The ancestral Pax6  gene controls development of this organ in all bilateral species, and the "light   switch" theory holds that evolution of it triggered the Cambrian Explosion.
In  July  2013,  Colombia  became  the  first  country  to  eliminate  transmission  of   onchocerciasis [AHN-kuh-sur-SY-uh-siss], the most common parasitic disease of this organ.  The ancestral Pax6  gene controls development of this organ in all bilateral species, and the "light   switch" theory holds that evolution of it triggered the Cambrian Explosion. Growth of new blood  vessels in the choroid can lead to the "wet" form of macular degeneration in this organ that requires  vitamin A for its rods to function. Glaucoma and cataracts impair what organ that detects light at  the retina to produce vision?
In  July  2013,  Colombia  became  the  first  country  to  eliminate  transmission  of   onchocerciasis [AHN-kuh-sur-SY-uh-siss], the most common parasitic disease of this organ.  
eye [accept onchocerciasis before it is mentioned]