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The ongoing debate between Darwinian evolution and Intelligent Design hinges on a vital question: Can we explain how living creatures originated and how their many intricately complex features came to be?

I have to admit that although most biologists believe that life arose out of ordinary, nonliving chemicals, this concept has not yet been proven by experiment. True, researchers have produced amino acids and other organic compounds out of non-living, inorganic chemicals, but none has yet produced even the simplest of living cells.

Does this mean that life’s origin is too complex for us to understand? That the best we can do is throw up our hands and admit the problem is beyond us? That we should attribute life’s origin to an Intelligent Designer who is far more intelligent and capable than we are?

I don’t believe so, but the subject is still open to debate — until some scientist succeeds in producing a living cell out of nonliving chemicals. That may happen soon, or it may not happen for another century.

Supporters of the Intelligent Design concept also point to individual features of living organisms and claim that they are too complex to be explained by natural causes. Many have suggested that the human eye could not possibly have arisen slowly, evolving gradually over many eons. “You can’t have half an eye,” they state.

Here the evidence is much clearer. Our eyes have a long evolutionary history. The evidence is there for anyone to see.

Vision is such an important way for an organism to gather vital information about its environment that many different kinds of creatures have developed organs that give them the ability to form visual images of the world in which they live. Different kinds of eyes have been developed by shellfish and squids, insects, snails, spiders and almost all the vertebrates, from the jawless fishes of half a billion years ago to Homo sapiens.

Many species of single-celled creatures such as amebas and paramecia have a special region that reacts to light, a photosensitive area that contains a pigment that undergoes a chemical change when light strikes it. The pigment is usually a protein that contains a carotenoid molecule. Carotenoids, which are sensitive to yellow light, are found in many vegetables. Animals obtain carotenoid as vitamin A by eating carotenoid-rich plants, which is why you should eat your carrots and other veggies if you want good vision.

From a pigment spot in single-celled organisms, multi-celled invertebrates moved a step further by developing a rudimentary kind of lens, backed by a collection of light-sensitive cells. The lens concentrates light so that the cells can react to weaker intensities of light. This also permits a finer discrimination between varying intensities of light.

Such a primitive eye can yield a completely new kind of information to the organism that possesses it: it can tell the direction from which the light is coming. If the light falls more intensely on the right side of the photosensitive cells, for example, it means the light is coming from the right. Such an eye can also tell if the source of light is moving even though no image is formed; the organism can’t tell if the object in motion is a predator or a chunk of food or a potential mate. But it can sense that something is moving out there.

Lenses started as merely a means of concentrating light on the photoreceptor cells, but they eventually became useful as a means of focusing an image on those cells. Of course, the number of photoreceptor cells had to increase enormously to handle the vastly increased amount of information that imagery entails. True eyes (such as ours) contain a retina, a curved surface lined with light-sensitive photoreceptor cells, and a lens to focus images on the retina.

One of the earliest steps toward true vision was taken by the lowly flatworms, which have eye cups that contain light-sensitive pigment. They can detect the presence or absence of light. Most flatworms are nocturnal, they move away from light.

Mollusks developed true vision. Some species of snails, certain shellfish such as scallops, squid and octopuses all have eyes with lenses and sophisticated retinas. It’s rather startling to see a row of bright blue eyes peeking out from the edges of a scallop’s shell.

Spiders have true eyes, but no way to focus them. Spiders have to creep to the right distance from an object to get it in focus, rather like a “fixed focus” camera that can give a sharp image only at a certain distance from the object.

Insects developed compound eyes, thousands of individual lenses packed together, each connected to photoreceptor cells by tubes, like miniature versions of a telescope.

The human eye is among the best of the image-forming type; biologists call our eyes “camera-type.” Human eyes are shaped like a globe. At birth, a baby’s eyes will be a little more than half an inch across; an adult eye is usually about one inch in diameter, and weighs a quarter of an ounce.

Interestingly, the eyes of the largest animal on Earth, the blue whale, aren’t much bigger than our own. While an adult blue whale can be 90 feet long, its eyes are about six inches across. This appears to be good enough to gather in the light the whale needs to see by. The whale’s eye is much like our own, and why not? We’re both mammals.

Our sharp-focusing eyes didn’t appear out of nowhere, like a rabbit popping out of a hat. They are the result of billions of years of evolution, starting from the pigment spots on bacteria. Through it all, evolution has adapted the same kinds of light-sensitive pigments, the same type of focusing lens, to produce what seems truly miraculous: our sharp-focusing, full-color vision.

Much of this column was adapted from Ben Bova’s nonfiction book “The Story of Light.” Dr. Bova’s Web site address is www.benbova.net.