Charles Darwin, author of the groundbreaking On the Origin of Species, is frequently quote-mined for a variety of reasons.
One of the most popular Darwin quotes frequently used by Creationists is:
To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree.
It is often forgotten that all quotes have an important context. This paragraph immediately continues with:
Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself first originated; but I may remark that several facts make me suspect that any sensitive nerve may be rendered sensitive to light, and likewise to those coarser vibrations of the air which produce sound.
Darwin continues with three more pages describing a sequence of plausible intermediate stages between eyelessness and human eyes, giving examples from existing organisms to show that the intermediates are viable. This process of gradual change has become known as Evolution.
In a recent BBC documentary renowned British Evolutionary Biologist and secular author Richard Dawkins is interviewed about the evolution of the eye as investigated by Darwin in light of what we know today.
The eye is often given as an example of something far too perfect and complex to have evolved from scratch.
Eons ago animals had to bump into things to know what they were. Imagine the benefits if they could develop an awareness of food before actually stumbling upon it, or of predators lurking nearby. Evolution has fortunately given animals the perfect tool for this: the eye.
An eye is very much like a camera. Light rays will come into the eye, be focused by the lens like the lens of a camera, then pass through the jelly of the eye to be focused onto the back of the eye, onto a structure known as the retina. The retina is lined by photoreceptor cells, which are similar to the film of a camera. These photoreceptor cells generate an electrical signal from the light energy. This electrical signal is then sent through the optic nerve to the brain, which translates the impulse into the image that we actually see.
Despite this complexity, reason tells us that if there were a series of gradual improvements then it would have been easy for the eye to have evolved by means of natural selection. All over the animal kingdom we find eyes in various stages of evolution. We have examples of these intermediate stages of evolution in modern day animals.
The first step is noticeable in the single-celled organism Euglena, of which over 1000 species have been identified. This organism just has a small eye spot at the end of the cell which is only sensitive to light, making it incapable of seeing anything or forming an image but which only allows it to tell whether it is light or dark, making this easily the first step in the evolution of the eye.
If you were to imagine that you had a sheet of cells, each of which is sensitive to light, more cells would react to light. If this flat sheet were to evolve a slight curve to be turned into a shallow cup, light shining from one end would be registered on the opposite end of the cup because the light would hit the opposite side of the bent sheet of the cup. This makes it possible to determine from which direction the light is coming from and perhaps be able to tell the direction from which a predator’s shadow is passing over.
This means that the cup-eyes of Planarian worms, non-parasitic flatworms, for example, have a evolutionary advantage in the long run over animals with just a flat eye.
So how do we progress from a cup-eye which is able to tell the direction light is coming from to an eye which is capable of forming an image?
Over evolutionary time the cup becomes progressively deeper until the ends at the top eventually fuse shut leaving a small hole at the top. You then have a pinhole camera. The pinhole camera is a pretty poor piece of work because it doesn’t show you much of an image, but it does show you a crude image, making it possible to make out shapes and detect movement. In reality, a literal pinhole camera presents its viewers with a blurry, upside-down image, but an image nonetheless.
A perfect example of this next stage in the evolution of the eye can be seen in many aquariums.
The mollusk nautilus, a relative of the extinct ammonite, has an eye on each side of its head that acts like a pinhole camera, which means they are capable of seeing blurry images.
The solution to the problem of discerning detail in an image is the lens. A proper man-made lens is a curved sheet of glass capable of refracting light. In nature, and old bit of gunge that is transparent will serve this purpose so long as it is approximately curved. If you were to place some water into a small polythene bag it would naturally fall into a curved shape. Hold this in front of the space of a pinhole camera and the blurry image suddenly becomes considerably clearer. This water represents a blob of gunge in the cup eye. Eventually this gunge or jelly would harden to form a proper lens and transmit a brighter, clearer image.
Sea snails have a blob of jelly that acts like a very simple lens, so they can focus on an object even if that image is still a bit blurry, but it does mean that they can make out food and predators.
Once you’ve got that, and because it works a bit better than nothing at all, you’ve got the raw material for natural selection to go to work. Over each generation there will be a slight improvement in the curvature and a slight improvement in the transparency resulting in a steady ramp of improvement all the way up to a proper lens such as you get in the vertebrate eye. The eye is an astounding example of a complex structure that evolved fairly quickly and easily from simpler structures.
What’s even more incredible is that as the eye evolved over millions of years, widely different species such as monkeys and molluscs, developed the same solutions to sight. This phenomenon is known as convergent evolution and examples of it are seen all over the world in a variety of forms, from the already mentioned eye to the fact that koala bears and humans have exactly the same fingerprints.
At the top of the mollusk evolutionary ladder is the octopus eye. It has a proper lens, it can adjust its own exposure and is pretty much like our own except that it comes from a completely different evolutionary line.
Scientists have done a lot of modeling to determine how long it took to get from a flat eye to a vertebrate eye. They have concluded that it took approximately 400 000 generations to get from the simplest light sensitive cell all the way to a fully functioning eye as we know it. Given that this all started in simpler aquatic organisms with very short life-cycles, this process would have happened in less than half a million years, which is fairly quick in evolutionary terms.
That is not the end of the story though. Another completely different type of eye has evolved as well: the compound eye.
The compound eye is a type of eye commonly found in arthropods, including many insects and crustaceans. It has a meshlike appearance because it consists of hundreds of thousands of tiny lens-capped optical units called ommatidia. Each ommatidium has its own cornea, lens, and photoreceptor cells for distinguishing brightness and colour. They are separated from each other by varying degrees of pigment and all relay electrical impulses back to the optic nerve.
With regards to the Argument from Design, the eye is an imperfect creation. The retina is back-to-front and it has a blindspot, amongst other things. P. Z. Myers discusses these arguments in more detail on his Pharyngula blog spot.
Liz, a BBC presenter, takes a look at how the compound eye evolved and why it would not work for us. This video is unfortunately not available in South Africa.