Denys writes:

> I suspect this was Wirt's answers to my questions.
>
>  Let me just say that saying photoreceptive eyes have evolved 39 times
>  certainly does nothing to explain to me HOW they did so.

>  I really look forward to that explanation.

The explanation is simple, and enormously simpler than I suspect that you may
believe it to be. The basic answer is that light is pronouncedly active
chemical catalyst, particularly light at the blue end of the spectrum. Blue light
interacts especially well with the electrons in the outer shells of atoms,
altering their proclivities to forms specific bonds with other atoms and
liberating free charge carriers (a blue light photon contains approx. twice as much
energy as a red light photon). It's the reason that your beer bottles are made of
dark colored glass, as are your medicine bottles.

Bruce Toback sent you a private response to this question, which he copied
me. Bruce said all of the right things, so read his response again if you don't
mind.

The fundamental problem of biochemistry, and similarly semiconductor
electronics, is making a reaction *not* photoreactive. The reason that integrated
circuits and individual transistors are are packaged in either dark plastic or
metal containers is to keep light out. These same circuits, when exposed even to
very weak room light, respond completely differently than they do in the dark.

The difference between a memory chip and a CCD photoreceptor lies more in its
English description than its manufacture. The only real difference is whether
the chip has a clear plastic cover on its surface or not.

In both chips, a capacitor is charged. If the charge is above a certain level
in a memory chip, it's said to hold a "1" in its cell. If it's below that
threshold, it's a "0". As the size of the capacitor is made smaller, the electric
potential that the cell holds increasingly leaks from the capacitor due to
thermally liberated migrating minority charge carriers, thus almost all modern
high-density memory chips need an auxiliary circuit to come around every so
often, read the charge, and then refresh the "1"'s with new charge.

A CCD array works in exactly the same way, but now the charge is purposefully
leaked from the cell proportionately to the intensity of light to which it's
exposed. Pixels are made sensitive to the three human color receptors by
simply placing color filters over each of the three forms of cells. Following
exposure, the voltages are read from each bit cell and the assigned a binary value.
After that, the entire array is recharged and then reexposed for the next
picture.

Biological tissue acts no differently, and in a multicellular organism, the
outer tissue layers are going to be the most photoreactive. You tan because
your skin is producing a pigment, not unlike the dark coloring in beer bottles,
in response to an overexposure to light to protect your outer dermal layers
from photo-induced damaging reactions, so clearly your skin can already
primitively "see" light.

But a flat surface such as the skin on your arms can't tell direction. If
you're going to build an "eye," no matter how primitively implemented, the skin
must be either invaginated, as it was done in molluscs and vertebrates, or it
must be everted, as it was done in the evolution of the arthropod eye.

Exaggerating the photoreceptivity of the cells that would now line the
interior of the invaginated tissue seems no great trick, just as suppressing the
photoreceptivity of the outer dermal layers seems easy enough. Evolution works
exactly as any engineering process does. It doesn't create the initial
phenomenon, such as light sensitivity, but through incremental improvements, it much
exaggerates the positive qualities it wants to emphasize while minimizing those
negative qualities that do it harm.

The first eyes in both the molluscs and the vertebrates were likely simple
pinhole cameras. We know this for certain with the cephalopods. The Nautilus
still retains this form of eye:

    http://www.weichtiere.at/Mollusks/Kopffuesser/nautilus.html

although its relatively close relative, the octopus, possesses a lensed eye
every bit as elaborate and functional as the one you bear.

Indeed, we can find examples of the complete progression of the development
of the eye among living organisms now, ranging from simple pits to deep
invaginations with pupliary actions to fully formed lensed cameras. Nor is the
innervation of the tissue any great mystery. The nerves were already there, at the
skin surface. Prick yourself with a pin to verify that presence.

As I mentioned earlier, the fully formed, precision image-forming eye has
evolved three times independently, once each in the molluscs, arthropods and the
vertebrates. The last common ancestor of these three taxonomic groups occurred
at the very beginning of the Cambrian, 520-570 Ma. This is one piece of the
many pieces of overwhelming evidence that there is an inevitablity to the
evolutionary process, and that certain designs and structures are essentially
predestined by their value to their possessors. Although the eyes are built
differently, they serve enormously similar functional processes.

But the convergences don't end there. The extraordinary convergences don't
lie in the mechanical construction of the eyes themselves but in the information
processing structures that lie right behind the eye.

I'll hold that lecture for later.

Wirt Atmar

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