How the eye evolved

Let us reason about this problem by answering a few questions.

1. Which part of the eye is most essential? The answer is obvious: the retina. If light cannot be detected, the remainder of the eye is useless.
2. Can a retina, on its own, without the rest of the eye, play a useful role? Obviously, yes. Many groups of not too complex animals have ocelli, photo-receptor cells that just react to the presence of light, but cannot form images. Of course, perceiving the presence of light offers advantage against being totally blind. The proof of this: ocelli have appeared independently in at least 40 different animal groups.
3. What is the next step? We also have traces among current animals. The Planaria is a Platyhelminth (flatworm) whose ocelli are located at the bottom of a concavity in its body. Thanks to this, the Planaria not only detects the presence of light, but also, to some extent, the direction it comes from. It is also obvious that being able to perceive the direction of light provides advantage to those who can, against those who cannot.

The eye as an argument about evolution

Schematic diagram of the human eye

The vertebrate eye consists of five parts: cornea (a transparent insulating layer); aqueous humor; crystalline lens (a lens surrounded by muscles that make it possible to achieve a variable focal length); vitreous humor; and retina. The light passes through the cornea and the aqueous humor, is focused by the lens, goes through the vitreous humor and impacts on the nerve cells of the retina, which generate electric signals that the optic nerve transmits to the brain, which forms from them an image of the external world, where the light rays came from. The brain even turns the image around, as it is inverted when projected on the retina.

The complex structure of the eye has always been a problem for evolutionists, and an argument for those opposed to the theory of evolution. Darwin, in Chapter 6 of The Origin of Species, whose title is significant (Difficulties of the theory) dealt with the problem of the evolution of the eye in the following words: