Editing Eye (section)

==Evolution==
{{main|Evolution of the eye}}
[[File:Diagram of eye evolution.svg|thumb|upright=1.5|Evolution of the [[mollusc eye]]]]

Photoreception is [[Phylogenetics|phylogenetically]] very old, with various theories of phylogenesis.<ref name=Autrum1979>{{Cite book|author=Autrum, H|editor=H. Autrum|chapter=Introduction|year=1979|title=Comparative Physiology and Evolution of Vision in Invertebrates- A: Invertebrate Photoreceptors|location=New York|series=Handbook of Sensory Physiology|volume=VII/6A|pages=4, 8–9|publisher=Springer-Verlag|isbn=978-3-540-08837-0}}</ref> The common origin ([[monophyly]]) of all animal eyes is now widely accepted as fact. This is based upon the shared genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 650-600 million years ago,<ref>{{cite journal | doi=10.1016/0959-437X(95)80029-8 | last1=Halder | first1=G. | last2=Callaerts | first2=P. | last3=Gehring | first3=W.J. | year=1995 | title=New perspectives on eye evolution | journal=Curr. Opin. Genet. Dev. | volume=5 | issue=5| pages=602–609 | pmid=8664548 }}</ref><ref>{{cite journal | doi=10.1126/science.7892602 | last1=Halder | first1=G. | last2=Callaerts | first2=P. | last3=Gehring | first3=W.J. | year=1995 | title=Induction of ectopic eyes by targeted expression of the ''eyeless'' gene in ''Drosophila''| journal=Science | volume=267 | issue=5205| pages=1788–1792 | pmid=7892602 | bibcode=1995Sci...267.1788H}}</ref><ref>{{cite journal | doi=10.1073/pnas.94.6.2421 | last1=Tomarev | first1=S.I. | last2=Callaerts | first2=P. | last3=Kos | first3=L. | last4=Zinovieva | first4=R. | last5=Halder | first5=G. | last6=Gehring | first6=W. | last7=Piatigorsky | first7=J. | year=1997 | title=Squid Pax-6 and eye development | journal=Proc. Natl. Acad. Sci. USA | volume=94 | issue=6| pages=2421–2426 | pmid=9122210 | pmc=20103 | bibcode=1997PNAS...94.2421T| doi-access=free }}</ref> and the [[PAX6]] gene is considered a key factor in this. The majority of the advancements in early eyes are believed to have taken only a few million years to develop, since the first predator to gain true imaging would have touched off an "arms race"<ref>Conway-Morris, S. (1998). ''The Crucible of Creation''. Oxford: Oxford University Press</ref> among all species that did not flee the photopic environment. Prey animals and competing predators alike would be at a distinct disadvantage without such capabilities and would be less likely to survive and reproduce. Hence multiple eye types and subtypes developed in parallel (except those of groups, such as the vertebrates, that were only forced into the photopic environment at a late stage).

Eyes in various animals show adaptation to their requirements. For example, the eye of a [[bird of prey]] has much greater visual acuity than a [[human eye]], and in some cases can detect [[ultraviolet]] radiation. The different forms of eye in, for example, vertebrates and molluscs are examples of [[parallel evolution]], despite their distant common ancestry. Phenotypic convergence of the geometry of cephalopod and most vertebrate eyes creates the impression that the vertebrate eye evolved from an imaging [[cephalopod eye]], but this is not the case, as the reversed roles of their respective ciliary and rhabdomeric opsin classes<ref name="Lamb"/> and different lens crystallins show.<ref>{{cite journal |author1=Staaislav I. Tomarev |author2=Rina D. Zinovieva |year=1988 |title=Squid major lens polypeptides are homologous to glutathione S-transferases subunits |journal=[[Nature (journal)|Nature]] |volume=336 |issue=6194 |pages=86–88 |doi=10.1038/336086a0 |pmid=3185725 |bibcode=1988Natur.336...86T|s2cid=4319229 }}</ref>

The very earliest "eyes", called eye-spots, were simple patches of [[photoreceptor protein]] in unicellular animals. In multicellular beings, multicellular eyespots evolved, physically similar to the receptor patches for taste and smell. These eyespots could only sense ambient brightness: they could distinguish light and dark, but not the direction of the light source.<ref name=Land1992/>

Through gradual change, the eye-spots of species living in well-lit environments depressed into a shallow "cup" shape. The ability to slightly discriminate directional brightness was achieved by using the angle at which the light hit certain cells to identify the source. The pit deepened over time, the opening diminished in size, and the number of photoreceptor cells increased, forming an effective [[pinhole camera]] that was capable of dimly distinguishing shapes.<ref name="ee">{{cite web |url=http://library.thinkquest.org/28030/eyeevo.htm |title=Eye-Evolution? |publisher=Library.thinkquest.org |access-date=2012-09-01 |url-status=dead |archive-url=https://web.archive.org/web/20120915061324/http://library.thinkquest.org/28030/eyeevo.htm |archive-date=2012-09-15 }}</ref> However, the ancestors of modern [[hagfish]], thought to be the protovertebrate,<ref name="Lamb">{{cite journal |author1=Trevor D. Lamb |author2=Shaun P. Collin |author3=Edward N. Pugh Jr. |year=2007 |title=Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup |journal=[[Nature Reviews Neuroscience]] |volume=8 |issue=12 |pages=960–976 |pmc=3143066 |doi=10.1038/nrn2283 |pmid=18026166}}</ref> were evidently pushed to very deep, dark waters, where they were less vulnerable to sighted predators, and where it is advantageous to have a convex eye-spot, which gathers more light than a flat or concave one. This would have led to a somewhat different evolutionary trajectory for the vertebrate eye than for other animal eyes.

The thin overgrowth of transparent cells over the eye's aperture, originally formed to prevent damage to the eyespot, allowed the segregated contents of the eye chamber to specialise into a transparent humour that optimised colour filtering, blocked harmful radiation, improved the eye's [[refractive index]], and allowed functionality outside of water. The transparent protective cells eventually split into two layers, with circulatory fluid in between that allowed wider viewing angles and greater imaging resolution, and the thickness of the transparent layer gradually increased, in most species with the transparent [[crystallin]] protein.<ref name="lenses come from">Fernald, Russell D. (2001). [http://www.karger.com/gazette/64/fernald/art_1_4.htm The Evolution of Eyes: Where Do Lenses Come From?] {{webarchive|url=https://web.archive.org/web/20060319050210/http://www.karger.com/gazette/64/fernald/art_1_4.htm |date=2006-03-19 }} ''Karger Gazette'' 64: "The Eye in Focus".</ref>

The gap between tissue layers naturally formed a biconvex shape, an optimally ideal structure for a normal refractive index. Independently, a transparent layer and a nontransparent layer split forward from the lens: the [[cornea]] and [[iris (anatomy)|iris]]. Separation of the forward layer again formed a humour, the [[aqueous humour]]. This increased refractive power and again eased circulatory problems. Formation of a nontransparent ring allowed more blood vessels, more circulation, and larger eye sizes.<ref name="lenses come from"/>

===Relationship to life requirements===
Eyes are generally adapted to the environment and life requirements of the organism which bears them. For instance, the distribution of photoreceptors tends to match the area in which the highest acuity is required, with horizon-scanning organisms, such as those that live on the [[Africa]]n plains, having a horizontal line of high-density ganglia, while tree-dwelling creatures which require good all-round vision tend to have a symmetrical distribution of ganglia, with acuity decreasing outwards from the centre.

Of course, for most eye types, it is impossible to diverge from a spherical form, so only the density of optical receptors can be altered. In organisms with compound eyes, it is the number of ommatidia rather than ganglia that reflects the region of highest data acquisition.<ref name=Land1992/>{{Rp|23–24}} Optical superposition eyes are constrained to a spherical shape, but other forms of compound eyes may deform to a shape where more ommatidia are aligned to, say, the horizon, without altering the size or density of individual ommatidia.<ref name=Land1989/> Eyes of horizon-scanning organisms have stalks so they can be easily aligned to the horizon when this is inclined, for example, if the animal is on a slope.<ref name="Zeil"/>

An extension of this concept is that the eyes of predators typically have a zone of very acute vision at their centre, to assist in the identification of prey.<ref name=Land1989>{{Cite journal
 | author=Land, M.F.
 | year=1989
 | title=The eyes of hyperiid amphipods: relations of optical structure to depth
 | journal=Journal of Comparative Physiology A
 | volume=164
 | issue=6
 | pages=751–762
 | doi=10.1007/BF00616747| s2cid=23819801
 }}</ref> In deep water organisms, it may not be the centre of the eye that is enlarged. The [[hyperiid]] [[amphipod]]s are deep water animals that feed on organisms above them. Their eyes are almost divided into two, with the upper region thought to be involved in detecting the silhouettes of potential prey—or predators—against the faint light of the sky above. Accordingly, deeper water hyperiids, where the light against which the silhouettes must be compared is dimmer, have larger "upper-eyes", and may lose the lower portion of their eyes altogether.<ref name=Land1989/> In the giant Antarctic isopod [[Glyptonotus antarcticus|Glyptonotus]] a small ventral compound eye is physically completely separated from the much larger dorsal compound eye.<ref name="Meyer-Rochow1982">{{cite journal|last=Meyer-Rochow|first=Victor Benno|title=The divided eye of the isopod Glyptonotus antarcticus: effects of unilateral dark adaptation and temperature elevation|journal=Proceedings of the Royal Society of London|date=1982| volume=B 215|issue=1201|pages=433–450|bibcode=1982RSPSB.215..433M|doi=10.1098/rspb.1982.0052|s2cid=85297324}}</ref> Depth perception can be enhanced by having eyes which are enlarged in one direction; distorting the eye slightly allows the distance to the object to be estimated with a high degree of accuracy.<ref name=Cronin2008/>

Acuity is higher among male organisms that mate in mid-air, as they need to be able to spot and assess potential mates against a very large backdrop. On the other hand, the eyes of organisms which operate in low light levels, such as around dawn and dusk or in deep water, tend to be larger to increase the amount of light that can be captured.<ref name=Land1989/>

It is not only the shape of the eye that may be affected by lifestyle. Eyes can be the most visible parts of organisms, and this can act as a pressure on organisms to have more transparent eyes at the cost of function.<ref name=Land1989/>

Eyes may be mounted on stalks to provide better all-round vision, by lifting them above an organism's carapace; this also allows them to track predators or prey without moving the head.<ref name=Cronin2008>{{cite journal | first1=T.W.| first2=M.L.| last2=Porter| title=Exceptional Variation on a Common Theme: the Evolution of Crustacean Compound Eyes | last1=Cronin | journal=Evolution: Education and Outreach | volume=1 | issue=4| pages=463–475 | year=2008 | doi=10.1007/s12052-008-0085-0 | doi-access=free }}</ref>