organism perceiving an image then the photoreceptor would need to be tied to the brain. But perceived images are not the only benefit of photoreceptors. And the association of nerve tissue and photoreceptors is almost inevitable. And if the original ” light-sensitive spot on the skin” were nerve tissue we skip a step.

    The PBS article is too vague to be of much use. But for anyone curious enough there is a wealth of information available. There’s also a wealth of misinformation and disinformation. Beware your own biases.

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  • “You see without the truth of the eyes the happy folk were blind”

    I have a feeling that a lot of “happy folk” will be gathering today and at least one of their theist leaders will be telling them that the eye is an example of why their deity must exist and how evolution is false. We can never prove how the eye evolved conclusively but we can demonstrate plausible steps from light sensitivity all the way to fully functioning eyes. However even the slightest gap in those steps will be taken as proof of God’s existence.

    We always come back to the same discussion, if the eye is implausible because of its complexity, just how implausible is a deity that can fashion such an eye? The happy folk truly are blind.

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  • 10
    Alan4discussion says:

    In reply to #3 by Stuart Coyle:

    These early creatures that evolved light sensitivity probably had nothing much resembling a brain to run nerves to. You could get a beneficial effect from simply connecting the light sensitive cells to whatever motor cells move the creature. I’m not a biologist and don’t know the details, but looking from a systems control point of view, you can easily create light seeking, constant light or light avoiding behaviour in a system without central control. You simply link up a feedback loop between the light sensor (eye spot) and the motor (cillia, fins, whatever).

    I also note that even plants show light sensitive movement and they have neither nerves nor brain.

    Modern plants do indeed illustrate this:

    http://en.wikipedia.org/wiki/Heliotropism
    >

    Heliotropic flowers track the sun’s motion across the sky from east to west. During the night, the flowers may assume a random orientation, while at dawn they turn again toward the east where the sun rises. The motion is performed by motor cells in a flexible segment just below the flower, called a pulvinus. The motor cells are specialized in pumping potassium ions into nearby tissues, changing their turgor pressure. The segment flexes because the motor cells at the shadow side elongate due to a turgor rise. Heliotropism is a response to light from the sun.

    The leaves of some trees also track the Sun.

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  • 11
    Alan4discussion says:

    @OP – How did the eye evolve?

    This title is misleading.

    Eyes have evolved many times in many different forms. – From simple light sensitive patches, through various pits and pin-hole systems to lenses, focussing lenses, and compound eyes.

    Some creatures such as shellfish or spiders, have more than one type of eye in the same organism.

    http://en.wikipedia.org/wiki/Mollusc-eye – Eyes have evolved independently between seven and eleven times in the molluscs,[3] which goes some way to explain the diversity of eye types observed. Molluscs have eyes of all levels of complexity, from the pit eyes of many gastropods, to the pinhole eyes of the Nautilus, to the lensed eyes of the cephalopods. Compound eyes are present in some bivalves, and reflective ‘mirrors’ have been innovated by other lineages such as scallops.[1] As well as varying in complexity, the eyes of molluscs span a huge range in size; they may be from 20 µm (0.02 mm) to 27 cm (11 in) across.

    Insect eyes can see ultraviolet light which humans cannot. Some snakes have sensory pits which can detect infra-red more accurately than heat sensitive nerves in ordinary skin.
    Cetaceans and bats can produce mental sonic images using no light at all, while other creatures like spiders are also sensitive to vibrations.

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  • 12
    Sjoerd Westenborg says:

    In reply to #10 by Alan4discussion:

    @OP – How did the eye evolve?

    This title is misleading.

    Eyes have evolved many times in many different forms. – From simple light sensitive patches, through various pits and pin-hole systems to lenses, focussing lenses, and compound eyes…

    Plus, eyes don’t only differ in their make-up (no pun intended), but also in origin. The eyes of squids and octopuses are very similar to ours, but evolved independently.

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  • 13
    God fearing Atheist says:

    Even some bacteria have a sensible response to light – Phototaxis. They don’t have brains, nerves or even eyes, just photosensitive pigments near the cell wall that set off a cascade of chemical reactions when hit by light which the cilia respond to. Eye evolution starts with those chemicals via a billion tiny steps.

    This is a typical creationist question, and from spider-man.

    Peter, if this is not a hit-n-run, if you want to learn about evolution, and you can program a computer, I suggest you look up “Computer science genetic algorithms” and experiment.

    And yes, mutation can add information – “The researchers also found that all Cit+ clones sequenced had in their genomes a duplication mutation of 2933 base pairs that involved the gene for the citrate transporter protein used in anaerobic growth on citrate, citT.

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  • 14
    CdnMacAtheist says:

    Hi Peter.

    As Nastika mentioned in Comment 4, there has been about 600,000,000 years since those short-lived urbilateria were around. http://en.wikipedia.org/wiki/Urbilaterian

    If you were to take the average lifespan of the known lineage from them to us to be 1 year – even today many small animals like mice only live 2 years and can reproduce after a few months – then there have been 600 million opportunities for evolution by natural selection to improve all the parts of these lifeforms.

    That’s a lot of accumulation of very tiny improvements – in all aspects of the organism – that each increased the chances of surviving at least until reproduction, and a whole bunch more than one child being born and developing, isn’t it?

    There is a massive and increasing amount of well-supported scientific information available that you can study to understand these facts.

    It takes a while – I’ve been at it 50 years since my teens – but the basics are relatively easy to understand for a non-scientist (like me) if you are a curious free-thinker who accepts reality and follows all the evidence found by real experts…. Mac.

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  • 15
    steve_hopker says:

    Hi Peter

    We’ve had a few posts about eyes as if they are a case to question evolution – I suppose it’s a long standing argument, dating back at least to Paley.

    Firstly, as others say, Darwin (or Dawkins) does not claim that major evolutionary steps happen over just one generation. We can see in our own species (and so perhaps more easily) that while children grow up to resemble their own parents, they are not the same as them. Much of that is due to the mixing of half each parent’s genes in the children, but occasionally a different gene arises from a mutation. Over many generations, myriad small changes resulted in larger changes seen only with the benefit of hindsight, which is what we are doing on a huge scale in the case of eyes, looking back over hundreds of millions of years and perhaps billions of generations.

    I think the actual facts about eyes seems to make the case for evolution stronger, not weaker. One can trace a line from the simplest light responsiveness (algae) to the most exalted (hawks). For even single celled organism can have light sensitivity. This article (http://learn.genetics.utah.edu/content/selection/eye/) summarises the topic. And this might be of interest (http://msdadmin.scican.net/mhs/Biology%20Class%20ONline/Flatworms.html). It points out that flatworms (a.k.a. platyhelmenthes) have two light sensitive spots each side of what one might call the head end, ie maybe the simplest thing one might link to later eyes. Flatworms are different to cniderians (jellyfish etc) in that, like most animals, they have three basic layers and bilateral symmetry, with a ‘head’-‘tail’ directionality. Jellyfish (double layered bags with tentacles) have a network of nerves and light sensitivity (see Zeugladon, post 5).

    So I think one can trace an evolutionary line in eyes (that correlates with other bodily features such as body plan) even in simpler creatures – from little blobs, to living carrier bags, to flattened tubes. This line is evident in modern organisms and fossils (rarer but not unknown in cniderians or platyhelmenthes). Also the human embryo in which these evolutionary stages are roughly repeated in the early stages of development (http://necsi.edu/projects/evolution/evidence/embryos/evidence_embryo.html). And from Nastika (4) the increasingly rich data from genes suggests a line can also be traced through DNA.

    Assuming that you are reading I hope you find these and other articles helpful (we do have occasional ‘fly-posters’).

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  • 16
    4as4is4 says:

    It’s not as difficult as the question makes out. Sensitivity to light does not need to be anywhere near as complicated as comprehensive, technicolour modern eyesight. Plants, after all, are phototropic. Even mustard and cress seeds bend towards sunlight (if ever they get a peek of it in stom-tossed Britain).

    Early light sensitivity probably had no focus at all, but was the activity of this super-abundant energy source (perhaps not yet an information source) on the outer surface of simple bacterial life. In water, of course, the journey to land comes after eyesight has developed. The bacteria evolved and formed colonies which also evolved in a context where it was always advantageous to have some idea of light and dark, day nd night. Start your story from there and it becomes easier to envisage.

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  • 17
    4as4is4 says:

    Thousand apologies DRA.

    I pressed “flag” for your comment instead of “like”. I am having trouble controlling the cursor.

    I’m really sorry.

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  • 18
    Alan4discussion says:

    @OP – But that would require a huge step of nerves going from the skin to the brain. How could this massive step happen in one child birth? Because the wires without the light-sensitive skin would be pointless and the light-sensitive without the connecting nerves would be pointless.

    This is a misconception put about by Creationists. Eyes did not evolve belatedly on already complex vertebrate animals. They evolved from simple light sensitive cells which evolved into nerves.

    Detecting light to avoid being left cold and inactive in shadow, or encouraging an organism to sink deeper into the water, or to crawl under a rock to avoid being roasted in the sun, has selective survival potential.

    Eyes have taken many generations to evolve in the many different organisms where they have evolved.

    Where complex eyes have evolved, they have progressed through the stages, gradually building slight improvements, one step at a time, from the very simple light-sensitive patches, to various complex camera-like forms with lenses and adjustments of light intensity.
    There are numerous examples of all these stages in present-day living animals, as well as in fossils.

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  • 20
    SaganTheCat says:

    shenanigans

    sounds like the standard “irriducible complexity” mantra.

    I wouldn’t mind but as animal organs go, the eye is rather a simple example. your entire body has complex networks of nerve cells. Vision is an extremely complex subject in itself but eyes? meh, there are people with prostetic instruments that can see with their tongues these days.

    in short. evolution through natural selection is the answer, what you beleive is too complex is merely a manifestation of the limitations of your own imagination.

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  • 21
    Alan4discussion says:

    In reply to #18 by Paris Price:

    For fun we used to catch jelly fish. For being blind they were very evasive.

    It is a myth that jellyfish are blind.

    http://en.wikipedia.org/wiki/Box-jellyfish

    Whereas some other jellyfish do have simple pigment-cup ocelli, box jellyfish are unique in the possession of true eyes, complete with retinas, corneas and lenses. Their eyes are located on each of the four sides of their bell in clusters called rhopalia. This enables them to see specific points of light, as opposed to simply distinguishing between light and the dark. Box jellyfish also have 20 ocelli (simple eyes), that do not form images but detect light and dark; they therefore have a total of 24 eyes

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  • 22
    Bill Crofut says:

    Re: “How did the eye evolve?”

    As an unlettered Traditional Roman Catholic, militant young-Earth Biblical creationist and geocentrist, my chosen status obviously posits me in a tiny minority of the population. My personal position regarding this issue is: Evolution, as a “process,” is delusionary mythology. Evolutionism, the only reality, is a false, atheistic religious belief system.

    Re: http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html

    “Zoologist Dan-Erik Nilsson demonstrates how the complex human eye could have evolved through natural selection acting on small variations. Starting with a simple patch of light sensitive cells…”

    Prof. Nilsson’s demonstration was the action of an intelligent agent with a specific purpose in mind. Where has anything resembling the demonstration been observed in the “natural world” without the action of an intelligent agent? What is the origin of the “simple patch of light sensitive cells?”

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  • 23
    DHudson says:

    In reply to #21 by Bill Crofut:

    Prof. Nilsson’s demonstration was the action of an intelligent agent with a specific purpose in mind. Where has anything resembling the demonstration been observed in the “natural world” without the action of an intelligent agent? What is the origin of the “simple patch of light sensitive cells?”

    From wiki: The earliest predecessors of the eye were photoreceptor proteins that sense light, found even in unicellular organisms, called “eyespots”. Eyespots can only sense ambient brightness: they can distinguish light from dark, sufficient for photoperiodism and daily synchronization of circadian rhythms. They are insufficient for vision, as they cannot distinguish shapes or determine the direction light is coming from. Eyespots are found in nearly all major animal groups, and are common among unicellular organisms, including euglena.

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  • 24
    Sheepdog says:

    Rather than being too complex to have evolved, the creationists common statement, the eye actually carries with it all the chunks of bad design that it has collected along the way.
    The retina is basically backwards, it has a big hole in the middle, the image is upside down, the focusing mechanism is crap. It goes on and on, as well as it does other parts of the body.
    The real remarkable thing is the Photoshop that goes on in the brain to correct the inherent problems. “Intelligent Design?” On the contrary, downright bad design, and when considered rationally, and with a very solid developmental history, does far more to demonstrate the accuracy of evolution, rather than argue against it.

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  • 25
    Alan4discussion says:

    In reply to #23 by Sheepdog:

    The retina is basically backwards,

    I would be careful with that feature. There is some evidence that a more deeply embedded retina gives it some protection from Solar radiation thus making it more durable and long lived than say Octopus eyes which are wired with the photoreceptors in front.

    it has a big hole in the middle, the image is upside down, the focusing mechanism is crap.

    The rest of your points are basically accurate. If you bought a camera of this quality, you would want your money back! (especially if you were a herbivore!) – Animals like horses can adjust the shape of their lenses as well but not to the extent that we can.

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  • 26
    Sheepdog says:

    In reply to #24 by Alan4discussion:

    In reply to #23 by Sheepdog:

    The retina is basically backwards,

    I would be careful with that feature. There is some evidence that a more deeply embedded retina gives it some protection from Solar radiation thus making it more durable and long lived than say Octopus eyes which are wired with the p…

    Point well taken. My other favourite example, as someone who is a martyr to sinusitis, is to wonder what “Intelligent Designer” would ever put the drain of a tank in the top of the tank rather than the bottom? Our sinuses are still structured to work properly when walking on all fours, or when supine, which is why we blow our noses in the morning when we wake up.

    It is one of those little details that there has never been real pressure on natural selection to correct. Or, possibly there is. I would be interested to see the genetically driven sinus structure of those individuals who, unlike me, have larger, or otherwise more effective sinus drains.

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  • 27
    Sheepdog says:

    In reply to #24 by Alan4discussion:

    In reply to #23 by Sheepdog:

    The retina is basically backwards,

    I would be careful with that feature. There is some evidence that a more deeply embedded retina gives it some protection from Solar radiation thus making it more durable and long lived than say Octopus eyes which are wired with the p…

    I knew that horse fences at equestrian facilities are always timber, and painted white, rather than the cheaper, simpler, and more common wire fencing generally employed because horses cannot distinguish wire, and injure themselves against wire fences. I suspect your point about imperfect “auto-focus” is the base reason.

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  • 28
    thomaskprakash says:

    The contemporary world presents a range of eyes from the simplest to the multi-lensed complex eyes possessed by a range of animals. The simplest eye need not have a brain to work.

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  • 29
    Markovich says:

    This question is adequately addressed by evolutionary biology. Instead of setting yourself up as an expert, take some courses. You will have to take a great many before you tackle this particular topic, however.

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  • 30
    nick keighley says:

    light sensitivity can work without a central nervous system. In fact there are some theories that the brain evolved from the optic nerve.

    Frogs change colour to match the pond they are living in. That might work without a nervous system. I could well imagine a colour sensitive spot releasing hormones or other chemical signals

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  • 31
    YesUCan says:

    In reply to #25 by Sheepdog:

    In reply to #24 by Alan4discussion:

    In reply to #23 by Sheepdog:

    The retina is basically backwards,

    I would be careful with that feature. There is some evidence that a more deeply embedded retina gives it some protection from Solar radiation thus making it more durable and long lived than say Oc…

    I think we are not in a position to criticize Nature for its results achieved by evolution. Prevailing physical laws, events, and biological and chemical processes determine the state of survival machines.

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  • 32
    hemidemisemigod says:

    But that would require a huge step of nerves going from the skin to the brain. How could this massive step happen in one child birth? Because the wires without the light-sensitive skin would be pointless and the light-sensitive without the connecting nerves would be pointless.

    The “huge step” of nerves going from skin to the brain happened over a huge period of time – in tiny steps. Modern plants and animals have receptor cells all over their bodies that are connected to their brains for sensing many different things: touch, heat, light, chemicals, etc. None of these appeared overnight. Fossils that show eyes go back over 500 million years. Light sensitive cells may have evolved many hundreds of millions of years before that. At that time there would be no complex skins or brains, just simple plants and bacteria.

    You might just as well ask how eggs happened “in one child birth” when there were no chickens to lay them.

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  • Gerald McDonald #33

    You seem to have the methodology back to front. The explanations of the evolution of eyes are based on observations of eyes in various organisms within the framework of the established theory of evolution by natural selection. A great array of different kinds of eyes are observed in organisms, and those observations are part of the abundant evidence in support of the said theory, which successfully explains the occurrence of the eyes observed and every other biological development. If another theory were proposed as a better one to supplant the present one, then it would have to be tested against the evidence to determine which theory were in fact the better.

    Your question, in any case, seems to require a predetermined, precise definition of ‘eye’, and you seem to think that some test needs to be done in order to determine whether what has been observed as actually been observed! But in evolutionary biology, an eye is any anatomical feature that enables an organism to distinguish variations in ambient light, however vaguely or sharply. The eyes that you and I are happy to own are not at all perfect, nor are they useless; they do a good enough job of enabling us to distinguish things around us by means of the ambient light. If people still exist one hundred thousand years from now, it is anyone’s guess what their eyes will be like. There is no finished result of the eye, so your wish to “test and demonstrate” that the evolutionary process has actually formed the eye makes no sense. Report abuse

  • Gerald, Hi. Welcome.

    As you might be expecting processes that takes thousands of generations to happen don’t get “demonstrated” of a piece. Like any detective case the explanations are built from the facts of many observations.

    Once we understand that different complexity eyes can be useful in different ways we can look backwards and see how  these simpler forms were useful at every stage in their evolution for creatures that lived like thus and so, hunted food like this avoided predators like that.

    The eye has evolved from little light sensitive patches (ocelli) many times so we see exactly these types of progression of eye and lifestyle in current living creatures.

    The mistake made by those mystified by the seeming complexity of the eye is to imagine there is some kind of “meant to be” about it. There is none, only current expediency. Eyes meet current demands well enough, with an ever present pressure to do a little better as the ecosystem itself changes.

    Such detective work is however consolidated by our ability to genetically relate living animals and their features and project backwards.

    Dawkins books are great here, but I also recommend Neil Shubin “Your Inner Fish” as giving great insight into how evolution uses the features currently expressed in the bodies of animals, say, to be adapted to other ends and relieve selection pressures with the least possible effort. Report abuse

  • Gerald McDonald says: –
    I would like to know just when the explanations of the processes given as to how the finished results of the eye . . .

    These links should give you the details of the evolution of the eye from nerves cells connecting simple light sensitive spots to the rudimentary brain, and the structural progression to the modern mammal eye.

    https://en.wikipedia.org/wiki/Evolution_of_the_eye

    In modern shellfish there is sill a range of types of eye from the simple eyes . through cup eyes, to camera eyes.

    https://en.wikipedia.org/wiki/Mollusc_eye

    Many types of eye have also evolved independently in spiders insects etc! – with sensitivity to different wavelengths – such as ultraviolet.

    https://owlcation.com/stem/Spider-Eyes

    https://animals.mom.me/insect-compound-eye-vs-human-eye-5728.html

    https://phys.org/news/2013-09-mantis-shrimp-world-eyesbut.html

    We even have reactions to light in plants, where some grow towards the sun, and those like Sunflowers, turn their heads to follow the sun.

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