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Is the human eye irreducibly complex?

by Josh Weed

December 24, 2013

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Donate Behold, the human eye.

There's a lot going on in there.The average human eye only weighs about 7 or 8 grams, but they are absolutely jam packed full of stuff. Chances are, you're the proud owner of two of these little beauties. Right now, they're busy using all of that intricate machinery to refract and focus light from your computer screen onto light sensitive rods and cones. From there, the light is being transduced into nerve signals, which are then being carried via your optic nerve into your visual cortex, where the raw information is getting filtered and patched together into something that you can make sense of.

How could this visual system have evolved, when it seems like all the components would have had to have been there from the very beginning for it to work?This question troubled Darwin, and those opposed to evolution are understandably fond of quoting the following passage from On the Origin of Species:
“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.”
However, they conveniently neglect to mention the remainder of the section, where he says that he has no problem believing that such a structure could have evolved:
“…If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case.”
But how could such a complicated piece of optical machinery arise through a process that has no foresight or intentionality?

Evolution of the Eye

First of all, the evolution of the eye was a step-by-step, cumulative process " it didn’t just spring into existence fully formed through some astronomically lucky macro-mutation. That’s impossible, and no one is seriously suggesting that that's how it happened. Things like that don’t happen by random chance, even over billions of years. Only with the help of a non-random process like natural selection can something like the eye come into being.

Fortunately, there are various creatures alive today that have proto-eyes, whosevery existence alone disproves the notion that having only part of an eye is useless. Because they represent different stages in a process that could lead to eyes like ours, evolutionary biologistscan use them to construct hypothetical evolutionary pathways that could be taken to arrive at the human eye.

Here’s an abbreviated version of the leading model:
  1. A mutation resulted in a single photoreceptor cell, which allowed the organism to respond to light and helped to calibrate circadian rhythms by detecting daylight.

  2. Over successive generations, possessing multiple photoreceptors became the norm in the gene pool, because individuals with mutations encoding for an increased number of photoreceptors were better able to react to their surroundings. An arms race began, fueling the evolution of the new sensory organ.

  3. Eventually, what was once just a single photoreceptor cell became a light-sensitive patch. At this point, the creature was still only able to distinguish light from dark.

  4. A slight depression in the patch created a pit, for the first time allowing a limited ability to sense from which direction light or shadow was coming from.

  5. The pit’s opening gradually narrowed to create an aperture " like that of a pinhole camera " making vision sharper.

  6. The aqueous humour formed. A colourless, gelatinous mass filling the chamber of the eye, it helped to maintain the shape of the eye and keep the light sensitive retina in place.

  7. At the front, a transparent tissue with a concave curvature for refracting light formed. The addition of this simple lens drastically improved image fidelity.

  8. A transparent layer evolved in front of the lens. This transparent layer, the cornea, further focused light, and also allowed for more blood vessels, better circulation, and larger eyes.

  9. Behind the cornea, a circular ring formed, the iris, with a hole in its centre, the pupil. By constricting, the iris was able to control the amount of light that reached the retina through the pupil.

  10. Separation of these two layers allowed another gelatinous mass to form, the aqueous humor, which further increased refractive power.

A further simplified diagram of the process:

This steadily increasing ramp of complexity is a logical progression from 1% of an eye to 100% of an eye, and each stage is useful to its possessor. The end result is the basic blueprint for all vertebrate eyes.Although wecan probably never know for sure whether it really happened that way (the fossil record isn’t a perfect catalogue of intermediary forms " and even if it was, eyes are composed of soft tissues and don’t readily fossilize), we have incredibly good evidence to support the model.


As I mentioned before, organisms with proto-eyes corresponding to every step in this evolutionary sequence have been found. With the right selection pressures, they could be navigating their environments with eyes similar to ours one day.

Uni-cellular protists of the genus Euglena posses a small stigma, or eye spot, that is capable of detecting light, but unable to form images.

(© CC 2011 Deuterostome)

Planarian worms have cup shaped eyes, capable of detecting the direction of a light source.

(© CC- 2011 Eduard Sol )

The nautilus has pinhole camera eyes, capable of seeing blurry images.

(© CC- 2012 Manuae)

Sea snails have a rudimentary lens in the form of a blob of jelly, giving them the ability to adjust their focus.

(© CC- 2007 Steve Childs)

Personal Incredulity

Darwin recognized that the idea of design arising in the absence of a designer defies intuitive common sense. In a later edition ofOn the Origin of Species,after he had had a chance to witness the publics reaction to his theory, he stated his opinion that their incredulity was just a failure of imagination:
“When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of vox populi, vox dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case and if such variations should be useful to any 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, should not be considered as subversive of the theory.”
Science has demonstrated over and over that the truth of things often defies common sense. But why is the evolution of complex structures like the eye so hard for us to swallow? As a species, we are hardwired to ascribe intention and agency in an attempt to predict the behaviour of other beings. However, we are hyper active in our propensity towards agency detection, to the point where we often ascribe agency to inanimate objects and natural forces. Another byproduct is a tendency to assume that anything that appears complex and purposeful must have been designed by an intelligent agent. This way of thinking is exemplified by the Teleological argument, which William Paley described using his watch maker analogy. He argued that in the same way a watch’s complexity implies a designer, the evident complexity of nature implies the existence of a designer.

In Paley’s day, it made more sense to make this argument, as a mechanism for how life could have evolved by itself had yet to be proposed. Today, the people making it really should know better.The theory of evolution, like the theory of gravity, provides a huge predictive capacity; it is a rigorous testable model of an observable phenomenon. We know that it is true, beyond all reasonable doubt, through a convergence of evidence from fields such as: palaeontology, geology, botany, zoology, comparative anatomy, embryology, bio geography, and genetics.

But still there remains a barrier of doubt that, for many, all the evidence in the world won’t surmount.To grasp the rather counter-intuitive fact that “design” can spontaneously emerge in the absence of a designer, I recommend checking outConway’s Game Of Life, a mesmerizing cellular grid program that models how complex patterns can emerge from the implementation of a few simple rules. As you watch the spiralling patterns and geometric shapes generate from practically nothing, I guarantee that your common sense will be highly offended.

Further Reading/Sources:


by Josh Weed

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