not enough time to seriously respond to everything in here, and DrL. and wellCooked have done a great job of it anyway.
I do want to add a few things:
1) in reference to photo-sensitivity-
a)given that most atoms/subatomic particle will absorb some wavelength of EM raditation or another, I would say that all matter is photo-sensitive. To get from an atom to an eye requires a grouping of matter which is photosensative to a similar wavelength of light, so that the "entire piture" can be built.
b)if you plave lipids into a water solution, and agitate, there is a very high chance that a lipid bubble will form, based solely on the physical properties of the lipid molecule. The lipid molecules can be heated by EM radiation, in particular visable and ultraviolet light. If you shine a light source on a lipid bubble, it will move away form the light source. This is easily explain by the excitation of the molecules on the surface of the bubble closer to the light source, however, we now a a fairly clearly non-living thing which is photo-responsive.
c)it has been shown in people suffering from sun withdrawl (in places such as finland and Alaska, where the sun may not rise for months at a time) respond better to UV-producing blubs attached to the backs of their knees than to UV light shone on their faces.
d)Vitamin D is produced by our skin in responce to photonic activity w/in a certain wavelength range. Our skin is, therefore, photo-sensitive and photo-reactive.
2)in refernece to sex/pleasure
a)chickens will sit on golfballs, which are similar in shape, size and color to eggs. There is no benifit in sitting on golf balls when it comes to reproduction, so if there were a consious nature to the universe, I would expect such useless behavior to not occur. If the chicken recieves pleasure from sitting on the golfball, I could easily see it happeneing. It have been shown that the smae areas of the brain activated during the act of mating is also triggered when a female chicken sits on an egg.
b)wolfs copulate for a while, and then the male's penis becomes so enlarged that he cannot siwth draw for up to 30 minutes. I'd say that the "average time for copulation" of 8 seconds is being thrown off by the sheer number of baterial and insect species. There are more insect species then there are species of every other more complex organism on the planet. and insects utilize external fertilisation, so even if there is a pleasure involved with reproduction, it does not help production to stay sexually transfixed. In the wolf population, the 30 hour stuck-together period will prevent another male from displacing the sperm of the current reproducing male. Same deal w/ people humping for 3-10 hours at a time. The longer a man can sexually last, the longer his sperm have to reach the female's ovum.
c)why do people and animals have sex more often than is needed for reproduction. because of the same thing that makes people and animals want to reproduce in the first place- pleasure. you, over millions of years, evolve a more and more profound sense of pleasure while reproducing. over time, you also develop a social structure which allows more and more time for copulation. if you don't have to stop, and you get a whole lot of pleasure out of it, why stop at all? If sexual pleasure has become more pronouced through evolution, then the act of sex will generally occur more often. those who like sex tend to have it more often, and tend to reproduce better
and on a sort of side-note:
you have two elephants. one of them has a gene mutation which slows aging, and lives to be 200 years old. But during that time period, it never reproduces (for what ever reason, it's ugly, it's in a zoo, whatever). Another elephant has a mutation which makes it a sex fiend. It is ready to have sex at the drop of a hat. It however only lives to be 2 years old, so it only sires one offspring.
Which of these two elephants was more successful? from an individual standpoint, the 200-year old lived the longest. From a species standpoint, the 2 year old was the only one which passe don it's genes. So it's the more successful one.
What happens if the offspring of the 2 year old never reproduces, because the females don't like ebing hit on so much? The original 2-year old was still just as successfull (he produced one ofspring), however, his genetic mutation was not (his genetic mutation only survived one egeration beyond him).
There are so many factors which play into evolution and the survival of a given gene, that you can't really say "*this* is what caused sight" or "*this* is what cause the extinction of the blue footed big ape". it's a whole crapload of stuff, which, at times, will come together to form a new pattern which can be amazing.
I recommend reding up on Chaos theory to really get a handle on the slow and explosive nature of the combined effects of selective reproduction and random mutation.
in particular is the string and button idea- take 100 buttons, and lay them on the floor. none of them are currently connected. then tie two of them (selected randomly) together with a string. you now have one string, 100 buttons, and one connection linking two buttons. tie two more buttons together randomly. you have now either (100 buttons, 2 strings, and 2 connected buttons - if you happen to pick the same two again), (100 buttons, 2 strings, and 3 connected buttons - if one button is the same), or (100 buttons, 2 strings, and 4 connected buttons -if none of the buttons are the same.)
continue this process, and graph out the number of buttons each button is connected to (directly and then subsequently, in a generational method - c1, c2, c3, c4 levels), and you get, most oft he time, a slow progression. Then suddenly, over the course of adding two or three strings, the number explodes! you get most of the buttons connected to most of the other buttons in the group fairly suddenly, as groups of connected buttons are suddenly connected to each other. These are the steps, IMO, which took us from .0000% to .00001% sight.
It's an idea that happens all around us in nature all the time, but we rarely notice it. Even within slow systems, their can be (and usually are) instances where things come together just right, and you see a huge jump ahead.