- Feb 10, 2007
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Hi there, many skeptics of evolution point out the eye, and how they assume it could not have possibly evolved. Here is an interesting article on PBS.com for the answers of how the eye evolved:
The human eye is
an organ of great
complexity, both in
structure and function. The case for evolution does not depend, even for a minute, upon a claim that living organisms are not complex or intricate. One case in point is a structure often cited as a perfect example of intelligent design: the human eye.
The eye, like a top-of-the-line modern camera, contains a self-adjusting aperture, an automatic focus system, and an inner surface that minimizes the scattering of stray light. But the sensitivity range of the eye, which gives us excellent vision in both sunlight and moonlight, far surpasses that of any film. Its neural circuitry enables the eye to automatically enhance contrast. And its color-analysis system enables it to quickly adjust to lighting conditions (incandescent, fluorescent, or sunlight) that would require a photographer to change filters and films.
The proponents of intelligent design assert that the combination of nerves, sensory cells, muscles, and lens tissue in the eye could only have been "designed" from scratch. After all, how could evolution, acting on one gene at a time, start with a sightless organism and produce an eye with so many independent parts, such as a retina, which would itself be useless without a lens, or a lens, which would be useless without a retina?
Cross-section of a
human eye
In a Darwinian world, the exquisite adaptations and specializations of living organisms are the products of natural selection, a process whereby the genetic variations -- such as size, shape, and coloration -- that give individuals the best chance to survive and reproduce are passed on to subsequent generations.
The pathway by which evolution can produce complex structures has been brilliantly explained in The Blind Watchmaker by Richard Dawkins, a biologist at Oxford University. The essence of Dawkinss explanation is simple. Given enough time (thousands of years) and material (millions of individuals in a species), many genetic changes will occur that result in slight improvements in a system or structure such as the eye. However slight that improvement, as long as it is genuine, natural selection will favor its spread throughout the species over several generations.
Birds have highly
developed eyes, in
some ways even
more so than
humans.
Little by little, one improvement at a time, the system becomes more and more complex, eventually resulting in the fully functioning, well-adapted organ that we call the eye. The retina and the lens did not have to evolve separately because they evolved together.
Evolution can be used as an explanation for complex structures if we can imagine a series of small, intermediate steps leading from the simple to the complex. Further, because natural selection will act on every one of those intermediate steps, no single one can be justified on the basis of the final structure toward which it may be leading. Each step must stand on its own as an improvement that confers an advantage on the organism that possesses it.
Building an eye
This step-by-step criterion can easily be applied to a complex organ like the eye. We begin with the simplest possible case: a small animal with a few light-sensitive cells. We could then ask, at each stage, whether natural selection would favor the incremental changes that are shown, knowing that if it would not, the final structure could not have evolved, no matter how beneficial. Starting with the simplest light-sensing device, a single photoreceptor cell, it is possible to draw a series of incremental changes that would lead directly to the lens-and-retina eye. None of the intermediate stages are unreasonable, since each requires nothing more than an incremental change in structure: an increase in cell number, a change in surface curvature, a slight increase in transparency.
This incremental process is the real reason why it is unfair to characterize evolution as mere chance. Chance plays a role in presenting random genetic variations. But natural selection, which is not random, determines which variations will become fixed in the species.
Critics might ask what good that first tiny step, perhaps only five percent of an eye, might be. As the saying goes, in the land of the blind the one-eyed man is king. Likewise, in a population with limited ability to sense light, every improvement in vision, no matter how slight, would be favored -- and favored dramatically -- by natural selection.
Design flaws
Another way to respond to the theory of intelligent design is to carefully examine complex biological systems for errors that no intelligent designer would have committed. Because intelligent design works from a clean sheet of paper, it should produce organisms that have been optimally designed for the tasks they perform. Conversely, because evolution is confined to modifying existing structures, it should not necessarily produce perfection. Which is it?
The eye, that supposed paragon of intelligent design, offers an answer. We have already sung the virtues of this extraordinary organ, but we have not considered specific aspects of its design, such as the neural wiring of its light-sensing units. These photoreceptor cells, located in the retina, pass impulses to a series of interconnecting cells that eventually pass information to the cells of the optic nerve, which leads to the brain.
Light passes
through the lens
to the retina,
and then to the
brain.
An intelligent designer, working with the components of this wiring, would choose the orientation that produces the highest degree of visual quality. No one, for example, would suggest that the neural connections should be placed in front of the photoreceptor cells -- thus blocking the light from reaching them -- rather than behind the retina.
Less-than-perfect vision
Incredibly, this is exactly how the human retina is constructed. Visual quality is degraded because light scatters as it passes through several layers of cellular wiring before reaching the retina. Granted, this scattering has been minimized because the nerve cells are nearly transparent, but it cannot be eliminated because of the basic design flaw. Moreover, the effects are compounded because a network of vessels, which is needed to supply the nerve cells with a rich supply of blood, also sits directly in front of the light-sensitive layer, another feature that no engineer would propose.
A more serious flaw occurs because the neural wiring must poke directly through the wall of the retina to carry the nerve impulses produced by photoreceptor cells to the brain. The result is a blind spot in the retina -- a region where thousands of impulse-carrying cells have pushed the sensory cells aside. Each human retina has a blind spot roughly a millimeter in diameter -- one that would not exist if only the eye were designed with its sensory wiring behind rather than in front of the photoreceptors.
The optic nerve
connects to the
brain through a
hole in the
retina, causing a
blind spot.
Do these design problems exist because it is impossible to construct an eye that is wired properly, so that the light-sensitive cells face the incoming image? Not at all. Many organisms have eyes in which the neural wiring is neatly tucked away behind the photoreceptor layer. The squid and the octopus, for example, have a lens-and-retina eye quite similar to our own, but their eyes are wired right-side out, with no light-scattering nerve cells or blood vessels in front of the photoreceptors, and no blind spot.nside-out development
Evolution, which works by repeatedly modifying preexisting structures, can explain the inside-out nature of our eyes quite simply. The vertebrate retina evolved as a modification of the outer layer of the brain. Over time, evolution progressively modified this part of the brain for light sensitivity. Although the layer of light-sensitive cells gradually assumed a retina-like shape, it retained its original orientation, including a series of nerve connections on its surface. Conversely, mollusk eyes are wired optimally because rather than evolving from brain cells, which have wiring on the surface, they evolved from skin cells, which retained their original orientation with the wiring below the surface.
The living world is filled with examples of many other organs and structures that clearly have their roots in the opportunistic modification of a preexisting structure rather than the clean elegance of design. This does not, despite the fears of "intelligent design" advocates, amount to evidence against the existence of a Deity. Properly understood, as Darwin himself pointed out, it only deepens our respect for the power and subtlety of the Creator's remarkable ways.
Thank you for taking time to read all of this, I think this is interesting and every skeptic of Evolution should read this.
The human eye is
an organ of great
complexity, both in
structure and function. The case for evolution does not depend, even for a minute, upon a claim that living organisms are not complex or intricate. One case in point is a structure often cited as a perfect example of intelligent design: the human eye.
The eye, like a top-of-the-line modern camera, contains a self-adjusting aperture, an automatic focus system, and an inner surface that minimizes the scattering of stray light. But the sensitivity range of the eye, which gives us excellent vision in both sunlight and moonlight, far surpasses that of any film. Its neural circuitry enables the eye to automatically enhance contrast. And its color-analysis system enables it to quickly adjust to lighting conditions (incandescent, fluorescent, or sunlight) that would require a photographer to change filters and films.
The proponents of intelligent design assert that the combination of nerves, sensory cells, muscles, and lens tissue in the eye could only have been "designed" from scratch. After all, how could evolution, acting on one gene at a time, start with a sightless organism and produce an eye with so many independent parts, such as a retina, which would itself be useless without a lens, or a lens, which would be useless without a retina?
Cross-section of a
human eye
In a Darwinian world, the exquisite adaptations and specializations of living organisms are the products of natural selection, a process whereby the genetic variations -- such as size, shape, and coloration -- that give individuals the best chance to survive and reproduce are passed on to subsequent generations.
The pathway by which evolution can produce complex structures has been brilliantly explained in The Blind Watchmaker by Richard Dawkins, a biologist at Oxford University. The essence of Dawkinss explanation is simple. Given enough time (thousands of years) and material (millions of individuals in a species), many genetic changes will occur that result in slight improvements in a system or structure such as the eye. However slight that improvement, as long as it is genuine, natural selection will favor its spread throughout the species over several generations.
Birds have highly
developed eyes, in
some ways even
more so than
humans.
Little by little, one improvement at a time, the system becomes more and more complex, eventually resulting in the fully functioning, well-adapted organ that we call the eye. The retina and the lens did not have to evolve separately because they evolved together.
Evolution can be used as an explanation for complex structures if we can imagine a series of small, intermediate steps leading from the simple to the complex. Further, because natural selection will act on every one of those intermediate steps, no single one can be justified on the basis of the final structure toward which it may be leading. Each step must stand on its own as an improvement that confers an advantage on the organism that possesses it.
Building an eye
This step-by-step criterion can easily be applied to a complex organ like the eye. We begin with the simplest possible case: a small animal with a few light-sensitive cells. We could then ask, at each stage, whether natural selection would favor the incremental changes that are shown, knowing that if it would not, the final structure could not have evolved, no matter how beneficial. Starting with the simplest light-sensing device, a single photoreceptor cell, it is possible to draw a series of incremental changes that would lead directly to the lens-and-retina eye. None of the intermediate stages are unreasonable, since each requires nothing more than an incremental change in structure: an increase in cell number, a change in surface curvature, a slight increase in transparency.
This incremental process is the real reason why it is unfair to characterize evolution as mere chance. Chance plays a role in presenting random genetic variations. But natural selection, which is not random, determines which variations will become fixed in the species.
Critics might ask what good that first tiny step, perhaps only five percent of an eye, might be. As the saying goes, in the land of the blind the one-eyed man is king. Likewise, in a population with limited ability to sense light, every improvement in vision, no matter how slight, would be favored -- and favored dramatically -- by natural selection.
Design flaws
Another way to respond to the theory of intelligent design is to carefully examine complex biological systems for errors that no intelligent designer would have committed. Because intelligent design works from a clean sheet of paper, it should produce organisms that have been optimally designed for the tasks they perform. Conversely, because evolution is confined to modifying existing structures, it should not necessarily produce perfection. Which is it?
The eye, that supposed paragon of intelligent design, offers an answer. We have already sung the virtues of this extraordinary organ, but we have not considered specific aspects of its design, such as the neural wiring of its light-sensing units. These photoreceptor cells, located in the retina, pass impulses to a series of interconnecting cells that eventually pass information to the cells of the optic nerve, which leads to the brain.
Light passes
through the lens
to the retina,
and then to the
brain.
An intelligent designer, working with the components of this wiring, would choose the orientation that produces the highest degree of visual quality. No one, for example, would suggest that the neural connections should be placed in front of the photoreceptor cells -- thus blocking the light from reaching them -- rather than behind the retina.
Less-than-perfect vision
Incredibly, this is exactly how the human retina is constructed. Visual quality is degraded because light scatters as it passes through several layers of cellular wiring before reaching the retina. Granted, this scattering has been minimized because the nerve cells are nearly transparent, but it cannot be eliminated because of the basic design flaw. Moreover, the effects are compounded because a network of vessels, which is needed to supply the nerve cells with a rich supply of blood, also sits directly in front of the light-sensitive layer, another feature that no engineer would propose.
A more serious flaw occurs because the neural wiring must poke directly through the wall of the retina to carry the nerve impulses produced by photoreceptor cells to the brain. The result is a blind spot in the retina -- a region where thousands of impulse-carrying cells have pushed the sensory cells aside. Each human retina has a blind spot roughly a millimeter in diameter -- one that would not exist if only the eye were designed with its sensory wiring behind rather than in front of the photoreceptors.
The optic nerve
connects to the
brain through a
hole in the
retina, causing a
blind spot.
Do these design problems exist because it is impossible to construct an eye that is wired properly, so that the light-sensitive cells face the incoming image? Not at all. Many organisms have eyes in which the neural wiring is neatly tucked away behind the photoreceptor layer. The squid and the octopus, for example, have a lens-and-retina eye quite similar to our own, but their eyes are wired right-side out, with no light-scattering nerve cells or blood vessels in front of the photoreceptors, and no blind spot.nside-out development
Evolution, which works by repeatedly modifying preexisting structures, can explain the inside-out nature of our eyes quite simply. The vertebrate retina evolved as a modification of the outer layer of the brain. Over time, evolution progressively modified this part of the brain for light sensitivity. Although the layer of light-sensitive cells gradually assumed a retina-like shape, it retained its original orientation, including a series of nerve connections on its surface. Conversely, mollusk eyes are wired optimally because rather than evolving from brain cells, which have wiring on the surface, they evolved from skin cells, which retained their original orientation with the wiring below the surface.
The living world is filled with examples of many other organs and structures that clearly have their roots in the opportunistic modification of a preexisting structure rather than the clean elegance of design. This does not, despite the fears of "intelligent design" advocates, amount to evidence against the existence of a Deity. Properly understood, as Darwin himself pointed out, it only deepens our respect for the power and subtlety of the Creator's remarkable ways.
Thank you for taking time to read all of this, I think this is interesting and every skeptic of Evolution should read this.