Saturday, September 2, 2017

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.

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