It is widely accepted based on shared anatomical and genetic features that all animal eyes have the same origin, a proto-eye that evolved approximately 540 million years ago. That being said, the majority of advancements are thought to have occurred in the past few million years, in a burst of evolution known as the Cambrian explosion. Once the first predators developed true imaging capabilities, this set off an “arms race” as predators and prey competed to evolve equal and better capabilities in order to survive. As a result, many types and subtypes of eyes developed in parallel.
Eyes display a number of variations based on the requirement of the organisms in which they are located. Such adaptations include acuity, range of wavelengths that can be detected, sensitivity in low light levels, ability to detect motion, and discrimination of colour.
Eyespots, or photoreceptor proteins that detect light, are the earliest predecessors of the eye. Found even in unicellular organisms, these proteins can detect light, enabling organisms to synchronize circadian rhythms, but not to distinguish shapes, nor to detect the direction of light.
The eyespot gradually changed to depress into a shallow cup shape, able to discriminate the direction of light. As the pit deepened, the number of photoreceptor cells increased and the opening diminished in size, thus enabling the eye to distinguish dim shapes.
A thin layer of transparent cells developed over the eye’s aperture to prevent damage, though this also enabled the contents of the eye chamber to specialize into a transparent humour to optimize colour filtering, block harmful radiation, and improve the refractive index of the eye. The layer of protective cells also split into two layers separated by fluid to improve viewing resolution and widen viewing angles.
In addition, a transparent layer and a non-transparent layer split forward from the lens, forming the cornea and iris, respectively. The separation of the front layer forms the aqueous humour, which again increases refractive power and helps with circulatory problems.