Darwinian natural selection operates on very small, incremental alterations to an organism’s genotype, which determines how that organism shall be – the genotype determines the phenotype. When we look at nature and at biology’s greatest achievements – birds’ wings, camera eyes, etc. – we witness thousands and millions and billions of years of gradual evolutionary development. This is the premise of natural selection and the evidence supports it. The complexity of the eye, for example, illustrates the immense “creativity” of non-random selection upon random genetic mutation.
However, some have doubted the capability of selection, notably, the famous physicist Sir Fred Hoyle, who compared the random emergence of a simple cell to “a tornado sweeping through a junk-yard [and] assembl[ing] a Boeing 747 from the materials therein.” So, if a simple cell could not randomly emerge, however could a wing or an eye? Richard Dawkins, in his sublime book Climbing Mount Improbable, compares this view of random emergence to a mountain called Mount Improbable. Looking at the mountain from the foot, we see a towering peak, a sheer vertical face rearing from the plain, upon which rests an eminently complex biological form, such as an eye. (Although for Fred Hoyle, it may have just as well been a simple cell.)
From this view of the proverbial mountain, the probability of the random emergence of the eye is so small as to be almost nil. Whilst there are freak mutations, wreaking large alterations in the genotype of an organism, one which led to the spontaneous emergence of an eye would be very, very unlikely. And besides, organisms undergoing huge genetic mutations are usually unstable and usually unsuccessful. So, does the eye, or even a simple cell defy evolution? It depends upon the angle at which you view the mountain. Let’s have a look round the back.
You will notice that the view is very different. The rearing vertical face has been replaced by a gradual upward path. As Dawkins says, the pathway leading up the back poses no great challenge to a climber. All that is demanded by this route is time. If you have enough time you may easily reach the once-forbidding pinnacles which rest upon the top of the great mountain. Mount Improbable is a parable for Darwinian evolution. Given enough time, the non-random selective processes of the Darwinian system will navigate the biological terrain and achieve complexity. Mount Improbabe embodies one of the prevalent misunderstandings of evolution – the one which Fred Hoyle had – which is that the emergence of something like the eye isan all-or-nothing eventuality. There is no genetic mutation which either results in an eye or chaotic nothing. Rather, there are lots and lots of small advantageous mutations, naturally selected, which result in the eye’s emergence. Evolution simply needs time.
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.
And yet, in a letter to an American colleague, he wrote,
The eye, to this day, gives me a cold shudder, but when I think of the fine known gradations, my reason tells me I ought to conquer the cold shudder.
And as Dawkins writes,
Darwin…saw his doubts as a challenge to go on thinking, not as a welcome excuse to give up.
So how did the eye evolve? To answer this question I am to look to Dawkins’ book, Climbing Mount Improbable, as this famous explanation is an excellent one.
Light is photons. When photons sent from the sun rain upon objects, some objects reflect light better than others and some objects absorb light better than others. When light is absorbed these objects appear (to an organism capable of forming images with a suitable eye of sufficient complexity) dark, whilst objects that reflect light appear light, of which an image can be formed. When we see something we are either seeing lots of photons being reflected from a something, or lots of photons being absorbed by the something. This absorption and reflection and degrees of absorbance and reflectivity allow a realistic image to be formed. But we have jumped ahead to eyes capable of forming compex images and visualising this natural habit. We need to go to the beginning.
The first “eyes,” Dawkins writes, would have consisted of coloured pigments which, as occurs in photosynthesis, when a light ray strikes an absorbent coloured pigment, an energy transference takes place generating a nerve impulse. These pigment molecules would not be anything like our eyes and would be far less proficient, but these “eyes,” as Dawkins says, would allow an organism to sense the passing shadow of a predator. Natural selection is contemporaneous; it is not concerned with future development; it doesn’t plan. It selects what is available at the time and maximizes opportunities, altering and improving the advantageous mutations, accumulating “survival traits.” Therefore, a pigment molecule capable of simplistically sensing light would be selected.
The old objectionable question What use is half an eye? does not provide any useful objection to the eye’s evolution. A hundredth, a thousandth of an eye is better than no eye. Being able to see a very small amount is unimaginably better than not being able to see at all. The other mistake in this question is the misunderstanding that evolution is striving to assemble a human’s excellent camera-eye, equipped with complex lenses and sophisticated neural circuitry. As I have said, without foresight or conscious engineering, selection cannot envisage an eye, such as ours’, and then attempt to assemble one. Evolution works with what it has – presumably, at first, something like pigment molecules, and then improves it and develops it contemporaneously with successive generations. There are many different eyes all over the animal kingdom, assembled differently and capable of creating different visual patterns. It is very anthropocentric to view the human eye as the definitve blueprint which all organisms must attempt to achieve. Our eye suits our physiology, and the combinatorial interplay between our physiology, brain and eye evolution have made our eye possible, but our eye would not suit an ant or a snail.
As you can see, a snail would not benefit from our excellent eyes, not because of the visual quality, but because of the quantity of the physical apparatus and the plethora of other apparatus necessary to compute and interpret the cacophony of the visual input.
So, pigment molecules would likely have been the first “eyes”, resting on the lower slopes of Mount Improbable’s massif and there is evidence from the animal kingdom that such pigment molecules exist capable of “seeing,” which are not connected to the neural-visual circuitry.
Photons are captured in human eyes, as well as in many other animals, by “rods and cones,” which Dawkins refers to as photocells.
The layers of pigment at the right-hand side of the diagram are responsible for catching photons. The more layers of the pigment that you have, the more photons you will capture and the more photons that you capture the better image you will be able to create. (We shall look at this in greater detail.) (Incidentally, the layers of pigment remind me of myelin, which is an electrically insulating substance which wraps itself in layers around neurons, enhancing the quality of the electrical signals sent along them (much as in fiber-optics).) Many of these photocells allow a greater proportion of photons to be captured, so two photocells are better than one, three than two, four than three, etc., and so allow a more realistic, complete image to be produced. Photocells lead us on to the next stage in eye evolution, which I shall call the Cup Phase.
A patch of coloured pigment or a batch of photocells would provide an immense visual advantage to an organism; it would be able to sense light and dark (or light and an absence of light). But so far these pigments would exist on a level surface. If the batch of photocells could move into the shape of a cup then the organism would be able to sense direction.
This diagram illustrates the great benefits of developing a cup eye. Rather than a level retina absorbing light rays over its whole surface, only a proportion of photocells would be capable of absorbing light and the impulses of just a portion of the retina would provide directional sensing. This would mean that as well as being able to “visualise” a predator’s presence, it would be able to sense the direction from which the predator was coming, and subsequently escape in the corresponding opposite direction. And all of this without necessarily creating an image, but just sensing light and dark.
The eye tells a fascinating evolutionary tale, one which I hope to continue recount to the best of my lacking ability, drawing from the work of Dawkins and others, in the next post, where we shall look at pinhole-cameras, lenses and the evolutionary massif of the eye.
Dawkins, R. (1996). Climbing Mount Improbable. (Viking: London).