How could something as complex as the human eye develop naturally? More importantly, how could it develop in gradual stages while still being functional? The eye's evolution is actually quite simple, starting as primitive light-sensitive cells with gradual improvements over about 364,000 years. Not only have scientists discovered how such a complex eye could have evolved from a light-sensitive cell, proving the process is not impossible, but every stage of the eye's development can be seen in nature!
Pigment Spot(flat piece of light-sensitive cells)
The most primitive type of eye can be found on euglenids, small single-celled organisms found in the greenish scum in water. The euglenid has a bright red structure called an "eyespot" that allows the organism to move in response to light.
A flat piece of light-sensitive cells on the skin helps an organism detect a light source.
Simple Pigment Cup(flat piece of light-sensitive cells)
As a flat piece of light-sensitive cells deepen it forms a simple cup shape like the eye of a flatworm.
A depression in the light-sensitive cells gives the ability to detect objects and the angle light is coming from.
Simple Optic Cup (flat piece of light-sensitive cells)
As the depression increases it forms a simple optic cup like the eyes of abalone shellfish.
The greater the cup shape of a depression the more precisely it can detect the angle of light. Edges shade the light so the angle can be based on what cells are shaded and what cells detect the light.
Pinhole Opening(flat piece of light-sensitive cells)
As the opening becomes even smaller it forms a pinhole opening like the eye of a chambered nautilus.
A smaller pinhole opening produces a sharper image and greatly increases the ability to detect the angle of a light source as less light-sensitive will be activated at a given moment.
Complex Lens Eye(flat piece of light-sensitive cells)
As part of the transparent liquid that fills the eye became denser it formed a lens. Little by little the image became sharper. This type of eye can be found in marine snails and the octopus.
A lens gives a sharper and clearer image.
Mammalian Eye(flat piece of light-sensitive cells)
As the slight and gradual changes continue just a bit more the dense liquid forming the lens also takes shape as the iris and we now have an eye similar to that of land mammals.
The new shape of the lenses gives an even sharper and clearer image still.