What is Evolution of the eye?
[[File:Diagram of eye evolution.svg|thumb|230px|Major stages in the evolution of the eye.]] [[File:Lobatus gigas eye.jpg|thumb|230px|The eye of a [[mollusc]]: the queen [[conch]].]] [[File:Kaldari Platycryptus undatus 03.jpg|thumb|230px|A jumping [[spider]]. Spiders have a number of eyes.]] [[File:NKN-2007-08-18 081218 GRS PLAN tete mante relg 1-2.jpg|thumb|230px|This [[praying mantis]] has [[camouflage]]d eyes]] [[File:Snail-front-0A.jpg|thumb|230px|Land [[snail]]s usually have two sets of [[tentacle]]s on their head: the upper pair have an eye at the end; the lower pair are for [[olfaction]].]] The '''evolution of the eye''' is an example of a [[homology (biology)|homologous]] [[Organ (anatomy)|organ]] which many animals have. Some components of the [[eye]], such as the light-sensitive [[opsin]]s, seem to have a common ancestry. They [[Evolution|evolved]] once, early in the evolution of animals. They control the conversion of [[photons]] into electrical signals. For example, an opsin found in the mammalian [[retina]], melanopsin, is involved in [[circadian rhythm]]s and [[pupil (eye)|pupil]]lary [[reflex]] but not in vision. On the other hand, complex image-forming eyes evolved some 50 to 100 times – using many of the same proteins and genetic toolkits in their construction.<ref>{{cite book|isbn=0-19-854980-6|author=Haszprunar |year=1995|editor=Taylor|chapter=The mollusca: coelomate turbellarians or mesenchymate annelids?|publisher=Oxford Univ. Press |location=Oxford |title=Origin and evolutionary radiation of the Mollusca: centenary symposium of the Malacological Society of London}}</ref><ref>{{cite journal|doi=10.1016/S1534-5807(03)00325-3 |title=Role of Pax genes in eye evolution: a [[Cnidaria]]n PaxB gene uniting Pax2 and Pax6 functions|pages=773–785|year=2003|author=Kozmik, Z|journal=Developmental Cell|volume=5|issue=5|pmid=14602077}}</ref><ref>Land M.F. and Nilsson D.-E. 2002. ''Animal eyes''. Oxford University Press, Oxford.</ref> Complex eyes seem to have first evolved in a few million years, in th