evolution, why do most things have 2 eyes? why didnt more eyes evolve?

Question

spiders, potatoes, ...what else....everytone else has 2 eyes. bugs, birds, fishes, us,....everyone i can think of. with the nature werld so rough & everybody constantly on high alert, why arent there more eyes? is it the capacity of the brain?

Answer 1

Your question is really about something called "bilateral symmetry" ... the fact that if you draw a line down the middle, our left and right sides are somewhat mirror images. (I say "somewhat" because this is true only in the overall body plan ... not true in the details.)

Two eyes, two nostrils, two ears, two symmetrical arms, two symmetrical legs and feet, two lungs, two kidneys ... even two hemispheres in the brain. It's all part of the same bilateral symmetry.

But meanwhile we have one nose, one mouth, one heart, one liver, one stomach/digestive tube, one spinal cord, and a lot of other singlet structures down the middle.

This is a basic body plan that is used by *all* vertebrates (mammals, birds, reptiles, amphibians, fish), and in fact, a lot of invertebrates too (like insects, spiders, and other arthropods). This is evidence that (a) all these organisms evolved from the same primitive ancestor with this bilateral body plan; and (b) this is extremely useful.

Symmetry is very useful in general. If you think of every organism as an embryo that grew up ... then this makes more sense. Every multicelled organism (every animal, every plant, every mushroom) is the result of a genetic "program" that has to build an embryo, which then develops into an adult organism. Evolution is the accumulation of changes to that "program" ... so that the embryo develops into something that has survival advantages.

In the simplest sense, an organism can just be totally symmetrical in all directions ... a sphere. But as this does not provide any opening for getting nutrients in, and waste products out, of the internal cells, only very small multicellular organisms can be truly symmetrical (so that the inner cells are close enough to the surface to exchange nutrients and waste). An example of this is a spherical organism called volvox.

So the next stage in evolution is what we call "radial symmetry" ... the embryo developed a single opening which allowed food and waste to be exchanged with the water. This body plan is still used by things like starfish, or sea urchins, or jellies ... and if they have senses, such as eyes or eyespots, these are usually distributed evenly around the central opening.

The next stage is still radial symmetry, but instead of a single opening, there is a tube with a separate mouth and anus. This provides the advantage of keeping waste products away from the mouth opening. The other thing that starts to develop is that sensory organs start to develop around the mouth region (for obvious advantageous reasons), and where there are sense organs, there are also the beginnings of concentrated nervous systems (the beginnings of a brain). However, the organism is still symmetrical in all directions around the tube. Modern worms still use this body plan.

The next development is what we call "dorsal-ventral asymmetry" ... once these tubes start to crawl around on the ocean floor, or swim in the ocean, they start to develop an "up" side (dorsal) that faces the surface, and a "down" side (ventral) that faces the floor. It may be colored differently to fool predators or prey coming from either above or below, or the up side may be tougher, while the down side is where the apendages are for movement (like fins). So there are advantages to dorsal-ventral (upside- downside) asymmetry. This is the body plan used by arthopods (insects, spiders, lobsters), fish, amphibians, and carried through to reptiles, dinosaurs, birds, and (in a separate branch) mammals, including humans (although once we stood upright ... our dorsal-ventral sides were no longer our "upside-downside", but became our "back-front").

(Incidentally, this dorsal-ventral asymmetry is mostly only useful for organisms that move around ... which is why is not as common in plants.)

However, while dorsal-ventral asymmetry is useful ... left-right asymmetry is not particularly useful. A predator is no more likely to come from the left than from the right. So few embryos have ever found much advantage to add that kind of left-right asymmetry of body plan to the "program." Of course, that central tube has gotten more and more complex (for example, developing an asymmetrical widening at one part of the tube that we call the "stomach", as the holding zone for food, so that the organism can eat food where it finds it, and then digest it later in a safer location). And other centralized organs (heart, liver, spleen) have gotten asymetrical in order to fit inside the cramped insides.

But the overall body plan (the early stages of the embryonic development) has not changed much. We are all basically just tubes with left-right symmetry, but mouth-anus assymetry, and dorsal-ventral asymmetry.

The bilateral symmetry of eyes is just a side-effect of this basic body plan. Two eyes has a huge advantage over one eye (e.g. it allows for better tracking of the *distance* to prey or predators) ... but three eyes would require a *massive* reorganization of our bilaterally symetrical nervous system.

Incidentally, this pretty much reflects the way all organisms are classified, and this sequence (sphere->tube->dorsal/ventral) reflects the early stages of *all* vertebrate embryo development, from fish to fishermen.

Answer 2

Science is a method; religion is a conclusion. When read literally, religion almost always contradicts reality. When read metaphorically, it's pretty much just pointless. What's the metaphor of there being a dome over the Earth which rain falls through.. of illness being caused by demons and curses.. of stars being tiny objects that can "fall" to the Earth.. etc? If these are metaphors, they're very strange metaphors, and they're not helpful. Another problem is that nowhere in the Bible (the source of religion in your examples) does it say where the metaphor starts or ends, or what part is literal. Although I've gotta completely agree that if a God exists, than he created nature, and therefore studying nature can only let us learn about God.. God's creation can't "contradict" God. The creationists are arguing for a deceptive God. The problem with getting rid of all the factual claims in religion is that there's not much left after that. It can be "true" in the same way that a Spider Man comic is "true". Sure there are some nice quotes in the Bible, but same with many other religions, and many non-religous writings. There are also some horrible immoral things. But as long as we're using our judgment to say what parts of the Bible are factually true (science), and using our moral judgments to cherry pick the parts that are moral, it's not much of a "source" for anything.

Answer 3

Actually, several animal lineages evolved more than one pair of eyes. Seastars have five simple eyes, box jellyfish have 24 eyes and there are types of clams with more than a hundred eyes. Spiders - which you mentioned - still have up to eight eyes, but those with good vision usually have one pair that is a lot bigger than the others. An extinct creature called Opabinia had five eyes, being the only known animal with an uneven number of eyes.

What is common for these creatures is that their eyes are fairly primitive, and vision quite poor if compared with our eyes. This might hint that processing power of the brain is involved. The better the eye, the more visual infromation for the brain to process. But there's probably more involved, since a lot of animals ended up with two eyes before any of them had good vision.

Evolution doesn't waste resources. If something can be done "cheaper", it usually will be. This might be why two eyes became the norm. You can get enough visual field with two eyes for the animal to be able to avoid predators or find prey. Two is also the minimum for stereoscopic vision. If having four eyes would not confer a benefit that would outweigh the resource cost of building two extra eyes and the extra brain tissue for dealing with the visual information, they wouldn't get selected for.

This is what I believe what determined the number of eyes in most of animals. The cost-effectiveness of two eyes is just so much better than having more eyes.

Answer 4

evolution seems to want only a pair of eyes in humans and other folks in our world. Potatoes eyes are for multiplying. Hey.if our stable eyes were used for that purpose it would be interesting . Write a theses on that.

Answer 5

10 points to tjinuski. She's spot on.