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The eyes have it

by Donald Prothero, Aug 08 2012

Since the days of Darwin, eyes and evolution have been an irresistible topic for scientists and amateur authors alike. British biologist St. George Jackson Mivart was initially a supporter of Darwin, but when his Catholic religion caused conflict with Thomas Henry Huxley in 1871, he changed to a critic. Mivart’s critique focused on the issue of the perfection of the human eye and how he could not possibly imagine how it could have evolved by natural selection and random chance (a point still raised by creationists today who know nothing about comparative biology). In later editions of On the Origin of Species, Darwin specifically addressed Mivart’s criticism and carefully explained how the incipient stages of complex structures like the eye could be useful, and could have evolved by small steps; it did not require a giant leap to the complexity to develop the human eye. As Darwin first showed, nature is full of examples of every kind of photoreceptor, from simple light-sensitive cells to eyespots to simple eyes with no lenses, to a variety of solutions of seeing with more and more complex eyes. Once you arrange these solutions in an array, it is only a small step from one to the next, more complex eye. (Indeed, many animals actually show this transition during their embryonic development as their eyes change, and in some organisms, the eyes develop differently in males and females). In fact, the passages where Darwin talks about the eye are one of the most frequently “quote mined” by creationists trying to distort Darwin’s meaning, because they quote only the beginning of the paragraph were Darwin is setting up the creationist position in order to shoot it down the in the rest of the passage (which creationists never quote). In full context, the quote reads:

To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree.[This is where the creationist quote-mine usually ends]. Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself first originated; but I may remark that several facts make me suspect that any sensitive nerve may be rendered sensitive to light, and likewise to those coarser vibrations of the air which produce sound. (Darwin, On the Origin of Species, 6th ed., 1872, 143-144).

The rest of Darwin’s chapter then goes into great length describing the full range of photoreceptor solutions in the animal kingdom—none of which any creationist ever bothers to read, let alone address.

Fast-forward 153 later to the culmination of this line of argument, represented by Ivan Schwab’s outstanding book Evolution’s Witness: How Eyes Evolved. There have been a number of scientific papers that have expanded on Darwin’s comparative sequence of ocular solutions, but none in the beautiful full-color coffee-table book format that extensively reviews photoreception across all of biology, as does Schwab’s book. Schwab is a professor of ophthalmology at University of California Davis, so he knows eyes in a way that few biologists do, but he also takes great trouble to study and image photoreceptors from nearly every group of living organisms. The book shows not only spectacular color photographs of a wide range of organisms and close-ups of their eyes, but many images of histological sections through the eyes and head (done by Richard Dubielzig, DVM), color reconstructions of prehistoric animals by renowned paleoartist John Sibbick, and microphotography of eye histology. Detailed discussions of the eye anatomy of many key living organisms are provided, along with speculation about the eye anatomy of fossils with excellent preservation. Some, like trilobites, have preserved their crystal calcite lenses unaltered, so we can actually see what they could see. The anatomical discussion might be heavy going for those without any background in biology, but the author provides good diagrams and definitions of every anatomical term, plus a glossary, so those who wish to dig in and learn the material will be rewarded.

The book also attempts not just a comparative biology exercise, but a fully chronological account of the geological factors that were in play when each type of eye arose prehistorically. Thus, rather than running through the full gamut of eye types in, say, phylum Arthropoda or phylum Chordata, the chapters jump from the kinds of eyes that existed in invertebrates of a given geological period to the contemporary vertebrates, and back again. This exposition may be a little hard to follow for some readers, but it does allow one to see when each type of eye arose and under what geological conditions. Thus, we can better understand, for example, how 300 m.y. old Carboniferous dragonflies with wingspans almost 3 feet across had eyes the size of golf balls! They, like the 9-foot Arthropleura (a sowbug relative on steroids) or the foot-long cockroaches of this period, were able to grow so large because atmospheric oxygen levels at that time were much higher; oxygen is a critical limiting factor not only for arthropod growth, but especially for eye development. Elsewhere, he describes the amazing dolphin-like marine reptiles known as ichthyosaurs, some of which had eyes the size of beach balls, the largest eyes ever known. These animals apparently were divers into the dark depths of the oceans where they must have hunted prey (possibly large squid like modern sperm whales hunt) that live at such depths.

Schwab starts the story at the very beginning, discussing the chemicals and pigments found even in bacteria and algae, and how many (besides chlorophyll) are sensitive to light. He shows how some non-photosynthetic light-sensitive pigments have non-light-sensitive precursors, and evolved before the advent of photoreceptors, then were later co-opted for light sensitivity. Schwab goes into the recent research on evolutionary development of eyes, and how certain genes turn on or off expression of certain eye features, or even the entire eye. He reminds us how many animals have no need for sensitivity to light at all, let alone photoreceptors or eyes, and dispels our anthropomorphic notion that eyes are essential to evolutionary success. In discussing the appearance of the first real eyes of the trilobites in the Cambrian, Schwab does not make the mistake that Andrew Parker made in his 2004 book In the Blink of an Eye: How Vision Caused the Big Bang of Evolution. Parker’s book was excoriated in the scientific community, not only for its execrable writing style and lack of proper references, but for the even simpler reason that the “Cambrian explosion” was no “explosion” (it took at least 20-80 million years in a series of steps) and that the trilobites and their eyes were one of the last events in this long slow process, so they are unlikely to have caused anything more than the late radiation of more trilobites. Finally, in his review of cephalopod eyes, Schwab shows how the structure of they fluid-filled eyeball of an octopus evolved independently in that lineage and in the vertebrates—but the octopus eye is actually designed better than our eye! It has no “blind spot” in the retina caused by the exit of the optic nerve, and its layers of photoreceptors are not buried under several other layers of cells, distorting the vision, as our retinas are. The next time you run into a creationist who rhapsodizes about how beautifully designed nature is, remind them of the flawed construction of our eyes compared to that of an octopus or squid, and ask what that tells them about the Designer!

In project this large in scope, small errors are bound to creep in, especially in areas far from the author’s expertise in eye anatomy. For example, he mentions the discredited notion that a meteorite impact might have something to do with the great Permian extinction; he follows the controversial idea that turtles are nested within Diapsida; but he retains the outdated notions that mesonychids are closer relatives of whales than hippos and other artiodactyls. These can be forgiven, because the author is making an attempt to reach across disciplines and paint a broad picture in a geological context, and such an effort is hard for anyone, regardless of specialty, to manage.

In the broader sense, the entire book is an outstanding antidote to scientific ignorance and creationist lies. It is beautifully written and illustrated, yet the writing is accessible to anyone with some biology background and a willingness to follow the details carefully. The evidence for evolution screams out at you page after page. Like Darwin did in 1859, Schwab pounds the case home with example and example in a way that no creationist can rebut. Among the overwhelmingly positive reviews the book has received in the professional journals and on the internet, there are three pathetic one-sentence negative reviews by creationist trolls on the book’s page who clearly show that they didn’t actually read the book and could not comprehend it. Indeed, one of them admits that he gave a bad review to the book after only reading the “look inside” feature on the site and skimming the few pages that are presented. If that doesn’t summarize creationist “scholarship” in a nutshell, nothing does!

18 Responses to “The eyes have it”

  1. Max says:

    I have a friend who believes in evolution, but can’t believe that it’s driven by random mutations. It seems to be a matter of timing. Had the human eye evolved from a simple eye in 1000 generations, it wouldn’t be enough time for Darwinian evolution to work. How many generations are enough? Don’t forget that you also need a visual cortex to process all those visual signals, which sounds like a chicken-and-egg problem.

    • trurl says:

      If you take a human generation to be 20 years, then 1000 generations is only 20,000 years. 20,000 years ago is still homo sapiens or close enough. Obviously 1000 generations back to a simple eye is off by many orders of magnitude.

      • Max says:

        Right, I wanted to make it obvious. For comparison, a mutation 6-10,000 years ago is thought to have created blue eyes.

        So how many generations does it take to create more significant changes? And what does it take for them to persist and not die out? Like, if the first blue-eyed person had been killed for being a freak, would there be blue eyes today?

      • LovleAnjel says:

        The first blue-eyed person may not have had a lasting contribution to the gene pool anyway. There may be more than one mutation that could lead to blue eyes, and each one of those mutations could arise independently multiple times. In addition, a recessive mutation could be carried through time even if the homozygous individuals are selected against. When that selection ends, homozygous individuals survive and it seems as though the mutation has suddenly spread through the population. In reality, it spread long ago in heterozygous individuals.

  2. Max says:

    Mantis shrimp have the most impressive eyes. Each eye can move independently, see in stereo, and perceive hyperspectral and both linearly and circularly polarized light. The latter may be a result of sexual selection, since the shrimp have body parts that circularly polarize light, which can be detected by potential mates but not by their predators.

    • Max says:

      That’s another chicken-and-egg problem. What evolved first, the perception of circular polarization or the circular polarizing body parts?

  3. Bjørn Østman says:

    FYI, the egg evolved before the chicken.

  4. kraut says:

    In view of human evolution as part of the evolution from a common ancestor of the primates it is rather nonsensical to speak of the evolution of the human eye.
    The human eye evolved not from what you call a “simple” eye – did you ever actually read anything about evolution? but from a already rather developed eye that we inherited from previous mammalian ancestors.

    There are really some stupid questions.

    • Max says:

      Use your imagination, kraut. Imagine if something as complicated as a human eye evolved from something as simple as a simple eye in 1000 generations. Imagine if humans had evolved from previous mammalian ancestors in 10,000 generations. This would be way too fast for Darwinian evolution, agree? What about 100,000 generations? How fast is too fast? That’s the question.

  5. kraut says:

    here a morphological comparison of the human eye vs. primate eyes

  6. kraut says:

    “This would be way too fast for Darwinian evolution, agree?”

    How fast is too fast? We have present day examples toward speciation among vertebrate animals with distinct characteristics in isolated populations within several hundred generations or less.

    Just look at the example of dog breeding, and what non natural selection within several tens of generations can – unfortunately – achieve. Or the whole area on traditional breeding of livestock and crop production, corn one of the prime examples.

    How fast is too fast is really not a question. We know through the paleontological record, through genetic analysis that there is a frequency in genetic change, but that this frequency is a constant only statistically and fast changes i.e. duplications of whole sets of genes with evolutionary potential can happen very fast, that the selection process – the non random aspect of evolution – depends heavily on the environment the population of animals lives in, the stresses a population encounters.
    If evolution does not happen fast enough to adapt to living within a given environment – there is an end to evolution for this particular species.

  7. James Robert Smith says:

    There are some excellent photographic records of the cascade of trilobite fossils showing the development of eyes from the earliest trilobites (which had only light-sensing spots) to the last trilobites that had complicated, highly developed, compound eyes. Want to see the evolutionary development of eyes? Look to the fossil record of that extinct class.

  8. Nils says:

    Just finished reading Richard Dawkins “The greatest show on earth”. A book that is easy to read/understand when your background is not in biology. Mine for example is in photography…Still I always believed in evolution but after this read I’m just amazed about the facts that are available. Overwhelmed in fact. So anyone that wants a good introduction to evolution, go Dawkins!

    • double-helical says:

      I agree! Dawkins’ book is a tour de force, and one should not be put off by its “weightiness.” (It’s a big book.) I loved it, and add my recommendation to yours.

  9. double-helical says:

    Dr. Prothero,
    Thanks for the excellent book review. This sounds like a handy book to keep, well, on the coffee table! I doubt that it will convince any true believers, but it should be good for the undecided.

  10. Insightful Ape says:

    Excellent book review. Just one point: telling creationists that the human eye has a blind spot while the squid eye doesn’t may not be helpful. When I’ve had this argument with them they point out (correctly) that human vision is better than squid vision (for example, photorecpetors looking away from light may actually account for better circulation). What really shuts them up is patterns of distribution: out of many tens of thousands of chordata, there isn’t a single one without a blind spot, just as out of dozens of squid and octopus species there isn’t a single one that does. All they can do when confronted with this is to throw up their hands and go “god did it that way”. (To trick us into thinking that evolution happened, maybe?)

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