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27 November 2010

Chickens Have Cellular Sunglasses


When light enters your eye it triggers a sequence of actions, ultimately causing a signal to be sent to your brain. Even a mere single photon can be detected in your vision system. It all starts with a photon interacting with a light-sensitive chromophore molecule. The interaction causes the chromophore to change configuration and this, in turn, influences the large, trans-membrane rhodopsin protein to which the chromophore is attached. This is just the beginning of the cellular signal transduction cascade. But before any of this begins, in some species the incoming light has already been filtered and focused.

The chromophore photoisomerization is the beginning of a remarkable cascade that causes action potentials to be triggered in the optic nerve. In response to the chromophore photoisomerization, the rhodopsin causes the activation of hundreds of transducin molecules. These, in turn, cause the activation of cGMP phosphodiesterase (by removing its inhibitory subunit), an enzyme that degrades the cyclic nucleotide, cGMP.

A single photon can result in the activation of hundreds of transducins, leading to the degradation of hundreds of thousands of cGMP molecules. cGMP molecules serve to open non selective ion channels in the membrane, so reduction in cGMP concentration serves to close these channels. This means that millions of sodium ions per second are shut out of the cell, causing a voltage change across the membrane. This hyperpolarization of the cell membrane causes a reduction in the release of neurotransmitter, the chemical that interacts with the nearby nerve cell, in the synaptic region of the cell. This reduction in neurotransmitter release ultimately causes an action potential to arise in the nerve cell.

All this because a single photon entered into the fray. In short order, this light signal is converted into a structural signal, more structural signals, a chemical concentration signal, back to a structural signal, and then back to a chemical concentration signal leading to a voltage signal which then leads back to a chemical concentration signal. There is, of course, a wealth of yet more detail which makes the information conversion process far more complicated.

Optical oil drops

And there is a seemingly endless stream of variations on this cascade. One fascinating variation is the use of special oil to filter and focus the incoming light before it ever reaches the first step of exciting the light-sensitive chromophore molecule. In chickens, for example, these brightly colored oil droplets seem to improve color discrimination as they focus the light toward the target chromophore molecules.

And these droplets are not just any kind of oil. The oil and its optical properties are finely tuned and specific to the different types of light sensitive cells. In fact the oil provides a convenient method for determining the cell type, which allowed researchers to map the optimal pattern the different cell types fall into. It is all part of what one writer called “a masterpiece of biological design.”

Evolutionists describe this as a consequence of evolutionary pressure:

Our results indicate that the evolutionary pressures that gave rise to the avian retina’s various adaptations for enhanced color discrimination also acted to fine-tune its spatial sampling of color and luminance.

Evolutionary, or selective, pressure is an internal contradiction in evolutionary thought. One can hardly blame evolutionists for their use of such action themes. It sounds better than the just-add-water account which holds that random biological variation produced nature’s marvels. But in fact there can be no such pressure in evolution’s the-world-is-a-fluke hypothesis.

The whole idea in evolutionary theory is that random mutations are, well, random. Biological variation is random with respect to need. There can be no “pressure” to produce any smart designs. And the evolutionist’s hole card, natural selection, doesn’t help. It only kills off the failures (of which there must have been a great many). Every mutation, one after the other, leading to nature’s gems such as the fine-tuned oil droplet in light sensitive cells, must have occurred for no reason. Only after they occurred could natural selection appreciate the brilliance of these random events. And that’s a fact. Religion drives science, and it matters.

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