Evolution of The Eye - One Origin or Many?

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 crystallin 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 opsins. 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 controls where the eye develops in organisms ranging from mice to humans to fruit flies. 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.