Eye



As an organ developed via the opportunistic twists and turns of evolutionary processes, the human eye is explainable. As an organ designed and created by an infinitely wise deity, the human eye is inexcusable. An eye is a differentiated sensory organ that detects light, and is very likely the organ you're reading this with.

There are many different types of eyes in use throughout the animal kingdom, ranging from the extremely rudimentary (i.e., light-sensitive patches of skin) to the exactingly precise (i.e., lens-and-retina). There are also eye analogs in unicellular eukaryotes (family Warnowiaceae, genus Chlamydomonas, and genus Euglena); eukaryote is the domain that also includes the animal, plant and fungus kingdoms. The human eye in particular is firmly in the 'exactingly precise' part of this spectrum.

Some creationists assert that the human eye (and in general, all eyes) is so complex/precise/advanced that it must have been designed by a designer, rather than being the current state of a continuing process of evolution.

However, given the fact that there are a number of different classes of eyes which, collectively, exhibit a wide range of degrees of complexity, it is difficult to understand what would absolutely prevent a lineage from acquiring a series of successive increases in the complexity of its eyes, eventually ending up with something akin to the human organ. Creationists tend to ignore this.

How did the eye evolve?
But even with these conservative assumptions, the time taken to evolve a fish eye from flat skin was minuscule: fewer than 400,000 generations. For the kinds of small animals we are talking about, we can assume one generation per year, so it seems that it would take less than half a million years to evolve a good camera eye. In spite of being a marvellous and complex organ, the evolution of the eye is actually not that difficult. In fact, ol' Charlie already described the basic process in On the Origin of Species in 1859 (see "" below).

The oldest fossil eyes that have been discovered are about 540 million years old (Schmidtiellus reetae in the trilobite subphylum), right at the start of the Cambrian explosion; some have suggested that the evolution of the eye caused an "evolutionary arms race". Simulations by Nilsson et. al. have shown that going from a basic light-sensitive patch to a complex eye can take as little as 360,000 generations, or about 364,000 years (or a mere 0.00802% of the Earth's lifetime). This is a base minimum and almost certainly took longer. Don Lindsay provides a simple explanation of the idea behind Nilsson's study.

The precise path that the evolution of the eye took isn't known. Traditionally, it has been assumed that eyes evolved about 40 to 65 times independently, but more recent genetic evidence seems to suggest that all the eye variations that exist today evolved from the same very simple eye (most famously, the Pax6 gene or variants (PaxB, PaxC) exists in all species with sight — from fruit flies to humans — suggesting a common ancestor). Whether this actually indicates a common ancestor is debated, as it may also be the case that the genes could have served a different function unrelated to the eye. This is still a topic of on-going research, but that doesn't mean we can, with a great deal of certainty, say some meaningful things about the evolution of the eye.

One possible(!) pathway is as follows:
 * 1) Light-sensitive proteins could evolve through a random mutation and re-use pre-existing cascade structures to signal other proteins and activate the cell. Natural selection would prefer organisms which do something useful with this mutation, such as improving the feeding pattern (i.e., during the day food may be more abundant, so conserving energy during the night would be advantageous).
 * 2) If one cell is evolutionary advantageous, then several cells may be more advantageous due to redundancy and due to the higher probability of successfully perceiving light.
 * 3) A flat patch of cells can be slightly improved by creating a small depressed area, which allows some amount of sense of where the light is coming from.
 * 4) Said depression can depress further until there is only a small slit of light coming in. This allows a more complex "picture" to be seen, rather than the more binary "light yes/no" of the flat patch and the "light on this side yes/no" of the depressed patch.
 * 5) Said depression may be covered by a membrane that would magnify the light coming in, which has obvious benefits.
 * 6) Muscles to move the "half eye" around would allow greater field of vision and more accurate determination of the source of the light.
 * 7) Detecting basic colors is more useful than detecting no colors (e.g., "is that the toxic blue flower or the edible yellow flower?").
 * 8) The ability to control the size of the slit (even a little bit) and its magnifier is useful to refocus and get just a little better vision at certain distances.

And with that, the hypothetical eye would be reasonably similar to the eye possessed by most mammals today.

Irreducible complexity and the eye
Creationists have claimed that "the eye is too complex to have evolved", meaning that the eye is only useful as a fully functional organ that it is today (in many animals), and taking away any part makes it useless.

Scientists' inability to explain eye evolution down to every microscopic detail does not demonstrate that the eye can't evolve or that the mechanisms of evolution are incapable of generating the complexity of the eye. This alleged inability also cannot invalidate other examples of evolution, either.

That complex and advanced eyes exist today doesn't mean that less complex eyes in various stages of evolution don't exist (Why are there still monkeys?); some eyes in various stages of evolution:


 * 1) Charles Darwin already listed many different stages of eye evolution in The Origin of Species. Trying to picture the eye as being far from any possible evolutionary path ignores well-known facts (see also, below).
 * 2) There is evidence that the eye in fact evolved. The squid and octopus have a retina that is the reverse of vertebrates' retinae. Our optic nerve and blood supply to the eye pass through the retina and feed it from the inside. The squid's eye is supplied and attached from the back. Squids and octopi lack the blind spot that all vertebrates have due to the entry point of the nerve. The superficial similarities of retinae between taxa, but structurally identical retinae within taxa is a good example of the type of hierarchy that common descent would produce. The use of rhodopsins for light detection across a wide-range of organisms is also evidence for common descent. Indeed, the general mechanism for photoreception is the same across metazoans (animals plus prokaryotes). The known organisms sharing the mechanism include all light-sensitive animals, from primitive eye-less Hydra species to humans, as well as single-celled prokaryotes such as Chlamydomonas and Euglena species.
 * 3) Scientists' inability to explain eye evolution in detail does not demonstrate that the eye can't evolve or that the mechanisms of evolution are incapable of generating the complexity of the eye. This alleged inability also cannot invalidate other examples of evolution, either.
 * 4) Molluscan eyes alone, present a huge spectrum of both primitive and complex eyes, from the primitive eye-cups of limpets, the lens-less eyes of scallops, and the pin-hole camera eyes of the nautilus, to the lens-camera eyes of squid, octopi and

Fallacies contained in this claim

 * Argument from incredulity: I can't understand how eye evolution worked, so it must be unevolvable.
 * Argument from design — "A complex watch has a designer therefore a complex eye has a designer."
 * God of the gaps: Science can't explain absolutely everything about the eye, so the eye was intelligently designed by God.

Darwin quotemine
Charles Darwin has been misquoted by almost innumerable creationists on evolution of the eye. The quotemine is so prolific that even some scientists get it wrong (although usually not to prove a point). The most common quote is "To suppose that the eye … could have been formed by natural selection, seems … absurd in the highest possible degree."

Yet, as Mark Isaak of Talk.Origins notes, "The quote is taken out of context. Darwin answered the seeming problem he introduced." Even CreationWiki &mdash; not known for abandoning creationist arguments &mdash; states, "most of those who use these quotes do not realise that they were lifted out of context by Watchtower Bible and Tract Society and they have seldom checked the original source" (though they then proceed to explain why quote mining isn't quote mining). The Christian Apologetics & Research Ministry, an unwaveringly Christian apologist group, also explains that the quote is a quotemine &mdash; and then argues that evolution is based on Darwin's imagination. Hell, WikiQuotes has an entire page for the misquote.

Moreover, as Babinski notes, even if the quote was valid (it's not), Darwin is not a "modern source". (Darwin's book is 0 years old!) It's as if these quotemining creationists were unable to find problems in modern evolutionist writings &mdash; and so attacked the easier-to-rebut literature of the distant past.

Darwin's original quote is collapsed below, due to its extreme length. Bolded sections are those actually quoted by creationists &mdash; to show how little is actually quoted. Judge for yourself whether the quotes below accurately describe what Darwin was trying to convey.

Here are several examples from creationists (in all cases emphasis added):

Watchtower
The Watchtower Bible and Tract Society misquoted Darwin in Life — How Did It Get Here? By Evolution or by Creation? (1985):

This misquote is asserted by CreationWiki to be the source of all the other misquotes.

Michael Griffith
Michael Griffith misquoted Darwin (and argued "what use is half of an eye") in Why the eye refutes evolution (1997):

Griffith tries to make the quote more believable by asserting (without evidence) that Darwin knew about the flaws of the evolution of the eye, but deceitfully argued for evolution anyway.

Ben Rast
Ben Rast misquoted Darwin in Lambert's Evolution (2002):

Jordan Niednagel
Jordan Niednagel misquoted Darwin in ''Monkey Business. Against Supposed Similarities'' (2002):

Answers in Genesis
Answers in Genesis proudly continues the tradition of misquoting of Darwin in "The Seeing Eye":

"Having abandoned his Christianity, Darwin was obliged to appeal to the absurd…" Oh, the irony.

Suboptimal design
If life was intelligently designed, the designer must have been thinking shortsightedly — eyes have innumerable flaws, many of these in the "favored" species, humans. In direct contrast to claims above that the eye is too complex or too perfect to have evolved, the eyes present in the world exhibit numerous flaws — which point towards evolution from a common ancestor &mdash; as Darwin suggested.

Vestigial eyes
Many organisms that live in perpetually dark environments (such as caves or the deep sea) are no longer subjected to evolutionary pressures that require them to respond to visual stimuli. As a result, their eyes have become nonfunctional. Many of these now-blind organisms, especially cave-dwelling salamanders, still retain their eyes as vestigial features, even though they can no longer see with them. It seems very unlikely that such organisms would be intelligently designed to have an almost totally useless organ.

Vertebrate eyes
Vertebrates have evolved a suboptimal eye (although only in the sense that any complex organ is suboptimal). In the eyes of vertebrates the photo-sensitive cells lie behind the optic nerve endings and a layer of blood vessels. These must emerge through the back of the retina, leaving a hole in the organ with no light-sensitive cells, forming a blind spot (find your own! ). Other animals which have evolved sight independently, such as cephalopods (e.g. squids and octopi) and insects, have much more effective eyes because they don't have a blind spot or nerves between the photo-sensitive cells and the light source. In cephalopods, the photocells in the retina each point towards the light, with their nerves out behind.

There are three major failings of the vertebrate eye from a design point of view:
 * 1) The blind spot in an eye is poor design (think about it!) that has consequences beyond the mechanical. We do not normally notice the blind spot, usually because each eye fills in the gap left by the other eye, but also because our brains fool our consciousness by filling in the missing bit, which means that extra brain processing is required to give us a full picture of the world. More design to correct the initial (flawed) design is surely not intelligent design!
 * 2) The second problem is a minor loss of visual acuity due to having the extra material between the retina and the light. The photons of light that hit the retina are scattered slightly before they hit the photoreceptors, causing a very slight blur.
 * 3) The worst result of the design is the relative ease with which the vertebrate retina can become detached from layer beneath it &mdash; the pigment epithelium. Each beat of the heart stretches and then relaxes the retina, pulling it away from this layer. Any retinal damage, usually a tear caused by direct trauma to the eye (e.g. a blow to the head) to the retina, is thus exaggerated by each and every beat of the heart. This would not occur if the blood vessels were on the "correct" side of the retina. In addition, trauma can cause fluid to build up between the retina and the pigment epithelium, forcing the retina to detach. Detached retina is one of the most frequent causes of visual impairment around the world and is entirely a result of this moronically bad design. The analogous cephalopod eye, with its "right-way-round" retina, cannot suffer from this because the retina is anchored into the blood-supplying tissue by the nerve wiring itself.

Additional failings are:
 * The retina is inside out, thus reducing image resolution.
 * Variations in eye shape cause common focusing problems (myopia and presbyopia). An estimated 60 percent of people need corrective vision aids.
 * Macular degeneration, which increases with age, but is caused by the structure of the eye. The existence of the macula is a "partial fix for the 'backward' arrangement of retinal layers with the nerve and blood vessels between the receptors and the direction of light", and unnecessary in cephalopods.
 * Angle closure glaucoma is a disease that is caused when the drainage of the vitreous humor becomes blocked. The drainage channel is "unnecessarily precarious and susceptible to blockage."
 * The human eye has six extraocular muscles that control the movement of the eye. In principle only three muscles would be necessary, but none of the six are redundant — the loss of any one causes eye movement impairment.

The vertebrate "design" can easily be explained by the theory of evolution — a proto-eye evolved which was simply an area of light-sensitive skin. By chance, nerve endings happened to be between the light sensitive area and the light sensitive cells — it (essentially) was a 50/50 chance where the cells were programmed to develop. Since the light sensitive cells gave the individuals which possessed them a slight selection advantage, they stuck, and developed into an eye.

Due to the role of melanin in eyesight for most mammals, including humans, albino individuals of those species are negatively affected to various degrees. Their eyes look deformed (in humans, violet and red eye color are exclusive to albinos since their blood vessels show through), they are far more prone to nearsightedness and/or blindness, all of which is prone to progression, and bright light badly damages their eyes at every exposure.

Human eyes are an even worse design than just having a back-to-front retina. Many animals have multifocal lenses and are able to magnify objects much farther away than humans are capable of seeing. If humans are the designer's favored creature, it doesn't show.

Creationist responses
Creationists have not yet produced an effective explanation for the design of the vertebrate eye, but they have produced many failed hypotheses.

By and large, creationists focus only on the first two problems of eye design, the blind spot and the minor loss of visual acuity. Since both of these problems are relatively minor, they can handwave them away with a shrug, usually while smugly accusing scientists of trying to make mountains from molehills. (However, it is not remotely realistic to brush off retinal detachment, the single most common cause of blindness in the world.) Another creationist claim is that the eye couldn't be designed any other way, or that the design actually isn't suboptimal:


 * A surprisingly common creationist claim is that the eye, for various reasons, couldn't be designed any other way. This crashes hard into the fact that other animals, such as gastropods and cephalopods, do have eyes designed another way, with the blood supply and nerves attached round the back. Creationists who are aware of this try to explain it by pointing out that cephalopods live in the ocean and try to salvage the situation from the different environments they and humans experience. However, fish are also found in the oceans and yet have the vertebrate-style eye, whereas terrestrial gastropods such as snails live in a similar environment to humans and yet have their retina arranged similar to those of cephalopods. If, for the sake of argument, we accept that the vertebrate eye was well-designed for land and the cephalopod eye well designed for the water, then fish and land snails both have badly designed eyes! (Perhaps the designer was just messy, and put the wrong design in the wrong animals?)


 * Another argument is that the arrangement was necessary for heat management reasons. It is often proposed that the choroid is responsible for acting as a heat sink, by taking away the heat that could threaten the retina. However, the choroid is a substantial producer of heat too (light that passes through the photoreceptors is absorbed by the choroid and the retinal pigment epithelium as heat). The photoreceptors in vertebrate eyes are located very close to these layers. This can be advantageous in low-temperature conditions, but also threatens the integrity of the retina if they overheat (ocular hyperthermia). The cells of the retinal epithelium produce the 11-cis retinal molecule required for sight, so it could be reasoned that these pigmented cells must be located near to the photoreceptors, and to counter this the choroid is required to remove the heat. However, in cephalopods, the photoreceptors produce their own 11-cis retinal molecule.


 * Other creationists, such as Jonathan Sarfati have claimed that the cephalopods have very poor vision, but their visual problems aren't due to the retinal design, rather certain problems with the lens they haven't evolved a solution for yet. Besides, their visual acuity isn't in general worse than that of fish, which have the vertebrate eye. And, pointing out another aspect of bad design would only make the "Intelligent designer" seem less competent!


 * Gurney also cites the evidence that the xanthophyll pigmentation of cells covering the retina — while allowing visible light through — absorbs blue and ultraviolet light and therefore protect the retina from photic damage. But given how little UV light is blocked by these thin layers of pigment (20-40%), this is a bit like praising the bullet-stopping power of cardboard. One wonders why the designer gave some animals this protection and not others.


 * One creationist response to the problems of the vertebrate eye is that one couldn't "build a better one. Build it perhaps, but it is not difficult to envision one (for the same reason that one can tell that a Rolls Royce is higher quality than a Yugo GV). If creationists really believe this, you should never ask them for shopping advice!


 * A slightly less common argument is that the orientation of the vertebrate eye helps to prevent light from being reflected off the the choroid onto the photoreceptors, which would cause blurry vision. While this would be the case if light was reflected, it is not clear from the creationist arguments exactly how the vertebrate eye provides any protection against reflection. It is clear that human eyes do reflect light, as evident by the 'red eye' effect in flash photographs, and even more so in other vertebrates (bats, cats, dogs, crocodiles, horses etc.) due to their reflective layer behind the retina known as the tapetum lucidum.

Nautilus eye
As photographers will tell you, a 'pinhole camera' is an effective way to form an image out of light; the small aperture at the front forms an image on the screen behind it. The smaller the hole, the sharper the image. The downside is that the smaller the aperture, the less overall light gets in. To solve the problem, camera designers add a lens. A lens is an obvious improvement, since it allows both a sharp and bright image. We might therefore expect the Intelligent Designer of organisms to use lenses in eyes.

For the most part, the 'designer' did. Octopus and squid eyes have lenses… vertebrate eyes have lenses. There is however one cephalopod mollusc, the nautilus (family Nautilidae), which is considered more primitive than octopuses and squid, since it has an external shell. It lives at considerable depths, where light is at a premium. The nautilus has a very good pinhole camera eye; considerable thought presumably went into its design. But the designer apparently saw fit not to give that eye a lens. Its eyes are therefore far less efficient than it easily could have been. Did the designer just forget?

What use is half an eye?
Thus the creationist's favourite question "What is the use of half an eye?" Actually, this is a lightweight question, a doddle to answer. Half an eye is just 1 per cent better than 49 per cent of an eye. Creationists ask, "What use is half an eye?" The answer is "a lot". After all, humans normally have two whole ones! However, they usually claim that half an eye is "not useful at all".

It is claimed that the eye is irreducibly complex and that all the parts of an eye are needed for any operation; and that any part "missing" would leave an eye defective. This is not true. You can still see when your lens is removed, and a dysfunctional iris doesn't lead to complete blindness, as anyone who is shortsighted or longsighted can tell you. Various forms of colour blindness is another fairly common eye failure (and may actually be an evolutionary advantage in certain conditions. )

This is really a different way of phrasing "The eye is too complex to have evolved", except in a straw man manner that makes the evolutionary case look more ridiculous. As a nice piece of equivocation "half" is typically interpreted as "some parts of the human eye removed", rather than "a simplification of the current eye".

This is an overgeneralization of evolutionary theory; the eyes developed over millions of years — not a blind slug one day growing half an eye.

Responses

 * 1) "Half an eye" can easily be a useful detector of light, just as pit vipers have useful detectors of infrared (see, below).
 * 2) What does "half an eye" look like? Eagles have incredibly acute vision, far more so than human beings; from an eagle's perspective, do humans have only "half an eye"? The question tries to give the impression of an eye which has been physically cut in half and which would be completely useless for vision. If the question is posed as "Would 50% of current vision still be useful?" then the problem simply does not exist.
 * 3) This is an overgeneralization of evolutionary theory. Rudimentary eyes could be produced with chance beginnings, like the ability to notice changes in light through a rudimentary retina in an animal — not a blind slug one day growing half an eye.
 * 4) One of the most rudimentary eyes known belongs to single-celled marine plankton known as warnowiids (family Warnowiaceae). In warnowiids, the eye-like ocelloid evolved from organelles: the lens/cornea evolved from mitochondria and the photorceptor evolved from a chloroplast. Due to the extremely small size of the photoreceptor and the wavelength of light that it receives, it is estimated that the photorceptor is roughly equivalent to one screen pixel. Besides being able to detect changes in light, the photoreceptor may also be able to detect changes in polarization.
 * 5) There is a documented progression from light-sensitive spots through the light sensors of clams to the human eye and further to the octopus eye, through at least five different lines of development. The octopus eye is arguably more developed than the human eye, as the human eye has blood vessels and nerves in front of the retina, obscuring it and giving the 'blind spot' &mdash; the octopus eye has neither problem. Despite octopi having highly-developed eyes, they also simultaneously have use for primitive eyes: their skin contains photoreceptors that enable them to cue camouflage changes.
 * 6) In those organisms that live in dark environments, especially caves or the deep sea, their eyes have become nonfunctional because these organisms are no longer subjected to evolutionary pressures that require them to respond to visual stimuli from their environments. Many of these blind organisms, especially cave-dwelling salamanders, still retain their eyes, even though they can no longer see with them. For more information on the topic, see vestigial features.
 * 7) Many humans can not see color, yet their eyes otherwise benefit them enormously.

Fallacies contained in this claim

 * Straw man: "Half an eye" is very subjective.
 * Equivocation: What does "half" mean?
 * Argument from ignorance: I can't understand how, so it can not be.
 * Suppressed evidence of useful partial eyes

Types of eyes
There are at least 9 known types of layouts of the eye in organisms, divided into simple or non-compound eyes (5) and compound eyes (4).