Vision is the most complex, highly developed
perceptual process found in
humans and other animals. The
eye is the sensory
organ which makes this process possible. How did this structure come into existence? What are its various types and parts?
Evolution of the Eye
The question of how the eye evolved is a complicated one. According to Charles Darwin, it was a complex process involving thousands of intermediate steps:
"To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting diffferent 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. 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; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to 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." 1
According to Darwin, then, the eye could evolve by a series of stages where each developed part of the eye was of some use to the life form in which it was manifested. Creationists often question how the first eye came into existence, and if it progressed in "sections", well, what good is half of an eye? The question is flawed it that it assumes that vision at a lower level than current complex vision is impossible. "Half an eye" would not mean cutting a human eye in half, rather, it would entail any visual system that allowed for light sensitivity, no matter how underdeveloped. These early systems would be very primitive in nature, yet would provide a basis for later evolution into more complex perceptual structures.
"Discovering the ancestry of a complex organ such as the eye is not simple. The conclusion I draw from the evidence at these multiple levels is that the image forming structures of eyes are convergent, but they share a monophyletic ancestral origin, from a simple eyespot or a cluster of photoreceptive cells. The evidence of ancestry from image forming structures such as the lens is inconclusive, and probably describes similar developmental mechanisms utilised rather than homology of simple and compound eyes. The evidence studied at higher levels than the photoreceptive cells suggests a polyphyletic origin of eye. In order to discover the ancestry of complex organs such as eyes, it is important to consider the multiple levels of the structure, since taken in isolation contradictory conclusions can be drawn." 2
The actual progression of visual capabilities is simple. The modern complex eye structure found in animals have origins in the primitive eyespot, an early photosensitive cell or group of cells that allowed vision in early forms of life. In order for such cells to exist, light sensitive chemicals must have been present in some form. According to Ma'ayan Semo:
"Another line [of] evidence for the ancestry of eyes comes from the methods of photon capture and phototransduction. To begin at the cellular level; photoreceptor types are broadly split into two main types, the cillary and the rhabdomeric (microvillar type). The cilliary are found more commonly in the deuterostomes, and the rhabdomeric in the protostomes. This evidence suggests that photoreceptor cells have polyphyletic origins. However exceptions to this rule have been found, photoreceptors which are both rhabdomeric and cilliary, as well as the interesting discovery that the classical rhabdomeric eyes of arthropods and cephalopods during early development form cilia transiently. This evidence suggests monophyly of photoreceptor cells, with a common ancestor of the cilliary type (Halder et al ,1995b). The absorption of photons and phototransduction gives evidence for the ancestry of molecules within the eye. Phototransduction in both vertebrates and invertebrates is mediated through a G-protein coupled cascade, of the opsin molecule, a in the seven-transmembrane receptor family which is covalently linked to a vitamin A derived chromophore (Ranganathan et al, 1995, and Halder et al,1995b). Comparison of DNA and amino acid sequences of opsins show that metazoan opsins must share common ancestry (Fernald, 1997, and Land & Fernald, 1992 )." 2
Vitamin A is crucial in understanding the possible monophyletic origins of visual process. The chemical that humans use for rod vision, rhodopsin, is simply a variant of Vitamin A. Many other animals possess chemicals like this one as well. An example is the chameleon, which uses melanopsin in their light-sensitive pigment cells. 3
Types of Eyes
- Simple Eyes:
- camera eye: This eye type is found in both invertebrates and vertebrates. In all vertebrates and some non-anthropod invertebrates, the lens is within the eye and spherical in shape. Due to the advanced structure of the lens, this eye type provides a bright picture that is high in optical quality.
- concave mirror eye: Found in some ostracod crustaceans, this eye type varies from the camera eye in the lens development. The lens is not nearly as spherical, and while it focuses light to create a bright image, the light is not refracted through the lens with the same precision, making a much more blurry image.
- pinhole eye: The best examples of this eye type are found in molluscs, particularly in the cephalopod Nautilusand and the abalone Haliotis. Light passes through a small opening at the front of the eye, but it not processed beyond that, hitting the back of the eye as unfocused beams that produce either a very blurry picture or a dim one.
- Compound Eyes:
- apposition eye: An eye type consisting of multiple ommatidia (lenses) which are each seperated by pigment cells, which surround them individually. Having the lenses seperate in this way creates the problem of poor photon reception, which led to the development of the superposition eye. The apposition eye is found in diurnal insects, some crabs and lower crustacea.
- superposition eye: Found in dipteran insects, crustacea and crabs this eye type is unique because the ommatidia aren't isolated from each other; together they produce a brighter image due to better photon reception. There are three types of superposition eyes:
The Human Eye
The human eye is the organ which gathers and focuses light. It can be classified as a simple eye, in that it has one lens, and it also falls under the subcategory of camera eye due to its spherical lens shape. Light first enters the cornea, a transparent bulge on the front of the eye. Next, it goes through the anterior chamber, a part of the eye filled with a clear liquid substance known as the aqueous humor. After this, the light passes through the pupil, an opening in the iris. The muscular iris regulates the amount of light entered through the pupil by pulling it open and closed. To further focus the light, the flexible lens shifts its shape according to how far away the object of focus is. The lens will grow thin to focus on far away objects, and thick to see closer ones. After this, light is filtered through the gel-like vitreous humor and finally striking the retina, a thin photosensitive sheet of tissue at the back of the eye. Once the light reaches the retina, it starts to be processed. There are two main types of photoreceptors:
- Rods: A type of photoreceptor which is concentrated along the periphery of the retina. They are most active in dim light. Rods do not produce sensations of color.
- Cones: A type of photoreceptor concentrated in the center of the retina which process bright light and create sensations of color.
The fovea is the area of the retina that contains dense concentrations of cones and is the point of sharpest vision in the eye. In this section of the eye, four types of cells are responsible for visual processing:
1 Charles Darwin. On The Origin of Species By Means of Natural Selection. p. 217. © Gramercy Books.
2 Ma'ayan Semo. Evolution of the Eye. http://www.maayan.uk.com/evoeyes1.html © 1998.
3 http://www.cs.colorado.edu/~lindsay/creation/eye_spot.html