the eyes have it

One wonders how a structure so complex might have evolved. Vision is an elegant sense, and the human eye has become an extremely specialized organ to its end. One has only to look in the standing water of a pasture to find a possible precursor within the Euglena. These small protists possess an eyespot apparatus; a primitive photo-receptive system which allows the motile Euglena to find a light source strong enough for photosynthesis. The Euglena holds chloroplasts within its transparent body which allow it to undergo this process to manufacture sucrose from water and carbon dioxide. Alga in Euglenaphyceae use a photoreceptor called a paraflagellar swelling near the body's main flagellum. This receptor is interesting in that it is a crystalline structure. The algae uses this dicroic crystalline detector along with a primitive absorbing screen to sense and respond to incident light.

The algal eye of Chlorophyceae contains optics, photo-receptors and elementary components of the signal transduction pathway. Though among the simplest eyes in nature, chlorophycean eyes are capable of detecting light intensity, color, and direction of incidence. Because of their small size, the optical systems on microalgae operate mostly on the basis of reflective surfaces and constructive interference. Although quite disparate macroscopically from the human eye, the chlorophycean eye also uses a rhodopsin chromophore. Retinal is present and used in a slightly different form, but this archaic chromophore is still similar to that used in the human eye. This fundamental system has been nearly conserved from even microalgae.

On early earth-orbital space flights, astronauts were surprised to see this structure staring back at them. It spans nearly thirty miles, and to date no definite scientific explanation has been established for its formation. Because of its scale, the Richat Structure is baffling in its symmetry and basic etiology. It seems that eyes both large and small, given enough time, can pop up in all sorts of places.

Hegemann, Peter. Algal Eyes
Ross, Greg. Futility Closet