Evolution of the Eye
150 years ago, Charles Darwin offered an explanation for the evolution of the eye in his book, On the Origin of Species by Natural Selection.
Nonetheless to this day those who oppose evolution argue that an organ as complex as the eye could not have evolved in small steps. In fact, they even quote Darwin saying that it seems difficult to explain the evolution of the eye and organs similarly complex:
To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different 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. (Origin of Species, ch. 6)
Quoting someone in this way is called "quote mining." Anti-evolutionists quote Darwin as though he were admitting that the evolution of the eye was a problem to his theory. It wasn't. Here is how he continued:
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 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. (ibid.)
So what Darwin was really saying is that even though the evolution of the eye seems exceptionally difficult to account for, it's really not. This is especially true for a naturalist like Darwin who was so familiar with life on this planet and thus so familiar with the eyes belonging to life on this planet.
That knowledge allowed him to write this very plausible scenario for the evolution of the eye of the human with all its complexities:
The Evolution of the Eye as Explained by Charles Darwin
The following explanation of the evolution of the eye is taken directly from Origin of Species, chapter 6.
In the Articulata [animals with segmented bodies, like worms and caterpillars] we can commence a series with an optic nerve merely coated with pigment, and without any other mechanism; and from this low stage, numerous gradations of structure, branching off in two fundamentally different lines, can be shown to exist, until we reach a moderately high stage of perfection. In certain crustaceans, for instance, there is a double cornea, the inner one divided into facets, within each of which there is a lens-shaped swelling. In other crustaceans the transparent cones which are coated by pigment, and which properly act only by excluding lateral pencils of light, are convex at their upper ends and must act by convergence; and at their lower ends there seems to be an imperfect vitreous substance. With these facts, here far too briefly and imperfectly given, which show that there is much graduated diversity in the eyes of living crustaceans, and bearing in mind how small the number of living animals is in proportion to those which have become extinct, I can see no very great difficulty (not more than in the case of many other structures) in believing that natural selection has converted the simple apparatus of an optic nerve merely coated with pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed by any member of the great Articulate class.
Darwin goes on to give a quite interesting and easier-to-understand explanation of how the evolution of the eye could have produced these variations in the Articulata:
If we must compare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with a nerve sensitive to light beneath, and then suppose every part of this layer to be continually changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form. Further we must suppose that there is a power always intently watching each slight accidental alteration in the transparent layers; and carefully selecting each alteration which, under varied circumstances, may in any way, or in any degree, tend to produce a distincter image. We must suppose each new state of the instrument to be multiplied by the million; and each to be preserved till a better be produced, and then the old ones to be destroyed. In living bodies, variation will cause the slight alterations, generation will multiply them almost infinitely, and natural selection will pick out with unerring skill each improvement. Let this process go on for millions on millions of years; and during each year on millions of individuals of many kinds; and may we not believe that a living optical instrument might thus be formed as superior to one of glass, as the works of the Creator are to those of man?
My attempt at Making Darwin's Words Easier
I'm not a scientist. I'm just a guy that loves to read about science and who is willing to take the time to really examine the claims of science and Christianity. So what I'm about to explain may not be perfect science, but it is a reasonable approximation of the real scientific explanation of the evolution of the eye.
Darwin had explained earlier that almost any nerve can become sensitive to light. So in order for an eye to evolve, one needs begin with only a nerve sensitive to light.
The outer layers of this nerve then adjust in density, thickness, and transparency, and even the distance from one layer to the next adjusts. This sort of thing is always happening in nature, and it only takes on organism with many descendants to produce millions of individuals with a specific slight mutation.
Natural selection then slightly favors each improvement. Over million of years, in this way, the Creator has formed many variations of eyes, some with lenses, some without, some with eyelids, some without, and I even recently saw a fish with a transparent head whose eyes were inside its head.
More Small Steps
Here's another picture of the evolution of the eye given to me by someone more modern than Darwin. He was just a guy on the internet, by the name of Bill Piper.
You begin with a light sensitive spot, common in nature. It becomes a depression, allowing it to catch more light. Transparent skin forms over it, and variations in thickness and shape allow that skin to become a lens. Muscles attached to that lens then allow it to focus. Similar mutations would produce first tear ducts, then an eyelid, then eyelashes to further filter contaminant particles, etc.
A lens is not as special a thing as you might think. It's just skin. Recently a woman had her sight restored literally "by the skin of her teeth."
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