“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.”
Charles Darwin, Origin of Species
With this statement, and like any good scientist, Charles Darwin provided a means by which his theory of evolution could be falsified. The claim from Intelligent Design advocates is that irreducibly complex systems falsify the theory of evolution and natural selection.
An irreducibly complex system cannot be produced directly (that is, by continuously improving the initial function, which continues to work by the same mechanism) by slight, successive modifications of a precursor system, because any precursor to an irreducibly complex system that is missing a part is, by definition, nonfunctional.
The human eye is often used as an example of an irreducibly complex system. If irreducibly complex systems are found in nature, then it follows that they must have been created, or designed, by some intelligent designer. The concept of irreducible complexity is one of the pillars for the theory of Intelligent Design.
In this article, we will review the evolution of the eye, an explanation of its function, and then step back to review if, indeed, it is irreducibly complex, and thus, possibly created by a designer.
An introduction to the evolution of the human eye. But first, a primer on evolution.
Evolution is the process by which groups of organisms and their descendants change over time. These changes in populations occur through changes in traits that are passed from parent to offspring. A trait is a characteristic of an organism that is determined by its genes.
As organisms reproduce, they pass their traits on to their offspring; this phenomenon is called heredity. The tendency for traits to differ slightly between individuals of a species is called genetic variation. Genetic variation is a random occurrence – that is, traits alter without preference for the surrounding environment. Variation of a trait or set of traits may make a group of organisms more or less likely to survive and reproduce.
Natural selection is the process by which these traits become more or less common in a population. Organisms with traits that make them more likely to survive and produce offspring are selected for positively. Organisms with traits that make them less likely to survive and produce offspring are selected for negatively. This is the Darwinian mechanism of evolution – natural selection makes different traits more or less common in a population as the members of the population reproduce. Even a slight advantage or disadvantage of a trait may have major consequences for a population of organisms.
By this process of evolution, nature has produced an incredible amount of diversity in life on Earth.
How might the eye have evolved?
For some time, the eye has been of interest to philosophers and scientists who sought to understand the origin of an organ as complex as the human eye. In his work, On the Origin of Species, Charles Darwin wrote,
“To suppose that the eye, with all its inimitable contrivances… could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree… Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist… and if any variation or modification in the organ be ever useful to an 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, can hardly be considered real.” Charles Darwin (1809–1882)
The topic of the evolution of the human eye has been studied at length, and the multiple steps between an “imperfect and simple” ancestral eye to a “perfect and complex” human eye alluded to in this often misquoted statement by Darwin has been demonstrated not only on an anatomical level, but also on a cellular and molecular level through advances in technology that have allowed researchers to investigate how vision operates in many species.
A currently proposed model of human eye evolution proceeds as follows: organisms that existed approximately 600 million years ago had a cluster of cells on their surface composed of light-sensing cells. This cluster of cells is called an eyespot. This eyespot gave these organisms an advantage in survival and reproduction over their relatives who lacked this trait.
As these ancestors to humans produced offspring, the number of individual organisms with eyespots grew. These eyespots can still be found on organisms today, including the group of flatworms called Planarians. With each generation, the opportunity for some offspring to develop another advantageous trait increased. For example, an inward folding of the photosensitive cells would make the eye sensitive to light in multiple directions, giving this population a survival advantage over organisms that lacked these inward-folding eyespots, also called “eyecups.”
As other differences accumulated between the organisms that had eyecups and their relatives with eyespots, the closely related groups diverged. This inward folding of the eyespot to an eyecup would represent the next stage of evolution for the vertebrate eye. Indeed, just as eyespots are found today in flatworms, eyecups are also found in a group called the hagfishes. Although these eyespots and eyecups provide an advantage to the organisms that have them, these sense organs are rudimentary – they do not even produce an image! Rather, their function is proposed to be to detect shadows or provide a body clock, also called the circadian rhythm.
By current estimates, image-forming eyes and visual systems emerged approximately sixty million years later, along with the relatively fast evolution of animal body plans that occurred during the Cambrian explosion. As the eyecup becomes deeper and the opening narrows, the resulting structure becomes something like a pinhole camera, allowing for greater sensitivity in the direction of the light source. Importantly, this eye is also able to produce a rudimentary image. The nautilus is a modern organism that makes use of this type of eye.
The next step in vertebrate eye evolution involves the development of a clear fluid within an enclosed chamber. This stage of evolution resulted in the formation of a lens and iris, with the original light detecting cells developing into a retina. During this stage of the evolutionary process, color vision may have developed from the evolution of cone cells on the early retina. By approximately 500 million years ago, this version of the eye would have existed in the common ancestor to vertebrates, as well as the lampreys, which have a similar eye structure today.
During the next stage in the process of vertebrate eye evolution, distinct lens and rod photoreceptors evolved. With both rods and cones present on the retina, as well as an adjustable lens, the organisms with these eyes enjoyed both daytime and nighttime vision. This type of eye is probably similar to the eyes of many fish that are alive today.
Finally, because many vertebrates live on land, having an eye exposed to air posed a challenge to the ancestors of today’s land-dwelling animals. Because air has a lower index of refraction than water, the eye has greater optical power on land than in the sea. The eye would also have to develop a method of protection against the dry environment to which it was exposed. To compensate for these changes, the lens developed into an elliptical shape while the eyelid evolved.
How the human eye works
Starting with the eye, the human visual system has incredible abilities in processing visual information taken from the world around it. The eye itself is a complex structure that receives photons of visible light from the environment, using cells located in the back of the eye, called photoreceptors, to convert information from light into electrical signals. It is estimated that the human eye can see up to ten million different colors! Using the retina to communicate with the brain, the eye is able to accurately receive and transmit a large amount of visual information, such as the location, color, and speed of objects in the surrounding environment, all from even just a quick glance.
The structure of the human eye
In biology, the structure of a system, like the kidneys or the heart, demonstrates how the system serves its functions. Likewise, the structure of the eye reveals its basic functions. The eye is a sphere of fluid surrounded by three layers of tissue, called the retina, the uveal tract, and the sclera. The retina does not completely enclose the eye, however, and is instead located closer to the back of the eye.
Neurons are found in the retina, which is the innermost of the three tissue layers. The purpose of a neuron, one of the basic cell types of the nervous system, is to transmit electrical information. The neurons found on the retina receive the image that the eye is looking at and transmits this image to the brain.
The uveal tract is the middle layer of tissue surrounding the eye. This layer provides nourishment to the cells of the eye, and also contains the iris, which is the muscular, colored portion of the eye that surrounds and adjusts the size of the pupil.
The outermost layer is called the sclera, which is made of a tough, white tissue except towards the front of the eye, where the cornea is formed. The cornea is a transparent tissue that allows rays of light to enter the eye. The fluid inside the eye is made of two, separated fluids called the aqueous humor, which is located between the cornea and the lens, and the vitreous humor, which is located behind the lens.
How is an image formed on the surface of the retina?
To form an image on the retina, the eye has to be able to allow light to pass through the tissues and fluid to reach the retina. The eye must also be able to focus light onto the retina to form an accurate image. In order to produce a focused image, the cornea and lens are able to refract light. Although the cornea is responsible for most of the refraction, the eye contains a muscle that adjusts the shape of the lens surrounds the lens. The adjustments to the lens due to this muscle allow the eye to focus on objects more effectively. For example, the muscle makes the lens flatter and thinner when objects are farther away, while for closer objects the lens becomes thicker and rounder. These changes in the shape of the lens in response to the environment are called accommodation. As a human ages, the ability of the lens to accommodate decreases. Changes in the size of the pupil also contribute to the clarity of an image formed on the retina. When the size of the pupil is small, objects that are farther away can be seen more clearly. However, because the pupil is small, less light is allowed to pass through into the eye. When the size of the pupil is large, more light is allowed to pass through, but the phenomena of spherical and chromatic aberration will blur the image slightly.
How Does the Retina Process Information From Light?
Two types of photoreceptors are located on the retina: rods and cones. These two cell types are specialized for different types of vision. Cones allow humans to see in color, while rods are very sensitive to light and provide black and white vision. Although cones are very useful for daytime vision, there are many more rods than cones on the retina. When light strikes a photoreceptor cell, the cell releases potassium to its external environment. This results in an electrical change inside the photoreceptor known as “hyperpolarization.” The more light that strikes the cell, the more hyperpolarized the cell becomes. When the photoreceptor is hyperpolarized, it sends a signal to a cell called a bipolar cell. These bipolar cells will then send signals to cells called ganglion cells, which transmit the information to the brain. If too much light strikes the photoreceptor, the molecular machinery inside the cell is able to turn the system “off” (this is why you see spots after staring at a bright light). This phenomenon is called “light adaptation.”
Disorders of the eye
The eye is far from “perfect”, as there are many, many disorders of the eye that a large portion of the population experiences. Take a look around you: how many people require glasses, contact lenses, or laser eye surgery?
Two common problems of vision that occur in human populations are myopia (nearsightedness) and hyperopia (farsightedness). Individuals with myopia and hyperopia have trouble focusing images directly onto their retina. Nearsighted individuals focus light in front of their retina, which may be caused by an elongation of the eyeball or increased curvature of the cornea. Farsighted individuals focus light behind their retina, which may be caused by shortness of the eyeball. Both myopia and hyperopia can be corrected using lenses, such as glasses and contacts, or through surgical means. Another common problem of vision, called presbyopia, occurs with aging. As humans age, their lenses become less elastic, and therefore less responsive to adjustments. This results in difficulty seeing close objects, which must be located farther away in order for the eye to focus the image onto the retina. Presbyopia may be corrected using reading glasses or bifocals.
Dichromacy, or “color blindness,” is a genetic disorder that prevents some individuals from detecting certain colors of light. Color blindness comes in two basic forms: protanopia, where the individual cannot detect red and similarly colored light, and deuteranopia, where the individual cannot detect green and similarly colored light. Other common eye disorders include age-related macular degeneration (AMD), glaucoma, and cataracts.
So, is the eye irreducibly complex?
Our current understanding of the evolution of the eye, and the examples we find in nature today, provide us with the building blocks for the gradual refinement and tuning to today’s vertebrate eye. It tells us that simpler “eyes”, beginning with a patch of photosensitive cells, could work and provide an adaptive role until we reach the modern eye. This, then, demonstrates that the eye is not irreducibly complex, does not provide evidence for the falsification of evolution, and does not require an intelligent designer.
To quote Dr. Steven Novella, off his Neurologica Blog:
It is ironic that creationists continue to use the eye as the example of the complex structure that defies evolutionary explanation – when in reality the various eyes that have evolved in nature represent one of the best lines of morphological evidence for evolution.
References and Further Reading
Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup
Trevor D. Lamb, Shaun P. Collin & Edward N. Pugh