Eyes
Humans often put most emphasis on sight, since it is arguably the most important of our senses. Many other animals rely heavily on sight. For living in an action-packed light-filled world, sight is very useful since there is nothing faster than light.
Previously it was mentioned that eyes have evolved on earth many times. This tells us that eyes are a very good invention and not so difficult to build or evolve. There is an awful lot of electromagnetic radiation (light) flying oround our universe. It is "relatively" safe, cheap, consistent and abundent. What's more, eyes are passive, meaning that you don't need to emit anything that might give you away when stalking prey or trying to hide.
The intricate workings of the eyes are often cited by "antievolutionists" as an example of proof of intelligent design. Their argument says that the function of the eye is totally dependent on the existence of many specialised structures. Take one away and the eye will cease to be useful. "Half an eye is not an eye," as they say. Since evolution proceeds in small random steps and requires everything to work throughout the lineage, something as complex as the eye could not have come about by chance alone. However, this line of argument is completely wrong and displays a fundamental misunderstanding of evolution.
Well, half an eye is an eye. Take out the lense from our eyes and although the image would be blurred we could still tell where safe shadows were, and see the direction of large, fast-moving blobs to avoid. Stretch the retina onto the surface of the skin and we lose the ability to tell direction. At least we can still tell the difference between light and dark, safety and danger and with two eyes we still have some kind of direction information.
If we look at the animal kingdom, we see many eyes, in all stages. There are advanced lensed eyes like you find in vertebrates and cephalopods and sophisticated compound eyes of insects. At the lower end, some bacteria display phototaxis (heading towards light) and worms have a very basic ability to tell the difference between light and dark, hardly what you would call eyes but at least they have the necessary first steps: light sensitive cells and information processing. There are also a range of eyes in between, such as those of the flatworm, which are more like simple pits of light sensitive cells. It is probably a eye like this that our early ancestors started out with, gradually deepening the pit and refining the structure to increase directionality, resolution, sensitivity, exposure control, focus and many other functions.
Not only are there many types of eyes, but they also come in all shapes and size and a wide variety of abilities. The largest eyes on earth belong to the giant squid and can measure over 10 inches in diameter. Four-eyed fish have eyes split into two, the top for focussing out of water and the bottom calibrated for looking through it. The mantis shrimp eye has ten colour channels, in contrast with our meagre three. Some birds have an extra fovea, giving them an additional zoomed image. Goldfish have and extended sensitivity to light wavelengths, they can see infrared and ultraviolet. Some spiders move their retinas around inside their head instead of swivelling their eyes. There are many more examples of extraordinary eyes.
Eyes on earth usually come in pairs. This is mainly because of the ubiquitous nature of bilateral symmetry. However, having eyes in pairs allows for binocular vision, which allows fairly accurate perception of depth, based on two images. A very useful ability. Having a single eye makes this almost impossible. Also, having eyes set in a "head" close to the "brain" makes the distance the singals have to travel shorter, thereby speeding up response time.
Most eyes focus light through a small hole and perhaps a lens to project an image of the world onto a sheet of light sensitive cells, which is then fed into the central nervous system. Biology is very good at creating rounded structures, which is perfect when it comes to making eyes. Thus it is not unreasonable to imagine aliens to have eyes that are similar to those found on earth. Since eyes are delicate they need to have protection like eyelids, it is a good idea to be able to move them, either swivelling around inside or waving on the end of stalks.
This is just a collection of facts and thoughts on the nature of eyes in the animal kingdom. I would certainly expect eyes to have evolved along similar lines on other planets. They may vary in shape, structure, size and sensitivity, but at a very basic level, there will be remarkable similarities.
Previously it was mentioned that eyes have evolved on earth many times. This tells us that eyes are a very good invention and not so difficult to build or evolve. There is an awful lot of electromagnetic radiation (light) flying oround our universe. It is "relatively" safe, cheap, consistent and abundent. What's more, eyes are passive, meaning that you don't need to emit anything that might give you away when stalking prey or trying to hide.
The intricate workings of the eyes are often cited by "antievolutionists" as an example of proof of intelligent design. Their argument says that the function of the eye is totally dependent on the existence of many specialised structures. Take one away and the eye will cease to be useful. "Half an eye is not an eye," as they say. Since evolution proceeds in small random steps and requires everything to work throughout the lineage, something as complex as the eye could not have come about by chance alone. However, this line of argument is completely wrong and displays a fundamental misunderstanding of evolution.
Well, half an eye is an eye. Take out the lense from our eyes and although the image would be blurred we could still tell where safe shadows were, and see the direction of large, fast-moving blobs to avoid. Stretch the retina onto the surface of the skin and we lose the ability to tell direction. At least we can still tell the difference between light and dark, safety and danger and with two eyes we still have some kind of direction information.
If we look at the animal kingdom, we see many eyes, in all stages. There are advanced lensed eyes like you find in vertebrates and cephalopods and sophisticated compound eyes of insects. At the lower end, some bacteria display phototaxis (heading towards light) and worms have a very basic ability to tell the difference between light and dark, hardly what you would call eyes but at least they have the necessary first steps: light sensitive cells and information processing. There are also a range of eyes in between, such as those of the flatworm, which are more like simple pits of light sensitive cells. It is probably a eye like this that our early ancestors started out with, gradually deepening the pit and refining the structure to increase directionality, resolution, sensitivity, exposure control, focus and many other functions.
Not only are there many types of eyes, but they also come in all shapes and size and a wide variety of abilities. The largest eyes on earth belong to the giant squid and can measure over 10 inches in diameter. Four-eyed fish have eyes split into two, the top for focussing out of water and the bottom calibrated for looking through it. The mantis shrimp eye has ten colour channels, in contrast with our meagre three. Some birds have an extra fovea, giving them an additional zoomed image. Goldfish have and extended sensitivity to light wavelengths, they can see infrared and ultraviolet. Some spiders move their retinas around inside their head instead of swivelling their eyes. There are many more examples of extraordinary eyes.
Eyes on earth usually come in pairs. This is mainly because of the ubiquitous nature of bilateral symmetry. However, having eyes in pairs allows for binocular vision, which allows fairly accurate perception of depth, based on two images. A very useful ability. Having a single eye makes this almost impossible. Also, having eyes set in a "head" close to the "brain" makes the distance the singals have to travel shorter, thereby speeding up response time.
Most eyes focus light through a small hole and perhaps a lens to project an image of the world onto a sheet of light sensitive cells, which is then fed into the central nervous system. Biology is very good at creating rounded structures, which is perfect when it comes to making eyes. Thus it is not unreasonable to imagine aliens to have eyes that are similar to those found on earth. Since eyes are delicate they need to have protection like eyelids, it is a good idea to be able to move them, either swivelling around inside or waving on the end of stalks.
This is just a collection of facts and thoughts on the nature of eyes in the animal kingdom. I would certainly expect eyes to have evolved along similar lines on other planets. They may vary in shape, structure, size and sensitivity, but at a very basic level, there will be remarkable similarities.
posted by Daley Paley at 10:15 PM
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