This is not related to ID theory. Just needed to understand what are the theories available to explain evolution of Eyes. "Darwin acknowledged from the start that the eye would be a difficult case for his new theory to explain. Difficult, but not impossible. Scientists have come up with scenarios through which the first eye-like structure, a light-sensitive pigmented spot on the skin, could have gone through changes and complexities to form the human eye, with its many parts and astounding abilities. Through natural selection, different types of eyes have emerged in evolutionary history -- and the human eye isn't even the best one, from some standpoints. Because blood vessels run across the surface of the retina instead of beneath it, it's easy for the vessels to proliferate or leak and impair vision. So, the evolution theorists say, the anti-evolution argument that life was created by an "intelligent designer" doesn't hold water: If God or some other omnipotent force was responsible for the human eye, it was something of a botched design. Bilogists use the range of less complex light sensitive structures that exist in living species today to hypothesize the various evolutionary stages eyes may have gone through. Here's how some scientists think some eyes may have evolved: The simple light-sensitive spot on the skin of some ancestral creature gave it some tiny survival advantage, perhaps allowing it to evade a predator. Random changes then created a depression in the light-sensitive patch, a deepening pit that made "vision" a little sharper. At the same time, the pit's opening gradually narrowed, so light entered through a small aperture, like a pinhole camera. Every change had to confer a survival advantage, no matter how slight. Eventually, the light-sensitive spot evolved into a retina, the layer of cells and pigment at the back of the human eye. Over time a lens formed at the front of the eye. It could have arisen as a double-layered transparent tissue containing increasing amounts of liquid that gave it the convex curvature of the human eye. In fact, eyes corresponding to every stage in this sequence have been found in existing living species. The existence of this range of less complex light-sensitive structures supports scientists' hypotheses about how complex eyes like ours could evolve. The first animals with anything resembling an eye lived about 550 million years ago. And, according to one scientist's calculations, only 364,000 years would have been needed for a camera-like eye to evolve from a light-sensitive patch." - http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html The flawed design argument seems to reject ID theory but it does not give any scientific evidence for evolution of eyes from light sensitive skin patches. or it Does.? if so, just want to know more of it. It would help other non-biologists to understand evolution in better light. Thanks in advance.
eyes didn't evolve only once! complex eyes evolved, completely independently, at least twice!!! in mammals and molluscs (octopods and squid)
Interestingly enough Cubozoa (box jellyfish, also called Cubomedusa) have a retina with a crude lens. The lens even contains similar structural proteins to the vertebrate eye, both seemed to have evolved seperately from the same class of proteins that are common in other biochemical pathways.
So, what would be the first stage of sight organs? Something like rudimentary eye (though even if they are similar, we shouldn't call it rudimentary eye for it means that it's out of use)?
That is fair enough. But i've difficulty in undertanding the appearance of lens to focus the light rays on to retina. Focussing could be done in 2 ways (as far as i know) either (i) by diffraction of light at the edge of small apertures or (ii) refraction of light rays through lens like transperant denser body. However i understand development of retina from light-sensitive spots. Appearance of lens confuses me, atleast. Is it accidental ? or the need for the lens induced ? (in that case, i understand induction works mostly to better the existing organs to morph to the best advantage, is it not.? lens comes from which organ.? if it is new addition then how evolution process knows about physics phenomena? and so forth. ) Rest in next. Thanks.
OK, Lets skip to the stage, after formation of Lens : "Could we use the composition of lenses to gain insight into eye evolution? Vertebrate lenses are formed from modified epithelial cells that contain high concentrations of soluble proteins known as crystallins because they are packed in a highly organized fashion. It is the change in relative concentration of these proteins from the periphery to the center of vertebrate lenses that produces the refractive index gradient necessary for a lens to be useful to the animal. In fact, the identity of the proteins seems not to be important since the crystallin proteins are not more transparent than others. Instead, the distribution of protein concentration as a function of radius is the key to a successful lens. Thus, the challenge in understanding lens evolution lies in discovering how the distribution of proteins within a lens is established and maintained." "Why might enzymes be recruited to make vertebrate lenses? Perhaps the robust regulation of enzyme production is advantageous for producing sufficient protein for a lens, but there is not much beyond speculation to support this notion. There may be some deeper reason, however, because this molecular opportunism seemed such a good idea, that certain invertebrates, e.g. mollusks, independently evolved the same strategy [8]. Squids have lenses whose protein content is nearly entirely the enzyme glutathione S-transferase. The common strategy of constructing lenses from different proteins seems to be a convergent evolutionary solution. This convergence of molecular strategy suggests that enzymes as lenses may have a functional meaning, or that it is easy to get lens cells to make a lot of enzyme, or there may be other as yet not understood reasons." - Evolution of Eyes, Russell D Fernald - http://www.karger.com/gazette/64/fernald/art_1_4.htm
A pinhole eye would be useful in localizing light sources whether or not there was a lens. It may be that the "light sensitive patch" slowly became indented into the organism's surface to allow better light localization, and that the skin of the organism eventually closed mostly over it to provide a pinhole for the greatest degree of accurace for localization. Once the pinhole model has developed, it's not quite as hard to picture how a lens could have developed - first a transparent covering would develop to keep stuff out of the "eye" cavity, and thereafter mutation might provide helpful refractive properties, which would be selected for. Or, the protective covering for the eye could have come about without the pinhole structure, and gained better refractive properties the same way.
In fact, though the pin hole helps focussing on the 'light sensitive patch', it is not as accurate as a little wider aperture with a lens like structure. The refractive properties could be achieved only by the proper density distribution of transperant proteins in the lens like structure. From the exctracts included in my previeous post, i understand that it is yet to be understood how this distribution is established and maintained. Ofcourse, mutations could be brought in, but we have also to consider that the development of eyes with lens like structure evolved several times independantly in different species at differnt places at different times. Seems a strange coincedence to me as a non-biologist. Perhaps you guys might be able to shed more lights on this. Thanks for your response.
