In the first part of this exploration of the evolutionary massif and, in particular, the evolution of the eye, we ended with the Cup Phase. Using Dawkins’ excellent book Climbing Mount Improbable we looked at how the first eyes would have consisted of simple, light-sensitive pigment molecules. These would have maybe just allowed an organism to sense light and dark. We then saw how if selection could craft a cup-shaped retina (like in the diagram below) an organism would be able to sense direction. In this post we next look at the pinhole-camera eye – the next stage in the Cup Phase.
Light Rays (Photons) Hitting a Cup-Shaped Batch of Photocells - a Retina (Figure 5.3, Climbing Mount Improbable)
So, we see when streams of photons pepper our retinas and are absorbed by our photocells. The cup-shape of a retina allows us to sense direction, but just as it can be difficult to see when there are too few photons, it is impossible to form an image when there are too many photons. Here enters the next development in the Cup Phase – the pinhole-camera eye.
How an Eye does not Work (Figure 5.5, Climbing Mount Improbable)
The above diagram shows how an eye does not work. The problem here is that whilst there are light rays doing what they are doing in the diagram, there are also thousands of other light rays going in all directions. The diagram below illustrates the problem.
Too many Photons do not Allow an Image to be Formed (Figure 5.6, Climbing Mount Improbable)
Here, if an image could be formed it would consist of an infinite series of dolphins. This would not be a suitable eye. The amount of light rays capable of reaching the retina must be reduced if any useful vision is to be allowed. This is where the pinhole-camera enters.
The Pinhole-Camera Eye (Figure 5.7, Climbing Mount Improbable)
If the cup can not remain like a cup, but can continue to extend its curvature, if the cup will come round to a kind of wine goblet shape and continue curving round so that it closes in to form an aperture, then the amount of light rays permitted to enter the retina will be greatly reduced and will allow a clearer image to be formed. Rather than an infinite number of light rays coming from infinite points of an object and creating an infinite series of visual images, the quantity of light rays entering the retina can be controlled by the aperture.
Various Pinhole-Camera Eyes from across the Animal Kingdom (Figure 5.8, Climbing Mount Improbable)
These eyes above, as Dawkins says, these examples must not be thought of as transitional forms; simply as examples gathered contemporaneously, but which can represent varying degrees of development, and how such a transition may have looked. These examples simply assist the visualisation of the evolutionary process, and offer some practical suggestions too. If you look at (a) Nautilus‘ pinhole eye, you can see how this early pinhole-camera eye may have looked. The pinhole-camera formation is limiting the number of photons showering the retina. But this eye could use an improvement, one which we see developing in (b) the marine snail’s eye and in (c) the bivalve mollusc and the (d) abalone.
Dawkins' Computational Imaginary "Magic Window" Lens Solution (Figure 5.9, Climbing Mount Improbable)
In the above eye connected to the bizarre contraption, the pinhole has been widened, which means more light and therefore a better, clearer image. The problem here, though, is the one which the pinhole principle sought to vanquish. However, the “magic window” as you will see is a cunning, opulently complex piece of technology – a piece of glass elaborately connected to a computer. The aim of the “magic window” is to converge light rays sent from, say, the dolphin’s tail to a corresponding point on the retina. The computer is running a programme which calculates this corresponding point at which the light sent from the dolphin’s tail should hit the retina.
Of course, the “magic window” is not a viable solution to the conundrum posed by the need to permit more photons whilst retaining the visual “clarity” of the pinhole-camera, as it would be hideously complicated to engineer and fabulously costly. But the real biological solution is far simpler, and far more practical. This solution is a lens.
Refraction of Light through Glass
The above diagram shows the refraction of light through a glass block. Light enters the glass at angle x and as it enters the block it is refracted to angle y. The rays travel through the block at angle y, but when they exit the block they are refracted back to their original angle, x. The problem to which the “magic window” was postulated as a solution can be solved by a transparent material capable of refracting light into a convergent point: a lens.The best shape for a lens would be a sphere, as a sphere allows light convergence to a single point. The light will enter the transparent sphere and as it leaves the light rays will converge into a single point on the retina. A lens means lots of light can enter the eye without the terrible side-effect of an infinite series of images.
Looking back to the diagram showing different examples of pinhole eyes from across the animal kingdom, you will notice the transparent vitreous mass (vm). The pinhole-camera formations are limiting the number of photons showering the retina. (a) Nautilus’ eye is limiting the number of photons, but could use an improvement, one which we see developing in (b) the marine snail’s eye and in (c) the bivalve mollusc and the (d) abalone. This development is the vitreous mass (vm). The vitreous mass is forming a lens: whilst not perfectly spherical (although the (e) ragworm’s eye is fairly spherical). The vitreous mass could have entered the eye for other purposes, but once it was found to benefit vision selection would have ensured that it remained there.
It didn’t have to be solely vitreous mass; it could be any transparent jelly-like substance which could form a nice rounded shape. As Dawkins says, a drop of water would make a lens, as it naturally forms a rounded shape and it is transparent. A transparent pebble which has been weathered into a curvaceous shape would form a lens too. They can’t help this, they just do.
Prof. Simon Conway Morris
Dawkins calls these kinds of forms “undesigned” and “accidental” and I wouldagree and disagree with this branding. I believe that there is intention in the universe – I do not believe that we are simply an accident (although that opens up the debate over the likelihood of the emergence of life, which we shall not go into here). I believe that God intended and created; I believe that we are His creation. But I believe that we have evolved, that God created fundamental constants which would allow the emergence of life. (The fine-tuning argument has been rubbished by some atheists, but it does contribute a salient view. Why is it that our universe is so unimaginably fine-tuned to allow us to exist? Did it have to be this way?) Simon Conway Morris’ view of convergent evolution, which says that if we re-ran the evolutionary tapes things would be very similar to what they are now, gives credence to the intentionality of God, and to the power of non-random selection on random mutation.
So, back to “undesigned” and “accidental.” Richard Dawkins believes that there is no Designer, but we don’t need to pooh-pooh his terminology instantly if we do believe there is a Designer. The “undesigned” and “accidental” are very useful terms, which confer the random emergences of mutation which benefit an organism. Pebbles and water droplets are accidental lenses and this terminology, whilst convinced that there is no Designer, no God, on the one hand, does assist the understanding of selection’s non-random control over random mutation.
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So, to summarise, the first eye would have consisted of a coloured pigment which could send nerve impulses which would produce “vision” (the ability to sense light and dark); the next step would have involved the development of photocells, which then moved round into a cup shape, which would have allowed an organism to sense where light was coming from. These cups would have then curved around, allowing less light in and therefore producing a more accurate image. But then something like vitreous mass would have entered the eye and formed a lens. This vitreous mass would become more and more spherical, allowing more light in, thereby increasing the quality of the image and converging the light rays to a point on the retina, eliminating the infinite series of images which form on the retina of an eye which allows too much light in without a lens. Succinctly, this is possibly how eyes evolved.
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In the next episode we shall look at the computer simulation of the eye produced by Nilsson and Pelger and further charter the evolutionary massif.
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Dawkins, R. (1996). Climbing Mount Improbable. (Viking: London).
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© Francis Smallwood 2010
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