Originally Posted by anthonyb

This is the fundamental misconception - that sex organs can't function until they are constructed in the fashion that we see today, or that poison delivery systems cannot function until they exist the way we see them in rattlesnakes, or that the eye cannot function until it is formed in the way we see it in the human form today.

The reason why this is a misconception is because you don't need sex organs to derive a benefit from sexual reproduction, you don't need hollow teeth before you can derive a benefit from excreting poison, and you don't need a human eye before you can derive benefit from sensing light.

Let's take the human eye for an example. It stands to reason that organisms that can visually perceive the environment around them would have a survival advantage compared to organisms that cannot. A number of simple, multi-cellular organisms have photoreceptors that can sense light and shadow. Not suprisingly, plants (which rely on photosynthesis) are also capable of sensing and tracking sunlight, despite the fact that they have no obvious "eyes."

It turns out the flatworms have optic "cups," with photoreceptors in the base of the cup that allow them to sense a directional component to light. Better than sensing just the presence of light or dark. The deeper that cup gets, the better the direction finding, and if the entrance of the cup narrows to a small circle (aka the "pupil," or in cameras the "iris" or "pinhole,"), you get shape perception. The smaller the pupil, the sharper the shape.

If you close off the front of the cup with a thin, transparent layer of skin cells, you provide protection to the light-sensing components AND you now have a substrate for the development of a mechanism to focus the light, etc. etc. etc.

A couple of interesting points about the eye - the human eye developed as an outpouching of the brain, and when we look at the back of the eye (as in your annual eye exam) we can actually detect abnormally high intracranial pressures because of that direct connection. The nerve fibers also stack on TOP of the light sensing cells (light from the outside has to pass through the nerve cells before it hits the photoreceptors). Mollusks, on the other hand, have a very structurally similar eye, but they evolved differently, and the nerve cells stack on the bottom of the eye. As a result, they have no blind spot (where the optic nerve enters the eye in humans).

Supposedly, the eye has been thought to have evolved independently about 50-100 times (according to wikipedia). I have no idea how accurate this is, but it makes sense in light of the immense survival advantage that vision plays in the environment.