This article was adapted from Scientific American July 2011 Evolution of the Eye, P48 Trevor D. Lamb

Creationists have long contended that the vertebrate eye is too complex to be a product of evolution. By comparing the structures and embryological development of the eye in vertebrate species, however, researchers have gained key insights into the organ's origin. These findings indicate that our camera-style eye has startlingly deep roots, and hint at how it might have functioned before it evolved all of the elements necessary for vision.

A possible pathway to the development of the mammalian eye is given below. Click on the blue text to reveal the image on the right.

The first primitive eye may have started as a set of nerve cells on the surface of an organism. Examples of such an eye are the pigment spots found on the limpet patella. This would offer a survival advantage to the organism. It may sense the length of day and night and so be able to mate at the right time of the year it would also sense the presence of predators when they cast a shadow.

Next came the formation of a pigment cup where the cells are placed in a circular cup-like depression at the surface of the organism. This is certainly an improvement on our previous model in that we can now detect direction. This exists today as in the slit shell mollusk Pleurotomaria. We know where the predator is coming from and our organism might be capable of swimming or moving away as opposed to moving at random.

What followed was a fluid filled cavity with a tiny opening. This pinhole camera effect could form a crude image on the light sensitive cells. This is seen in the nautilus

As the fluid filled cavity evolved to house a membrane bound thick fluid that bends light and focuses it on the increased number of nerve cells, the organism was better able to make out shapes and determine threatening and friendly organisms. This was the start of the lens.

Later modifications gave rise to the camera type eye we see in the octopus today.

The human eye is an exquisitely complicated organ. It acts like a camera to collect and focus light and convert it into an electrical signal that the brain translates into images. But instead of photographic film, it has a highly specialized retina that detects light and processes the signals using dozens of different kinds of neurons. So intricate is the eye that its origin has long been a cause célèbre among creationists and intelligent design proponents, who hold it up as a prime example of what they term irreducible complexity—a system that cannot function in the absence of any of its components and that therefore cannot have evolved naturally from a more primitive form. Indeed, Charles Darwin​ himself acknowledged in On the Origin of Species​—the 1859 book detailing his theory of evolution by natural selection—that it might seem absurd to think the eye formed by natural selection. He nonetheless firmly believed that the eye did evolve in that way, despite a lack of evidence for intermediate forms at the time.

Direct evidence has continued to be hard to come by. Whereas scholars who study the evolution of the skeleton can readily document its metamorphosis in the fossil record, soft-tissue structures rarely fossilize. And even when they do, the fossils do not preserve nearly enough detail to establish how the structures evolved. Still, biologists have recently made significant advances in tracing the origin of the eye—by studying how it forms in developing embryos and by comparing eye structure and genes across species to reconstruct when key traits arose. The results indicate that our kind of eye—the type common across vertebrates—took shape in less than 100 million years, evolving from a simple light sensor for circadian (daily) and seasonal rhythms around 600 million years ago to an optically and neurologically sophisticated organ by 500 million years ago. More than 150 years after Darwin published his ground breaking theory, these findings put the nail in the coffin of irreducible complexity and beautifully support Darwin’s idea. They also explain why the eye, far from being a perfectly engineered piece of machinery, exhibits a number of major flaws—these flaws are the scars of evolution. Natural selection does not, as some might think, result in perfection. It tinkers with the material available to it, sometimes to odd effect.

Some of the defects include:
- an inside out retina that forces light to travel though cell bodies and nerve fibers before reaching the photoreceptors;

- blood vessels that sprawl across the retina's inner surface;

- nerve fibers that gather together and push through the retina to become the optic nerve thus creating a blind spot.

- The arrangement of the muscles that move the eyes, is also difficult to explain. In order to move the eye through 360 degrees 3 musclse are necessary, arranged in a tripod fashion, however, the eye has six. Since there are more muscles than are necessary you would expect that there is some functional redundancy. Despite the extra three muscles, the loss of function of any one muscle causes an impairment of eye movement and results in double vision. Surely a three muscle design is more efficient and provides less parts that can break down..

These flaws are not inevitable in the evolution of a camera style eye because the octopus and squid have evolved camera like eyes without these defects. These absurd shortcomings are not a product of intelligent design but make sense when viewed from an evolutionary point of view.

What is irreducible complexity ?

How do creationists apply irreducible complexity to the eye?

A set of pigment cells and nerve cells on the surface of an organism is thought to be the first light detecting organ. This can be seen today in a simple mollusc, the limpet. Explain how this primitive eye may have given a survival advantage to those that had it.

The nautilus has a simple key-hole camera type of eye. Explain the advantage of this type of eye over the light detecting cells of the limpet.

Is the eye a perfectly designed organ and therefore designed by a creator? Explain how the eye is a fine example of evolution.

Why is it so hard to document the evolution of the eye as compared to the skeleton?

What are the imperfections of the eye that point to evolution than to a carefully designed organ?

View the video on the right.

What is the benefit to the organism of a pit of light sensitive cells as opposed to a layer of light sensitive cells on the skin surface of the organism?

What is one theory of how the lens evolved?

What creatures exist that have rudimentary eyes?