Google| Subject | Yet *another* evolution thread; was: Queen mother (of Britain) has died |
| From | Raven |
| Date | 2002-04-10 20:51 (2002-04-10 20:51) |
| Message-ID | <%53t8.1260$Rd4.3889@news.get2net.dk> |
| Client | |
| Newsgroups | alt.fan.tolkien |
| Follows | Joy |
| Followups | rand mair fheal (8h & 58m) > Raven |
"Joy" <queen_yoj@hotmail.com>skrev i en meddelelse news:29ff3ad6.0204091002.a6dacc8@posting.google.com...
JoyI believe that what Darwin admitted was that if no intermediate steps between no-eye and the very complex mammalian eye could be found or construed, his theory of evolution would have a problem. But as it is, such an evolutionary path can be easily imagined. Starfish have no real eyes. What they do have is a light-sensitive patch at the tip of each arm. These "eyes" are of course of very limited abilities, but for a slow animal such as a starfish, it is of some use to know eg. if some of its arms are in shadow while others are in sunlight, or if the light level changes swiftly at one side of it. So imagine an early chordate - the chordates include fish and certain primitive, fish-like animals, amphibians, reptiles, birds and mammals. Chordates are animals with a dorsal chord: they are elongated and have a thick nerve - the dorsal chord - running along the body. Your dorsal chord is inside your spine, and if it is damaged, you'll die or become lame in your lower body. A chance mutation makes our primitive chordate able to produce a light-sensitive protein, similar to but much simpler than our opsine. This chemical interacts with such rudiments of a nervous system as the animal has, and it changes its behaviour according to whether it is in sunlight, shadow or dark of night. This is not a very large evolutionary step: the chemical would change its form when struck by sunlight, just as rhodopsine does, and there would be a chemical change of some nature within the nervous system of the animal. That would have to have some effect. Also it changes behaviour if there is a sudden change in lighting: a change in lighting triggers some action, or cessation of action. Some of these actions are harmful, and swiftly die out. Others are beneficial, eg. that a sudden darkening causes the animal to wriggle more vigorously - this automatic response would enable the animal to escape predation more easily, and there would be a strong selection pressure for both the ability to discern between light and shadow and for that particular automatic response. Within few generations of these primitive animals "eyesight" is an established ability. Now imagine that another mutation causes the light-sensitive protein to be produced primarily in one end of the animal. Now the animal has a vague sense of where the light comes from, just as I have when I close my eyes. This permits the animal to change its course when a shadow suddenly appears before it, and while its more primitive brethren may wriggle right into the claws of an approaching predator, the New and Improved (tm) animal can consistently swim away from danger. And in a short time all animals of this line have the rudiments of a *localized* eye. Since proteins require food to be produced, there will be a selection pressure towards concentrating the light-sensitive protein in one or a few small patches, rather than all over the skin. If the animal is already right-left symmetric, it will come natural to have two light-sensitive patches of skin, one on each side of the body, rather than one patch on eg. the left side. This also permits the animal even more accurately to discern where a change in lighting occurs. These light-sensitive patches are of course more easily harmed than the rest of the animal's skin. So a mutation that causes the proto-eyes to sit in little hollows in the skin will be beneficial. But once this hollow is deep enough, the proto-eye is somewhat occluded by the skin around it: light that comes towards the eye at an oblique angle will only hit part of the light-sensitive layer. Now the eye can form crude images. Soon all the animals of this line will have "eyes" retracted somewhat in the skin. Another way to protect the proto-retina from damage is to cover it with transparent skin. Once a mutation enables this, this mutated gene will multiply. The deeper the retina is retracted into the proto-eye socket, the more detailed images can the eye discern. But it is helpful if the cavity that this leaves between the transparent skin and the retina is filled with some transparent material. Now the eye has a real eyeball. The transparent skin that was originally a protection for the sensitive retina becomes more specialized: it becomes a lens that can focus light. Muscles, first just the general muscles of the skin, later specialized ones, permit the lens to change shape, and therefore change focus, such as from near to far. Since the animal now has a fairly advanced pair of eyes, it becomes beneficial if these eyes and the mouth are fairly close, so that the animal may eat more efficiently. For an elongated animal the best place for the mouth and eyes is in one end rather than near the middle. So if mouth and eyes were at this stage not already near one end, they "move" towards it, and this end becomes the forward end and the head.
Looking at a building, I don't need to be able to touch/smell/see an architect to know that there was one. My eyeball is infinitely more complex, and Darwin himself admitted that the formation of the eye through natural selection seemed highly absurd. (Something like "If you didn't have photosensitive cells, how would you know about light? If you didn't know about light, why would you develop photosensitive cells?")
This is not necessarily how the chordate eye was developed. Indeed it is probably not. But this outlined scenario provides an existence proof of a path leading in small steps, each of which beneficial, from total inability to sense light to the eyes of an attacking barracuda --- and my own eyes reviewing what I've just written.
Jon L. Beck.
2002-03-30-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
