The concept

Irreducible complexity is the concept that a system that is made of individual parts cannot operate properly if even a single component piece is removed from the system.

The classic mousetrap is an example (image taken from Wikipedia):

The mousetrap in the above image has (at least) eight pieces and would fail to operate (as a mousetrap) if a single one of those pieces was removed from the system.

Creationists use the concept of irreducible complexity as a “gotcha” argument that they believe proves that a designer is needed. For example, they state that the human eye could not have evolved because each distinct component of the eye is pointless if the other distinct (complex) pieces are not there to permit the eye to function as it does.

Creationists are correct when it comes to something like the above mousetrap, however they are incorrect when they extend the argument to a system such as a human being, or even to a part of a human being, such as the human eye.

The eye

Not only is the development of the eye well known from the fossil record, it is well known by studying animals that are alive today. Yes, that’s right, the evolution of the eye, in all its various stages, is alive and kicking today. In other words, the eye has developed independently along many evolutionary paths. To the horror of the religious, the eye is actually one of the strongest supports for evolution – when it is understood.

The following shows the evolution of the eye (link to a Wikipedia article). The diagrams below that demonstrate the stages of evolution of the eye were taken from that article.

The earliest “eyes” were nothing more than patches of photoreceptor cells that obviously had the evolutionary advantage of allowing a primitive animal to seek light or dark environments as need arose. The first fossils of primitive “eyes” date from about 555 million years ago, however they could have existed quite a bit earlier.

The patch of photo-receptor cells then became cupped in a small depressed area, thus allowing for a limited sensitivity to the direction of light. The planarian is one of many flatworms (alive today) that have such light-sensitive cells. They also have a ganglion under the eye-spots which is a mass of nerve tissue that reacts in similar ways to the EEG activity of other animals. From the ganglion two nerve cords extend, producing a ladder-like nerve system that traverses the body of the animal. It’s not a huge stretch to see that set up as a primitive optic nerve.

That cupped area then became bigger and the front of the cup largely closed to make a small opening in front of the photo-receptor cells. That is a construction that any competent photographer will tell you makes a basic pinhole camera – thus permitting finer directional sensitivity and limited imaging. The nautilus is a marine mollusk alive today that has just such a pinhole construction.

The enlarged cup structure then developed layers of transparent cells to cover the small opening at the front, largely to prevent contamination and parasitic infestation. The interior of the protected socket could then gradually specialise into a transparent aqueous humour for optimisations such as colour filtering, higher refractive index, blocking of ultraviolet radiation, or the ability to operate in and out of the water.

The next stage of the evolution involved the formation of a lens at the front of the eye, which had the advantage of being able to make the light entering the eye more intense and thus permitting the animal to operate in darker conditions – which is a better strategy for survival.

Finally, the lens became more sophisticated and allowed the animal to focus accurately, which again is a wonderful adaptation to increase the chances of survival. With an eye that is more in focus, predators could be better avoided and food sources better attained.

This is strong evidence of the key point of evolution – the animals that happened to develop advantageous characteristics had better chances of survival, and therefore to pass on their characteristics to subsequent generations. Other developments in optics such as color-vision, polarization-vision, better placement of eyes, and the ability to alter the position of focus, were all just icing on the evolutionary cake.

However it is not all good news for some animals because it is known that eye-loss or eye-reduction can occur if an animal becomes subjected to an environment with decreased amounts of light, for example in subterranean, deep sea, and nocturnal habitats. This is a well-studied phenomenon, and even though eye function can be greatly diminished over the generations, the remnants of the visual system can still be there to study.

Some animals, such as the cave salamander in Europe have perfectly normal eye development initially, but then something goes wrong and the cornea and lens regress to produce an eye that is strongly reduced and sunken into the orbits. Evolution had given them the means to have good eyesight, but their environment has meant genetic diminishment in being able to use their eye mechanisms. Why would a creator do that?

The real reason for the diminishment is that natural selection in such darkened environments does not favor strong optical characteristics. For example, creatures with genetic problems with their eyes may still win out if they have better hearing or sense of smell, and that way they are better able to survive to pass on their faulty optical genes.

Everyone knows that we can only see in the “visible” spectrum and not in the ultraviolet or infrared ends of the electromagnetic spectrum, however it’s interesting to ask the question why that is? For example, it would have been of great benefit if we could have seen in the infrared because we could have detected the heat of nearby predators or food sources; but, sadly it wasn’t to be. But why? Well, it is likely that a key reason is because the earliest species to develop photosensitivity were aquatic, and water filters out much of the spectrum in which we can no longer see. In other words, we haven’t dragged ourselves out of our evolutionary past. But fear not because evolution is still happening, so at some time in the future, humans are likely to develop sensitivities to electromagnetic radiation outside of the current visible spectrum. But sadly for us, it will be a long process.

Some animals have evolved eyes that are superior to humans. The bird lineage seems to be eye-centric and most birds have eyes that are larger than their brains. For example, eagles can resolve objects about eight times further away than humans. Can you spot a rabbit at about 3.5 kilometres (2 miles)? Well, an eagle can. They can also see a wider range of colours, including into the ultraviolet range. Due to a reflective surface behind the retina, owls (and other animals) have vastly superior night-vision to humans. In fact, the colour vision of humans greatly diminishes in low light. Humans have three types of cones in our eyes, but that’s nothing because mantis shrimp have 16 types of cones, giving them a more advanced colour-recognition system over humans.

To the religious reader, why is it that your god didn’t put the best eyes into the sockets of his most important and beloved creation? Are you saying that he couldn’t have done it? If you’re not saying that, then why didn’t he do it? Just forgot perhaps?

Perhaps you are still not convinced because you have heard that even Charles Darwin doubted that evolution could produce the eye? Wrong. In his work On the Origin of Species he did indeed write “To suppose that the eye … could have been formed by natural selection, seems, I freely confess, absurd in the highest degree”, however what the quote-mining religious folk will never admit is what he wrote immediately after that sentence. It’s worth reading the section in full:

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.

So, no, Darwin did not doubt that evolution could produce the eye.

The answer

The underlying problem with creationists using irreducible complexity as a reason to support an intelligent designer is that they are failing to differentiate between the the things that are created and the things that are not created. On the contrary, they are starting with the faulty assumption that everything must have been created and then they go looking for problems with a theory (evolution) that can no longer be relevant.

There are elements of both argument-from-incredulity and argument-by-strawman in their approach. They are in effect saying that evolution principles are impossible in something like the eye simply because they don’t believe that evolution happened. They are simply starting with the end state and ignoring how that state could have been achieved (besides being created).

The answer for creationists is to relax their thinking and entertain the notion (as demonstrated above) that a complex system can indeed have many simpler stages in its history. We don’t try to leap from ground level to the first level in a building, we take many incremental steps to go up one floor. It’s not so incredulous to believe that “irreducible complexity” is not a thing when analyzing the individual steps of something.

Instead of hoping that something like the eye is too complex to have evolved (which it clearly isn’t), it would be much better if creationists took the time to read in detail about the extremely interesting evolution of the eye. There are many sources available. They’ll be amazed at how much detail science has discovered that religious “leaders” have somehow failed to explain.