|MadSci Network: Evolution|
The evolution of eyes has many fascinating components. Because light carries such a vast wealth of information about an organism's surroundings, the ability to sense changes in light can add greatly to an organism's fitness. Because it is so important, sight can be used to understand how evolution by natural selection works.
Before I describe the evolutionary principles behind your question, I might clarify one point: many animals do not have two eyes. Some animals, such as sponges have no eyes. Others, such as arthropods, can have many eyes (although insects usually have two compound eyes supplemented with more simple eyes). Also, the sightlessness of cave animals is one of the most interesting areas of evolutionary study. Yet your observation remains valid. Why do so many animals have two eyes? There are two evolutionary principals at work: convergent evolution and homology.
First let's discuss homology. Many animals have five fingers on each limb. Salamanders, armadillos and humans all have highly divergent types of fingers, but they each have exactly five. Losing a finger really doesn't lower the fitness of an organism, and upping the number to six wouldn't have a very big impact either, so why five? Quite simply, all the organisms with five fingers are decedents of the same five- fingered ancestor. Since the development of those five digits, the selective force on the number of digits has been inconsequential. Instead, different types of fingers were selected for: gripping fingers for the salamander, digging fingers for the armadillo, and grasping fingers for humans. The various types of fingers are said to be homologous (derived from the same ancestral source). (For more information on homology see another MAD Scientist's description of pentadactally.)
This is the same process at work in maintaining two eyes for the Chordates, which all have "camera eyes". This group includes all vertebrates such as fish, birds, amphibians, and mammals. (Camera eyes are eyes using lenses to focus an image.) These animals have two eyes because they retained the trait from a common ancestor. As in pentadactally, the eyes have been modified for different uses by different animals. Some of these eyes are specialized for seeing at night. Some see different colors, or only in black and white. (See this MAD Scientist's remarks about animals with color vision.) But the number of eyes within this monophyletic group (all derived from a single ancestor) is always two.
You mentioned that animals with differently shaped faces would not have the same uses for their eyes. In fact, the placement of the eyes on an animal's face is an excellent example of homology in eyes. As a general rule, animals that are subjected to considerable predation have one eye on either side of a narrow face, as in thin fish and rabbits. By maximizing the field of view, the animal can more quickly sense a predator coming for it. Animals with eyes placed side by side on the front of the face are predators. By overlapping the fields of view of the two eyes, depth perception is obtained. This is of great benefit when hunting down a running prey or judging the distance before jumping out and ambushing the prey.
Why don't we evolve a third eye on the back of our heads now? There are several reasons. First, complex eyes like we have are very energetically expensive for an organism. Not only is there the physical eye to grow and maintain, but a substantial portion of the brain has to be devoted to interpreting the incoming signals. It's much more efficient to simply rotate your head than to grow and maintain a whole new eye. The other factors preventing the evolution of a new eye in Vertebrates involves constraints of evolution. Another MAD Scientist has written in more detail on the requirements for evolving new eyes.
So among the Chordates, we have two eyes because we are all descendents of a single ancestor with two eyes. But why did our common ancestor evolve exactly two eyes? The reasons for evolution of binocular vision in the ancestor of the vertebrates are the same reasons for evolution of two eyes in invertebrates.
The eyes of invertebrates are not genetically related to the eyes of vertebrates. In fact, the different eyes did not even evolve from the same kinds of cellular structures. When different organisms evolve similar characteristics based on similar selective forces, it is called convergent evolution. While arthropods can have many additional simple eyes (ocelli), many have two, more highly developed compound eyes. Why do they have two?
Having two eyes lets you compare light coming at you from two different directions. Even when the fields of view don't over-lapped, comparison of left and right can orient an organism. The animal can direct itself directly towards light by turning until both eyes are receiving equal, maximal amounts of light stimulation. Likewise, an animal can direct itself away from light by turning until both eyes are receiving an equal, minimal amount of light stimulation. This can be particularly important for animals that need to stay out of the light, either because they can easily dry out in the sun, or because they want to hide from predators. For such orientation, only two eyes are needed.
As a point of interest, development of camera eyes similar to those of vertebrates has occurred within a group of invertebrate mollusks called the Cephalopods. Cephalopods include octopuses and squid. Development of the camera eye two separate times is one of the greatest examples of convergent evolution.
In summary, animals that have two eyes have them because of two reasons. All vertebrates with eyes have two because the common ancestor of vertebrates had two eyes. That common ancestor had two eyes for the same reasons that invertebrates with eyes tend to have only two complex eyes: two eyes are better than one because they allow comparison of light coming from two directions, while a third eye doesn't give enough extra information to make it worth the costs of having it.
If you're confused about how the different groups of animals all fit together, I highly recommend visiting the Tree of Life's page on Metazoa (animals).
Thank you for your excellent question!
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