The eye’s intricate structure has long been a focal point in the creationism vs evolutionism debate, serving as a key example in the argument of irreducible complexity. More recently, methods of simulating the evolution of the eye have emerged, which claim to provide significant support for Darwinism. Who is right?
Even before evolutionary theory emerged, the eye of vertebrates was at the centre of the search for evidence of God’s existence in nature. In 1802, the Christian theologian William Paley declared that the eye was a miracle of “design“, supporting the idea that such perfection could only have been achieved by a Creator. Almost half a century later, Charles Darwin anticipated the problems of irreducible complexity raised today, stating that the evolution of the eye through natural selection seemed absurd at first analysis. However, he expressed confidence that a series of small, gradual adaptive changes could produce such a complex organ.[1] Darwin was also the first to propose an evolutionary sequence from a primitive eye consisting of only a few photosensitive cells to a fairly sophisticated eye, using only examples of visual organs found in existing organisms.
Irreducible complexity
In his book, Darwin’s Black Box, American molecular biologist Michael Behe revives part of Paley’s older argument in favour of a creator of life in the form of “irreducible complexity”. In short, Behe showed that most complex biomolecular organs and functions are either logically incompatible with an evolutionary origin or so unlikely as to be essentially impossible. Such organs and functions are usually made up of biological or molecular components that, when taken separately, have no function and therefore no selective value for evolution. All of the components must be present, fully formed, and fully integrated for the organ’s biological function to be expressed.
Behe’s argument is that, in these cases, there is no “plausible” sequence of gradual evolutionary changes in which each step provides increasing functionality to be favoured by natural selection. Biological utility appears suddenly in such a scenario and therefore cannot be produced by gradual evolution. The modern theory of irreducible complexity has found a prime example in the eye. A complex eye cannot function without a motor system (muscles), a coordinator (nerves), a central nervous system to interpret the image, a photoreceptor system, an optical system, and so on. All of these vision subsystems must be present at the same time, be functional, and be fully coordinated for the biologically useful function of vision to be achieved.
The evolutionary perspective
The idea that the eye gradually developed in a series of useful intermediate steps seems unlikely. From an evolutionary perspective, however, the eye’s subsystems developed simultaneously, with the complexity of their components and integration gradually increasing over time.
This theory has already been successfully explained in detail: it is thought that a patch of photosensitive tissue can only detect the presence and intensity of light, but not the direction from which it comes. Nevertheless, this provides an important advantage in discerning at least the day-night cycle. Most organisms have at least such primitive visual organs, and even many bacteria have photosensitive areas for this purpose. The next evolutionary step would be to deepen the flat surface of the photosensitive patch to create a concave surface. This configuration has the added advantage of providing information about the direction from which light is coming, as it creates a lit area and a shaded area on the layer of photosensitive cells. The effect is stronger the more pronounced the concavity, so it is assumed that evolution will encourage deeper and deeper concavities until a spherical enclosure with a narrow opening through which light enters is formed. This type of eye is very good at detecting light intensity and direction, but cannot yet form an image as we humans perceive, for example.
In order to form an image, light must be focused through a lens. It is thought that the lens forms by modifying the transparent layer of cells that protects the eye. First, a primitive lens emerges that can only produce blurry images. Gradually, this evolves into a lens with a continuous gradation of refractive index that can produce a clear image. At the same time, the lens migrates to its optimal position and size within the eye, producing the clearest possible image on the retina (the initial photoreceptor layer). From Darwin to the present day, this has broadly been the proposed process for the evolution of a complex eye. In support of this theory, many of the intermediate forms of eyes in this evolutionary process can indeed be found in various organisms in nature, each fulfilling a limited purpose. While this scenario ignores complications involving the nervous and muscular systems that coordinate the eye (among other things), it does highlight a progressive selective value for eye types in the order in which they appear.
Nilsson and Pelger
The Swedish zoologist Dan-Eric Nilsson made a significant contribution to the support of Darwin’s ideas when he theorised that the evolution of the vertebrate eye had occurred in four stages, beginning with just a few photoreceptor cells.[2] In 1994, Nilsson and his colleague Susanne Pelger published a highly influential study on the evolution of the fish eye entitled “A Pessimistic Estimate of the Time Required for an Eye to Evolve”, in which they attempted to simulate this process in order to estimate how long it would take.[3] This study was extremely influential and was cited extensively in subsequent works. We will therefore discuss it in detail below, first as reflected in most secondary sources (reports by other authors on the study in question).
Secondary sources
The computer simulation started from a simplified model of the eye consisting of only three types of tissue: an opaque layer of cells; a patch of photosensitive cells; and a transparent layer. Spontaneous genetic mutations were simulated as percentage changes in the size and shape of the cell layers, or in their optical properties. The variations produced by these mutations were not artificially directed towards a particular evolution of the eye, but were left to undergo simulated natural selection. Natural selection was simulated as a selective pressure favouring the clearest possible vision (visual acuity). To avoid favouring evolution too much through a priori direction, the researchers set the selective pressure to a minimum level. Therefore, even if a 1% improvement in the performance of a simulated organism’s eye was observed, an organism without this improvement would still have over 99% of the former’s survival chances. The authors deliberately chose the selective pressure and several other parameters related to population genetics to be “pessimistic”, so as not to be accused of artificially favouring an unreasonably rapid evolution of the eye.
Charles Darwin anticipated the problems of irreducible complexity that are discussed today. He stated that the evolution of the eye by natural selection seemed absurd at first glance. The eye has become a benchmark for the modern argument of irreducible complexity.
As soon as they published their research results, Nilsson and Pelger received an enthusiastic review from Richard Dawkins in the journal Nature entitled “The Eye In A Twinkling”.[4] In this article, Dawkins discusses two computer simulations based on random mutations: one to simulate the evolution of the eye, and another to estimate the time required for this process. Both simulations are based on random mutations and plausible selective pressures. Dawkins concludes: “The results were swift and decisive. A trajectory of steadily improving acuity led unhesitatingly from a flat beginning through a shallow cup [of the primitive eye—ed.] to a steadily deepening cup. The transparent layer thickened to fill the cup and smoothly curved its outer surface. And then, almost like a conjuring trick, a portion of this transparent filling condensed into a local, spherical subregion of higher refractive index—not uniformly higher, but a gradient of refractive index such that the spherical region functioned as an excellent graded-index lens [an extremely advanced type of lens by modern optical engineering standards—ed.]. Best of all, the ratio of the focal length of the lens to its diameter settled down at a close approximation to Mattiessen’s ratio, long known to be the ideal value for a graded-index lens. . . Turning to the question of how long the evolution might have taken, Nilsson and Pelger had to make some plausible population-genetic assumptions. . . Their guiding principle in choosing such numbers was pessimism. For each assumption they made, they wanted to err in the direction of overestimating the time taken for the eye to evolve. They even went so far as to assume that any new generation differed in only one part of the eye: simultaneous changes in different parts of the eye, which would have speeded up evolution, were banned. But even with these conservative assumptions, the time taken for a fish eye to evolve from flat skin was under 400,000 generations. Assuming typical generation times of one year for small animals, the time needed for the evolution of the eye, far from stretching credulity with its vastness, turns out to be too short for geologists to measure. It is a geological blink.”
Checkmate?
Nilsson and Pelger’s simulation therefore seems to accurately and independently reproduce the expectations of evolutionists from Darwin to the present day. Changes occur gradually, with visual acuity increasing step by step to confer selective advantages for natural selection. The result is an eye with all the features of a complex eye as found in nature, and the time required is practically a geological blink of an eye. If we accept the proposed estimate, then in the 540 million years since primitive visual organs are believed to have first appeared in the early Cambrian, there would have been ample opportunity for the complex eye to evolve over 1,500 times. The icing on the cake for evolutionists is that the key argument for irreducible complexity—the implausibility of the evolution of the complex eye—is proven wrong beyond any doubt. With the enthusiastic endorsement of renowned biologists such as Dawkins, Nilsson and Pelger’s work has become the definitive reference on the evolution of the eye and is cited in numerous articles and subsequent research. Even today, famous professors[5], popular bloggers[6], and presenters of widely watched evolutionary science series[7] use the information distilled from Dawkins’ review to support devastating tirades against creationism.
Primary source
The only problem is that Nilsson and Pelger’s paper does not actually mention any computer simulations. Furthermore, the two simulations referred to by Dawkins do not exist, and there is no computer analysis of any aspect of the paper. Furthermore, the simulation presented by Nilsson and Pelger[8] is not based on random mutations under selective pressure, but on a strictly constrained and predefined evolutionary scenario.
Below are the two myths about the simulation of eye evolution that have spread from Dawkins’s blatantly incorrect review, which was accepted without verification by the world’s most prestigious scientific journal:
- The first myth is that the article in question is based on two computer simulations of the evolution of the eye. In reality, there is no such thing.
- The second myth is that the simulations in the article would produce the evolutionary sequence of the eye starting from random mutations under natural selection. In reality, however, the evolutionary sequence of eye types presented in the article is not the result of a simulation; it is the premise of the simulation. The random factors are strictly constrained by the authors to ensure this sequence is followed.
Both myths have been extensively documented and are obvious to anyone who reads the original paper. They have also been confirmed as false by the authors themselves, Nilsson and Pelger.
Why did Dawkins get it so wrong? Commentators favourable to him have tried to defend him by trivialising his mistakes and correcting them on the spot, granting him the presumption of innocence with regard to his intention to deceive. In the absence of a statement from Dawkins himself, we can only do the same in the spirit of goodwill. However, Nilsson and Pelger’s paper remains an extremely interesting and well-written scientific endeavour when we consider its actual content. Let’s examine what the paper actually contains.
At most, Nilsson and Pelger’s study can be used as an argument in favour of evolution to support intelligently guided evolution (or theistic evolution), i.e., still a form of intelligent design, not Darwinian naturalistic evolution.
Firstly, when the authors posed the question, in 1994, of estimating the time required for the evolution of the eye, they were most likely not trying to counter creationist arguments. Rather, they were responding to the repeated failures of phylogenetic studies to reconstruct a coherent evolutionary history of the eye. Due to these consistent shortcomings in integrating the visual organ into a coherent evolutionary tree, it is now believed that the eye has evolved independently 50–100 times.[9] Given this, it is easy to see why it is important for the eye to evolve quickly and easily, and Nilsson and Pelger set out to discover whether this is possible. In fact, a careful reading of the article shows that this is the main conclusion that the authors are trying to draw: that the eye has the potential to evolve rapidly. They have proposed a possible (though not necessarily probable) evolutionary sequence to illustrate this potential. But how solid is this conclusion?
Interestingly, the authors attempt to constrain the parameters of their model as “pessimistically” as possible so as to avoid accusations of selecting convenient parameters to achieve a short evolution time. However, if the pessimistic parameters of the model give a time of only 364,000 years for the evolution of the eye, what estimate would we get by introducing more realistic parameters into the equation? Amateur biologist VJ Torley did just that, obtaining a realistic time for the evolution of the eye of only 3,640 years—short enough to have occurred between the time of Pharaoh Tutankhamun and the present day.[10] While this extremely short time may seem absurd even to the most convinced evolutionist, let’s examine where Nilsson and Pelger’s model may be flawed.
First, a closer look at the paper reveals that there is no such thing as a “simulation”. What the authors actually did was create a model of eye evolution and estimate how long it would take. The well-known evolutionary sequence of the formation of the complex eye is a premise of this model, not a result, as Dawkins has mistakenly and enthusiastically propagated. Therefore, the model does not confirm the evolution of the eye as imagined by evolutionists; rather, it asserts this as a premise for estimating the timescale of evolution, which is the goal of the study. According to correspondence with Torley, Nilsson openly admits that the evolutionary stages of the eye in their simulation were pre-selected to produce a continuous route from a flat eye without the ability to create an image, to a complex eye with a lens. This ensures that every change on this route, no matter how small, generates an improvement in visual acuity. Furthermore, the authors selected the parameters which were to vary “randomly”, as well as the order in which they would be subjected to selective pressures. This indicates beyond doubt that Nilsson and Pelger’s study can at most be used to support the idea of intelligently guided evolution (or theistic evolution), i.e. a form of intelligent design rather than Darwinian naturalistic evolution.
Also very important is the impact of this study on the creationist argument of irreducible complexity. The authors have certainly built a solid case for “gradualism” in their study because the proposed evolutionary sequence successfully defines a series of steps for building a complex eye with constant improvements in visual acuity.
But demonstrating that something could happen in a purely theoretical sense is far from saying that it is likely to happen or to have happened. The main problem with the model is that it merely demonstrates a theoretical possibility without estimating the probability of that possibility materialising. It merely starts from an evolutionary sequence provided “ready-made” by the authors and estimates the time it could be transposed into reality by strongly constrained random factors and natural selection. However, this approach does not demonstrate the validity of the Darwinian position in any way, and, to their credit, nor do the authors insist on this. From this point of view, the argument of irreducible complexity retains its secular validity, perhaps only requiring a clearer explanation that it refers to the extreme improbability, rather than the impossibility, of evolutionary processes.
From this perspective, the artificiality of the “pessimistic” estimate of 364,000 years for the evolution of the complex eye becomes apparent. Ultimately, this is neither better nor worse than the “optimistic” yet utterly absurd estimate of just 3,640 years. Both estimates are flawed in that they are based on processes that are only theoretically possible without any analysis of context, or plausibility.
Conclusions
So who is right? Nilsson and Pelger deserve credit for designing a solid and honest study of the theoretical possibility of eye evolution. However, the evolutionary perspective from which they analyse it leads them to suggest that the time estimate they calculated could be probable, which cannot be supported. Their study remains a reference on the evolution of the eye, even more than 30 years later. Unfortunately, a “misunderstanding” on the part of a renowned biologist led to a misguided popularisation of this study which persists to this day and is used as a false argument against creationism.