What are sodium channels?
Nerve cells have a long tail which carries an electronic impulse. The tail can be several feet long and its signal might stimulate a muscle to action, control a gland, or report a sensation to the brain.
Like the many telephone wires wrapped into a cable, nerve cells are often bundled together to form a nerve. Early researchers considered that perhaps the electronic impulse traveled along the nerve cell tail like electricity in a wire. But they soon realized that the signal in nerve cells is too weak to travel very far. The nerve cell would need to boost the signal along the way for it to travel along the tail.
After years of research it was discovered that the signal is boosted by membrane proteins. First, there is a membrane protein that simultaneously pumps potassium ions into the cell and sodium ions out of the cell. This sets up a chemical gradient across the membrane. There is more potassium inside the cell than outside, and there is more sodium outside than inside. Also, there are more negatively charged ions inside the cell so there is a voltage drop (50-100 millivolt) across the membrane.
In addition to the sodium-potassium pump, there are also sodium channels and potassium channels. These membrane proteins allow sodium and potassium, respectively, to pass through the membrane. They are normally closed, but when the electronic impulse travels along the nerve cell tail, it causes the sodium channels to quickly open. Sodium ions outside the cell then come streaming into the cell down the electro-chemical gradient. As a result the voltage drop is reversed and the decaying electronic impulse, which caused the sodium channels to open, is boosted as it continues on its way along the nerve cell tail.
When the voltage goes from negative to positive inside the cell, the sodium channels slowly close and the potassium channels open. Hence the sodium channels are open only momentarily and, now with the potassium channels open, the potassium ions concentrated inside the cell come streaming out down their electro-chemical gradient. As a result the original voltage drop is reestablished.
This process repeats itself until the impulse finally reaches the end of the nerve cell tail. Although we’ve left out many details, it should be obvious that the process depends on the intricate workings of the three membrane proteins. The sodium-potassium pump helps set up the electro-chemical gradient, the electronic impulse is strong enough to activate the sodium channel, and then the sodium and potassium channels open and close with precise timing.
Toxic to evolutionary theory
Sodium channels are a great target for a biological toxin such as tetrodotoxin. Introduce a compound that clogs the channel and nerves and muscles lose function. That brings on paralysis, respiratory failure, and even death. Tetrodotoxin wreaks its havoc by binding to the opening of the sodium channel.
But for all its lethality, tetrodotoxin can be neutralized with merely a few changes to the sodium channel’s amino acid string. In fact, even swapping in a single new amino acid can do the job.
Such minor changes are found in various species, including three garter snakes, Thamnophis atratus, Thamnophis couchii and Thamnophis sirtalis, as detailed in research published last year.
These minor changes are found in segments of the sodium channel gene which otherwise is highly conserved across a wide range of species. From the garter snake to humans, these segments are identical, or nearly so.
For evolutionists, such strong similarity across so many species suggests strong selection at work. That is, very little variation in the amino acid sequence can be tolerated. As the authors explain:
Amino acid sequences within the [sodium channel segment] are nearly invariant across garter snakes and relatives and are almost identical to mammalian sequences, suggesting the locus is under strong purifying selection because of its critical functional role.
But if so little variation can be tolerated, then how did the sodium channel evolve in the first place? Vague evolutionary speculation, such as here and here, of course does not address this awkward question.
Also, how could those three lucky garter snake species survive the few mutations that must have occurred according to evolution. In other words, with evolution we must be believe that one or a few mutations occurred in the sodium channel segment which apparently cannot tolerate such change. These mutations would have been handy when the snake eventually consumed a newt, but in the meantime the mutations should not have been tolerable according to evolution.
Finally, the evidence suggests the multiple mutations work together. Alone, some of the mutations have little affect on helping the snake resist the tetrodotoxin, but together the mutations have a tremendous effect. The weak mutations alone would have been less likely to have been selected and therefore, according to evolution, essentially simultaneous mutations are more likely to have occurred. But this dramatically reduces the likelihood of such an event occurring at all. Religion drives science, and it matters.