We do not typically think of eyes as adaptations, at least not in the same way we think of that term as it applies to the spines of a cactus or the toxin on a frog's skin. Perhaps we find it easier to identify the survival advantage through characteristics that are unique to relatively small groups of organisms. In the animal kingdom, eyes are the rule rather than the exception. And yet, the eye has evolved to its present state in much the same way that other adaptations have come into being—by a succession of small modifications over a long period of time. In fact, the evolution of eyes over the course of about 550 million years has provided survival advantages to organisms in countless numbers of different habitats.

The human eye is just one result of millions of years of eye evolution. As one of the most complex organs in the human body, it combines a number of specialized tissues and structures that enable us to process detailed information about our environment. Visual images enter the eye in the form of light. Light passes through the cornea, the clear outermost protective layer of the eye. From here, light travels through a clear fluid called the aqueous humor, then passes through the pupil, which controls the amount of light that reaches the back of the eye. Just behind the pupil is the lens, a clear flexible structure that changes shape to focus images on the retina, a layer of light-sensitive cells at the back of the eye. Two types of cells—rods and cones—make up the retina. Rods, which are more numerous than cones, are highly sensitive to light (but only to shades of bright or dark, rather than to color). Cones are sensitive to colors, but function only in bright light. The optic nerve transmits electrical impulses from these cells to the brain, which is ultimately responsible for interpreting the visual information that the eye receives.

Even among vertebrates, variations on this basic eye plan depend on the environments and habits of particular animals. For example, the eyes of nocturnal animals have evolved to maximize the amount of light they receive and their sensitivity to that light. Most nocturnal animals have eyes that are inordinately large relative to their body size; in addition, these creatures are able to dilate their pupils far wider than humans can. The retinas of nocturnal eyes are also equipped with a high percentage of light-sensitive rods. Although rods provide relatively poor visual acuity and do not allow for color vision, it is more important for nocturnal animals to see well in low-light conditions than it is for them to distinguish between colors.

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