The eye is a complex organ that detects light from the surroundings to enable vision. Different eyes have developed in various organisms with the passage of time whether by intelligent design or evolution.
Basic Eyes
The simplest of eyes also known as ocelli, exist in organisms like snails. These eyes have cells that are sensitive to light but are not equipped with an image projection system. Ocelli are hence only able to distinguish between light and dark or day and night thereby allowing snails to avoid direct sunlight. Such eyes are incapable of vision.
Compound Eyes
Insects and spiders have compound eyes and these are made up of many sensors that act like mini eyes. Each sensor is equipped with its own lens and light sensitive cells hence when taken as a whole and depending on the organism, they may give rise to a pixel-type image or multiple image vision. Such sensors are arranged hexagonally, able to give 360-degree field of vision and can number up to the thousands per eye.
Complex Eyes
Vision is an ability also shared by complex organisms such as mammals, birds, reptiles and fish. The two eyes commonly found in each organism may exist on different planes. This allows the monocular vision experienced by rabbits and chameleons through the production of two separate images. The two eyes may also exist on the same plane as occurs in humans. This allows the experience of binocular vision from the interpretation of a single three-dimensional image.
Common Origins
The common origin of animal eyes despite differing photoreceptor systems can be traced to the common reliance on a family of proteins called opsins. These pigmented light receptor proteins are responsible for the generation of specific electrical impulses from the eye that are then interpreted by the brain to construct an image. Colour vision is believed to have developed as a result of receptor cells having developed multiple pigments. Night vision and day vision developed as these light receptors differentiated. With time, different species developed different focusing capabilities according to their environmental needs. In some, the lens was moved back and forth, in others it was stretched flatter or relaxed to thicken. Another mechanism for focusing images was through the controlled growth of the eye.
Better Imaging, Better Survivability
The development of imaging is expected to have taken at least several million years in the history of life on earth. The first creatures to develop true imaging had a significant advantage in survivability. This advantage however relied strongly on genetic and environmental stimuli. In the modern day human, a different form of vision enhancement through laser vision correction like that of LASIK may provide the next advantage.
