At the bottom of Mount Improbable?
Eye evolution, a case study
One major criticism of Dawkins’ scenarios is that they presuppose an enormous level of complexity to start with. Indeed, Dawkins is repeating the error of Darwin—as Behe shows, Darwin was ignorant of the complexity of even the simplest cell that modern biochemistry has discovered.
When ‘explaining’ the origin of the eye, Darwin started with a light sensitive spot. Similarly with Dawkins’ chapter on eye evolution. He relies on a computer simulation of gradual eye evolution by Dan Nilsson and Susanne Pelger, which claims, ‘it would take less than 364,000 years for a camera eye to evolve from a light-sensitive patch.’ 26 They start from a light-sensitive layer, with a transparent coating in front and a light-absorbing layer behind.
First, this layer bends gradually into a cup, so it can tell the direction of light rays increasingly well. This continues until it is curved into a hemisphere filled with the transparent substance. Secondly, bringing the ends together, closing the aperture, would gradually increase the sharpness of the image, as a pinhole camera does, because a smaller hole cuts out light, and as there are diffraction effects if the hole is too small, there is a limit to this process. So thirdly, the shape and refractive index gradient of the transparent cover change gradually to a finely focusing lens.
However, Behe has shown that even a ‘simple’ light sensitive spot requires a dazzling array of biochemicals in the right place and time to function. He states that each of its ‘cells makes the complexity of a motorcycle or television set look paltry in comparison’ and describes a small part of what’s involved:
When light first strikes the retina a photon interacts with a molecule called 11-cis-retinal, which rearranges within picoseconds to trans-retinal. (A picosecond [10 –12 sec] is about the time it takes light to travel the breadth of a single human hair.) The change in the shape of the retinal molecule forces a change in the shape of the protein, rhodopsin, to which the retinal is tightly bound. The protein's metamorphosis alters its behavior. Now called metarhodopsin II, the protein sticks to another protein, called transducin. Before bumping into metarhodopsin II, transducin had tightly bound a small molecule called GDP. But when transducin interacts with metarhodopsin II, the GDP falls off, and a molecule called GTP binds to transducin. (GTP is closely related to, but different from, GDP.)
GTP-transducin-metarhodopsin II now binds to a protein called phosphodiesterase, located in the inner membrane of the cell. When attached to metarhodopsin II and its entourage, the phosphodiesterase acquires the chemical ability to “cut” a molecule called cGMP (a chemical relative of both GDP and GTP). Initially there are a lot of cGMP molecules in the cell, but thephosphodiesterase lowers its concentration, just as a pulled plug lowers the water level in a bathtub.27
And the eye-cup sounds simple enough when Dawkins describes it, but dozens of proteins control the structure of cells and their arrangement, and needs molecular supports to hold the structure in place.
A major objection to the Dawkins scenario is that the ability to perceive light is meaningless unless the organism has sophisticated computational machinery to make use of this information. For example, it must have the ability to translate ‘attenuation of photon intensity’ to ‘a shadow of a predator is responsible’ to ‘I must take evasive measures’, and be able to act on this information for it to have any selective value. Similarly, the first curving, with its slight ability to detect the direction of light, would only work if the creature had the appropriate ‘software’ to interpret this. Perceiving actual images is more complicated still. And having the right hardware and software may not be enough—people who have their sight restored after years of blindness take some time to learn to see properly. It should be noted that much information processing occurs in the retina before the signal reaches the brain.
Nilsson and Pelger admit that ‘an eye makes little sense on its own’ and that they ‘avoid the more inaccessible problem of photoreceptor evolution’ 28 although they remain convinced that eye evolution is possible. But the fact remains that, far from starting at the bottom of the mount, Dawkins starts very high up a sheer cliff, even if he, and Nilsson and Pelger, were right that there is a gentle plateau from there.
Also, the videos do not state where the various structures evolve from. They are simply, arbitrarily added to the model.
The ‘beginnings of vision’
In support of the notion that the eye could have evolved in small increments, Dawkins argues that 5% or 1% of normal vision is better than none at all and so by a process of small steps the eye’s performance could improve gradually over many generations.
‘Vision that is 5 per cent as good as yours or mine is very much worth having in comparison with no vision at all. So is 1 per cent vision better than total blindness. And 6 percent is better than 5, 7 better than 6, and so on up the gradual continuous series.’
This simplistic logic glosses over at least two major considerations. First, as indicated above, it ignores the principle of irreducible complexity. Let us imagine, for the sake of argument, some lowly creature for whom the avoidance of light would give it some supposed evolutionary advantage to evade a predator; and that it had acquired by evolution a single photoreceptor cell. Such an acquisition would, of itself, be a major achievement in that it is so specialized. But for that receptor to be of any value to the creature, a neural or humoral mechanism for communication and coordination within its body would have to be in place at the same time. All this immensely reduces the possibility of such a system ever evolving, not to mention the insuperable biochemical obstacles to the first appearance of a single cell. 9–11 Furthermore, any predator of such a creature would need a sophisticated visual or other sensory system to detect the presence of the prey which then begs the question as to how the predator evolved! If there were no predator then the potential prey would not need the protection such a primitive visual system might afford. The visual system would then be eliminated by natural selection as being unneeded!
Secondly, any improvement in an animal’s genetic endowment as part of the supposed evolutionary process requires the incorporation of more information in its genome. This, as a spontaneous phenomenon, i.e. without intelligent intervention, has yet to be observed or reproduced experimentally and so remains a supposition, not an established fact. Observed mutations have never generated new information but, rather, they represent corruption of the genetic data, i.e. there is a loss of information. 12
A recent editorial article 13 has reviewed research with a bearing on the question of whether vision reduced to only perception of light is of value to those so afflicted. Workers who train and assist the visually handicapped agree that even this extremely poor level of vision is useful for orientation and mobility. A single light source can guide and orientate the subject. Light is differentially reflected off various surfaces, e.g. more from concrete, painted walls or crosswalks than from darker areas such as grass etc; this can provide clues for the handicapped. However, nonetheless, the fact remains that the severely visually impaired are very vulnerable and dependent on community goodwill. They are unable to avoid obstacles in their path when relying solely on their vision without the other senses.
Furthermore, unless the handicapped person can at the same time perceive the direction of light rays (known as light projection) entering the eye, the light in practice will be virtually no help for the purpose of mobility. Accurate light projection requires a large number of functioning photoreceptors suitably disposed to receive an image, together with their connections to the brain; 14 it represents a huge advance in complexity over bare perception of light with no projection, not only in the eye but more so in the brain.
The same can be said for each successive stage in the supposed evolutionary development of our visual system. This would require the simultaneous acquisition of new structures in the eye plus increases in the complexity and organisation of the brain; one without the other would confer no competitive advantage in natural selection. It would entail progressing from perception of light with projection to the vaguest perception of movement, to form sense (the ability to perceive shapes), to colour vision, to binocular depth perception etc. Each stage is an increase in complexity of several orders of magnitude involving both eye and brain, to say nothing of the six muscles controlling the position and movement of each eye.
For any degree of worthwhile form sense, the eye’s optical system must form an image on the retina and this requires for vertebrates a clear refracting medium through which the light must pass before reaching the retina. This is provided in the human eye by the lens and the cornea.
To illustrate the two principles mentioned above (irreducible complexity and genetic information gain) it is instructive to consider the cornea, the eyelids and the tears.