Evolution of the eye

Evolution of the eye

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The evolution of the eye has been a subject of significant study, as a distinctive example of a homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

 organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...

 present in a wide variety of taxa. Certain components of the eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...

, such as the visual pigments, appear to have a common ancestry – that is, they evolved
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 once, before the animals radiated
Evolutionary radiation
An evolutionary radiation is an increase in taxonomic diversity or morphological disparity, due to adaptive change or the opening of ecospace. Radiations may affect one clade or many, and be rapid or gradual; where they are rapid, and driven by a single lineage's adaptation to their environment,...

. However, complex, image-forming eyes evolved some 50 to 100 times – using many of the same proteins and genetic toolkits in their construction.

Complex eyes appear to have first evolved within a few million years, in the rapid burst of evolution known as the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...

. There is no evidence of eyes before the Cambrian, but a wide range of diversity is evident in the Middle Cambrian Burgess shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...

, and the slightly older Emu Bay Shale
Emu Bay shale
The Emu Bay Shale is a geological formation in Emu Bay, South Australia, containing a major Konservat-Lagerstätten . It is one of two in the world containing Redlichiidan trilobites...

. Eyes show a wide range of adaptations to meet the requirements of the organisms which bear them. Eyes may vary in their acuity, the range of wavelengths they can detect, their sensitivity in low light levels, their ability to detect motion or resolve objects, and whether they can discriminate colours.

History of research



The complex structure of the eye has been used as evidence to support the theory that they have been designed by a creator
Creator deity
A creator deity is a deity responsible for the creation of the world . In monotheism, the single God is often also the creator deity, while polytheistic traditions may or may not have creator deities...

, as it has been said to be unlikely to have evolved via natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

. In 1802, philosopher William Paley
William Paley
William Paley was a British Christian apologist, philosopher, and utilitarian. He is best known for his exposition of the teleological argument for the existence of God in his work Natural Theology, which made use of the watchmaker analogy .-Life:Paley was Born in Peterborough, England, and was...

 called it a miracle of "design". Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...

 himself wrote in his Origin of Species, that the evolution of the eye by natural selection at first glance seemed "absurd in the highest possible degree". However, he went on to explain that despite the difficulty in imagining it, this was perfectly feasible:

...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.


He suggested a gradation from "an optic nerve merely coated with pigment, and without any other mechanism" to "a moderately high stage of perfection", giving examples of intermediate grades of evolution. Darwin's suggestions were soon shown to be correct and current research is investigating the genetic mechanisms responsible for eye development and evolution.

Rate of evolution


The first fossils of eyes that have been found to date are from the lower Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 period (about ). This period saw a burst of apparently rapid evolution, dubbed the "Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...

". One of the many hypotheses for "causes" of this diversification, the "Light Switch" theory of Andrew Parker
Andrew Parker (zoologist)
Andrew Parker is a visiting member of the University of Oxford Department of Zoology since 1999. He is a Royal Society University Research Fellow, an Ernest Cook Research Fellow, and a Research Associate of the Australian Museum and University of Sydney...

, holds that the evolution of eyes initiated an arms race that led to a rapid spate of evolution. Earlier than this, organisms may have had use for light sensitivity, but not for fast locomotion and navigation by vision.

Since the fossil record, particularly of the Early Cambrian, is so poor, it is difficult to estimate the rate of eye evolution. Simple modelling, invoking small mutations exposed to natural selection, demonstrates that a primitive optical sense organ based upon efficient photopigments could evolve into a complex human-like eye in approximately 400,000 years.David Berlinski
David Berlinski
David Berlinski is an American educator and author of several books on mathematics. Berlinski is a Senior Fellow of the Discovery Institute's Center for Science and Culture, the hub of the intelligent design movement. Though he criticizes the theory of evolution, Berlinski who is an agnostic,...

, an intelligent design proponent, questioned the basis of the calculations, and the author of the original paper d Berlinski's criticism.


One origin or many?


Whether one considers the eye to have evolved once or multiple times depends somewhat on the definition of an eye. Much of the genetic machinery employed in eye development is common to all eyed organisms, which may suggest that their ancestor utilized some form of light-sensitive machinery – even if it lacked a dedicated optical organ. However, even photoreceptor cells may have evolved more than once from molecularly similar chemoreceptors, and photosensitive cells probably existed long before the Cambrian explosion. Higher-level similarities – such as the use of the protein crystalin in the independently derived cephalopod and vertebrate lenses – reflect the co-option of a protein from a more fundamental role to a new function within the eye.

Shared traits common to all light-sensitive organs include the family of photo-receptive proteins called opsin
Opsin
Opsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical...

s. All seven sub-families of opsin were already present in the last common ancestor of animals. In addition, the genetic toolkit for positioning eyes is common to all animals: the PAX6 gene
PAX6
Paired box protein Pax-6 also known as aniridia type II protein or oculorhombin is a protein that in humans is encoded by the PAX6 gene.- Function :PAX6 is a member of the Pax gene family...

 controls where the eye develops in organisms ranging from mice to humans to fruit flies
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...

. These high-level genes are, by implication, much older than many of the structures that they are today seen to control; they must originally have served a different purpose, before being co-opted for a new role in eye development.

Sensory organs probably evolved before the brain did—there is no need for an information-processing organ (brain) before there is information to process.

Stages of eye evolution



The earliest predecessors of the eye were photoreceptor proteins that sense light, found even in unicellular organisms, called "eyespot
Eyespot apparatus
The eyespot apparatus is a photoreceptive organelle found in the flagellate cells of green algae and other unicellular photosynthetic organisms such as euglenids. It allows the cells to sense light direction and intensity and respond to it by swimming either towards the light or away from the...

s". Eyespots can only sense ambient brightness: they can distinguish light from dark, sufficient for photoperiodism
Photoperiodism
Photoperiodism is the physiological reaction of organisms to the length of day or night. It occurs in plants and animals.Photoperiodism can also be defined as the developmental responses of plants to the relative lengths of the light and dark periods...

 and daily synchronization of circadian rhythm
Circadian rhythm
A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...

s. 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
Euglena
Euglena is a genus of unicellular flagellate protists. It is the best known and most widely studied member of the phylum Euglenozoa , a diverse group containing some 44 genera and at least 800 species. Species of Euglena are found in fresh and salt waters...

. The euglena's eyespot, called a stigma, is located at its anterior end. It is a small splotch of red pigment which shades a collection of light sensitive crystals. Together with the leading flagellum, the eyespot allows the organism to move in response to light, often toward the light to assist in photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

, and to predict day and night, the primary function of circadian rhythms. Visual pigments are located in the brains of more complex organisms, and are thought to have a role in synchronising spawning with lunar cycles. By detecting the subtle changes in night-time illumination, organisms could synchronise the release of sperm and eggs to maximise the probability of fertilisation.

Vision itself relies on a basic biochemistry which is common to all eyes. However, how this biochemical toolkit is used to interpret an organism's environment varies widely: eyes have a wide range of structures and forms, all of which have evolved quite late relative to the underlying proteins and molecules.

At a cellular level, there appear to be two main "designs" of eyes, one possessed by the protostomes (molluscs, annelid worms and arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s), the other by the deuterostomes (chordate
Chordate
Chordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...

s and echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....

s).

The functional unit of the eye is the receptor cell, which contains the opsin proteins and responds to light by initiating a nerve impulse. The light sensitive opsins are borne on a hairy layer, to maximise the surface area. The nature of these "hairs" differs, with two basic forms underlying photoreceptor structure: microvilli
Microvillus
Microvilli are microscopic cellular membrane protrusions that increase the surface area of cells, and are involved in a wide variety of functions, including absorption, secretion, cellular adhesion, and mechanotransduction....

 and cilia
Cilium
A cilium is an organelle found in eukaryotic cells. Cilia are slender protuberances that project from the much larger cell body....

. In the protostomes, they are microvilli: extensions or protrusions of the cellular membrane. But in the deuterostomes, they are derived from cilia, which are separate structures. The actual derivation may be more complicated, as some microvilli contain traces of cilia — but other observations appear to support a fundamental difference between protostomes and deuterostomes. These considerations centre on the response of the cells to light – some use sodium to cause the electric signal that will form a nerve impulse, and others use potassium; further, protostomes on the whole construct a signal by allowing more sodium to pass through their cell walls, whereas deuterostomes allow less through.

This suggests that when the two lineages diverged in the Precambrian, they had only very primitive light receptors, which developed into more complex eyes independently.

Early eyes


The basic light-processing unit of eyes is the photoreceptor cell, a specialized cell containing two types of molecules in a membrane: the opsin
Opsin
Opsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical...

, a light-sensitive protein, surrounding the chromophore
Chromophore
A chromophore is the part of a molecule responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others. The chromophore is a region in the molecule where the energy difference between two different molecular orbitals falls...

, a pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

 that distinguishes colors. Groups of such cells are termed "eyespots", and have evolved independently somewhere between 40 and 65 times. These eyespots permit animals to gain only a very basic sense of the direction and intensity of light – enough to know when they are safely in a cave, for example, but not enough to discriminate an object from its surroundings.

Developing an optical system that can discriminate the direction of light to within a few degrees is apparently much more difficult, and only six of the thirty-something phylaThe precise number varies from author to author. possess such a system. However, these phyla account for 96% of living species.


These complex optical systems started out as the multicellular eyepatch gradually depressed into a cup, which first granted the ability to discriminate brightness in directions, then in finer and finer directions as the pit deepened. While flat eyepatches were ineffective at determining the direction of light, as a beam of light would activate exactly the same patch of photo-sensitive cells regardless of its direction, the "cup" shape of the pit eyes allowed limited directional differentiation by changing which cells the lights would hit depending upon the light's angle. Pit eyes, which had arisen by the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 period, were seen in ancient snail
Snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

s, and are found in some snails and other invertebrates living today, such as planaria
Planarium
Planarian is the common name given to many non-parasitic flatworms of Turbellaria class. It is also the common name for a member of the genus Planaria within the family Planariidae. Sometimes it also refers to the genus Dugesia....

. Planaria can slightly differentiate the direction and intensity of light because of their cup-shaped, heavily-pigmented retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

 cells, which shield the light-sensitive cells from exposure in all directions except for the single opening for the light. However, this proto-eye is still much more useful for detecting the absence or presence of light than its direction; this gradually changes as the eye's pit deepens and the number of photoreceptive cells grows, allowing for increasingly precise visual information.

When a photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

 is absorbed by the chromophore, a chemical reaction causes the photon's energy to be transduced into electrical energy and relayed, in higher animals, to the nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...

. These photoreceptor cells form part of the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

, a thin layer of cells that relays visual
Visual perception
Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...

 information, including the light and day-length information needed by the circadian rhythm system, to the brain. However, some jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...

, such as Cladonema, have elaborate eyes but no brain. Their eyes transmit a message directly to the muscles without the intermediate processing provided by a brain.

During the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...

, the development of the eye accelerated rapidly, with radical improvements in image-processing and detection of light direction.


The "pinhole camera
Pinhole camera
A pinhole camera is a simple camera without a lens and with a single small aperture – effectively a light-proof box with a small hole in one side. Light from a scene passes through this single point and projects an inverted image on the opposite side of the box...

" eye was developed as the pit deepened into a cup, then a chamber. By reducing the size of the opening, the organism achieved true imaging, allowing for fine directional sensing and even some shape-sensing. Eyes of this nature are currently found in the nautilus
Nautilus
Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in two genera, the type of which is the genus Nautilus...

. Lacking a cornea or lens, they provide poor resolution and dim imaging, but are still, for the purpose of vision, a major improvement over the early eyepatches.

Overgrowths of transparent cells prevented contamination and parasitic infestation. The chamber contents, now segregated, could slowly specialize into a transparent humour, for optimizations such as colour filtering, higher refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

, blocking of ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 radiation, or the ability to operate in and out of water. The layer may, in certain classes, be related to the moulting
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...

 of the organism's shell or skin.

It is likely that a key reason eyes specialize in detecting a specific, narrow range of wavelengths on the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

—the visible spectrum
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...

—is because the earliest species to develop photosensitivity
Photosensitivity
Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light.- Human medicine :Sensitivity of the skin to a light source can take various forms. People with particular skin types are more sensitive to sunburn...

 were aquatic, and only two specific wavelength ranges of electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

, blue and green visible light, can travel through water. This same light-filtering property of water also influenced the photosensitivity of plants.

Lens formation and diversification


Lenses evolved independently in a number of lineages. Simple 'pit-eyes' probably developed lenses to improve the amount of light that reached the retina; the focal length of an early lobopod with lens-containing simple eyes focussed the image behind the retina, so while no part of the image could be brought into focus, the intensity of light allowed the organism to see in deeper (and therefore darker) waters. A subsequent increase of the lens's refractive index probably resulted in an in-focus image being formed.

The development of the lens in camera-type eyes probably followed a different trajectory. The transparent cells over a pinhole eye's aperture split into two layers, with liquid in between. The liquid originally served as a circulatory fluid for oxygen, nutrients, wastes, and immune functions, allowing greater total thickness and higher mechanical protection. In addition, multiple interfaces between solids and liquids increase optical power, allowing wider viewing angles and greater imaging resolution. Again, the division of layers may have originated with the shedding of skin; intracellular fluid may infill naturally depending on layer depth.

Note that this optical layout has not been found, nor is it expected to be found. Fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

ization rarely preserves soft tissues, and even if it did, the new humour would almost certainly close as the remains desiccated, or as sediment overburden forced the layers together, making the fossilized eye resemble the previous layout.
Vertebrate lenses
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...

 are composed of adapted epithelial
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

 cells which have high concentrations of the protein crystallin
Crystallin
In anatomy, a crystallin is a water-soluble structural protein found in the lens and the cornea of the eye accounting for the transparency of the structure. It has also been identified in other places such as the heart, and in aggressive breast cancer tumors....

. In the embryo, the lens is living tissue, but the cellular machinery is not transparent so must be removed before the organism can see. Removing the machinery means the lens is composed of dead cells, packed with crystalins which must last the life of the organism. The refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 gradient
Gradient index optics
Gradient-index optics is the branch of optics covering optical effects produced by a gradual variation of the refractive index of a material. Such variations can be used to produce lenses with flat surfaces, or lenses that do not have the aberrations typical of traditional spherical lenses...

 which makes the lens useful is caused by the radial shift in crystallin concentration in different parts of the lens, rather than by the specific type of protein: it is not the presence of crystallin, but the relative distribution of it, that renders the lens useful.

It is biologically difficult to maintain a transparent layer of cells. Deposition of transparent, nonliving, material eased the need for nutrient supply and waste removal. Trilobites used calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

, a mineral which has not been used by any other organism; in other compound eyes and camera eyes, the material is crystallin
Crystallin
In anatomy, a crystallin is a water-soluble structural protein found in the lens and the cornea of the eye accounting for the transparency of the structure. It has also been identified in other places such as the heart, and in aggressive breast cancer tumors....

. A gap between tissue layers naturally forms a biconvex shape, which is optically and mechanically ideal for substances of normal refractive index. A biconvex lens confers not only optical resolution, but aperture and low-light ability, as resolution is now decoupled from hole size—which slowly increases again, free from the circulatory constraints.

Independently, a transparent layer and a nontransparent layer may split forward from the lens: a separate cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

 and iris
Iris (anatomy)
The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...

. (These may happen before or after crystal deposition, or not at all.) Separation of the forward layer again forms a humour, the aqueous humour
Aqueous humour
The aqueous humour is a clear, gelatinous fluid similar to plasma, but containing low-protein concentrations. It is secreted from the ciliary epithelium, a structure supporting the lens. It is located in the space between the lens and the cornea...

. This increases refractive power and again eases circulatory problems. Formation of a nontransparent ring allows more blood vessels, more circulation, and larger eye sizes. This flap around the perimeter of the lens also masks optical imperfections, which are more common at lens edges. The need to mask lens imperfections gradually increases with lens curvature and power, overall lens and eye size, and the resolution and aperture needs of the organism, driven by hunting or survival requirements. This type is now functionally identical to the eye of most vertebrates, including humans. Indeed, "the basic pattern of all vertebrate eyes is similar."

Color vision


The ability to see colors presents distinct selective
Selection
In the context of evolution, certain traits or alleles of genes segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the...

 advantages for species, such as being better able to recognize predators, food and mates. Indeed, it is thought that simple sensory-neural mechanisms may selectively control general behaviour patterns, such as escape, foraging, and hiding. Many examples of wavelength-specific behaviour patterns have been identified, in two primary groups: less than 450 nm, associated with natural light sources, and greater than 450 nm, associated with reflected light sources. As opsin molecules were subtly fine-tuned to detect different wavelengths of light, at some point color vision
Color vision
Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths of the light they reflect, emit, or transmit...

 developed when photoreceptor cells developed multiple pigments. As a chemical adaption rather than a mechanical one, this may have occurred at any of the early stages of the eye's evolution, and the capability may have disappeared and reappeared as organisms became predator or prey. Similarly, night and day vision emerged when receptors differentiated into rods and cones, respectively.

Focusing mechanism


Some species move the lens back and forth, some stretch the lens flatter. Another mechanism regulates focusing chemically and independently of these two, by controlling growth of the eye and maintaining focal length. Note that a focusing method is not a requirement. As photographers know, focal errors increase as aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...

 increases. Thus, countless organisms with small eyes are active in direct sunlight and survive with no focus mechanism at all. As a species grows larger, or transitions to dimmer environments, a means of focusing need only appear gradually.

Location


Prey generally have eyes on the sides of their head so to have a larger field of view, from which to avoid predators. Predators, however, have eyes in front of their head in order to have better depth perception
Depth perception
Depth perception is the visual ability to perceive the world in three dimensions and the distance of an object. Depth sensation is the ability to move accurately, or to respond consistently, based on the distances of objects in an environment....

. Flatfish are predators which lie on their side on the bottom, and have eyes placed asymmetrically on the same side of the head. A transitional fossil
Transitional fossil
A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...

 from the common symmetric position is Amphistium
Amphistium
Amphistium is a fossil fish which has been identified as an early relative of the Flatfish, and as a Transitional fossil. In a typical modern flatfish, the head is asymmetric with both eyes on one side of the head...

.

Evolutionary baggage


The eyes of many taxa record their evolutionary history in their imperfect design. The vertebrate eye, for instance, is built "backwards and upside down", requiring "photons of light to travel through the cornea, lens, aqueous fluid, blood vessels, ganglion cells, amacrine cells, horizontal cells, and bipolar cells before they reach the light-sensitive rods and cones that transduce the light signal into neural impulses, which are then sent to the visual cortex at the back of the brain for processing into meaningful patterns."

The camera eyes of cephalopod
Cephalopod eye
Cephalopods, as active marine predators, possess sensory organs specialized for use in aquatic conditions. They have a camera-type eye, which consists of a lens projecting an image onto a retina. Unlike the vertebrate camera eye, the cephalopods' form as invaginations of the body surface , and...

s, in contrast, are constructed the "right way out", with the nerves attached to the rear of the retina. This means that they do not have a blind spot. This difference may be accounted for by the origins of eyes; in cephalopods they develop as an invagination
Invagination
Invagination means to fold inward or to sheath. In biology, this can refer to a number of processes.* Invagination is the morphogenetic processes by which an embryo takes form, and is the initial step of gastrulation, the massive reorganization of the embryo from a simple spherical ball of cells,...

 of the head surface whereas in vertebrates they originate as an extension of the brain.

Further reading


illustration
  • Land, M. F., & Nilsson, D-E, Animal Eyes, Oxford: Oxford University Press, 2002 ISBN 0-19-8509685 "The origin of vision", Chapter 1, pages 1–15
  • Journal Evolution: Education and Outreach Volume 1, Number 4 / October 2008. Special Issue: The Evolution of Eyes. 26 articles, free access.

External links