The Evolution of the Human Eye

When people want to attack Darwin's theory, they often point to the human eye. How could something so complex, they argue, have developed through random mutations and natural selection, even over millions of years?

If evolution occurs gradually, how could it have created the separate parts of the eye: the lens, the retina, the pupil..., since none of these structures by themselves would make vision possible? In other words, what good is part of an eye?

Darwin acknowledged from the start that the eye would be a difficult, but not impossible, case for his new theory to explain. Scientists have come up with possibilities through which the first eye-like structure, a light-sensitive pigmented spot on the skin, could have gone through changes to form the human eye, with its many parts and abilities.

Through natural selection, different types of eyes have emerged in evolutionary history, and the human eye isn't even the best one. Because blood vessels run across the surface of the retina instead of beneath it, it's easy for the vessels to increase in umbers or leak, and impair vision.

Biologists use the range of less complex light sensitive structures that exist in living species today to hypothesize the  evolutionary stages eyes may have gone through.

Some scientists think some eyes may have evolved by the  light-sensitive spot on the skin of some ancestral creature giving it some tiny survival advantage, perhaps allowing it to avoid a predator. Random changes then created a depression in the light-sensitive patch, a deepening pit that made "vision" a little sharper. At the same time, the pit's opening gradually narrowed, so light entered through a small aperture, like a pinhole camera.

Every change had to give a survival advantage, no matter how slight. Eventually, the light-sensitive spot evolved into a retina, the layer of cells and pigment at the back of the human eye. Over time a lens formed at the front of the eye. It could have evolved as a double-layered transparent tissue containing increasing amounts of liquid that gave it the convex curvature of the human eye.

In fact, eyes corresponding to every stage in this sequence have been found in existing living species. The existence of this range of less complex light-sensitive structures supports scientists' hypotheses about how complex eyes like ours could have evolved. The first animals with anything resembling an eye lived about 550 million years ago. And, according to one scientist's calculations, only 364,000 years would have been needed for a camera-like eye to evolve from a light-sensitive patch.