Eye Evolution

At least three times during evolution, eyes with lenses have developed independently in animals eyes as widely different as insects, mollusks, and vertebrates. Fish move the whole lens closer to the retina when focusing on distant objects. Mammals, including humans, have evolved a more complex method of focusing by changing the curvature of the lens, flattening it for close objects, thickening it for distant ones. Predatory birds have an effective strategy of keeping the prey in focus while sweeping down on it; instead of adjusting the lens, they quickly change cornea, which is a transparent membrane covering the lens and also supporting the eyeball.

Another essential refinement, color perception, also evolved independently several times. Among mammals only humans, primates, and a few other species can recognize colors clearly. Birds, on the other hand, have a color perception superior to that of humans. Among insects, honeybees can be trained to distinguish colors, but they are color blind to red. Similar training experiments have shown that at least some teleosts, or bony fish, can discriminate colors, but elasmobranches, such as sharks, cannot.

Finally, evolution resulted in the gradual, development of binocular vision, the shifting of the eyes position from the side of the head to the front; this permitted that fusion of the images in each separate eye into a single, three dimensional image in the brain.

Animals Eye

Almost all animals can perceive and respond to light, but eyes are as varied as the animals that posses them.

Eyes that form definite images are found only in some mollusks, mainly squids, octopuses and cuttlefish; in a few worm; in most arthropods, including insects, spiders, lobster, and most crabs; and in vertebrates. Except for most insects, these animals have eyes that are similar in structure and function to a camera. The eye uses a single Lens to focus a picture on a surface of densely packed cells called photoreceptors. The receptor surface, called the retina, functions like a piece of film. An external object is pictured on the retina like the points of newspaper photograph. The picture later received in the Brain, however is not the same simple point by point image. Exactly what this picture is remains unknown, but perception is a process that takes place in the brain not in the eye. Information from the eye, like the piece of a puzzle, is analyzed in the brain and fitted into meaningful forms.

Most insect eyes are built on an entirely different principle from the described above and are called compound eyes. Thousands of densely packed lenses are spread like a honeycomb over a spherical surface so that a mosaic image is formed. Each lens is associated with relatively few receptor cells, and the entire unit is called an ommatidium. No structure, therefore, is strictly analogous to the retina of other animals. What kind of image this arrangement conveys to the insect depends on the complexity of the structure.