Wednesday, March 2, 2011

Unexpected Role for Ciliary Photoreceptor Cells in a Brachiopod

New research has discovered ciliary photoreceptor cells providing directional light detection in the brachiopod Terebratalia transversa. This unexpected finding further complicates the already circuitous evolutionary narrative of the origins of vision at the cellular level.

Background

At the molecular level vision begins with a complex signal transduction cascade. As photons enter your eye they interact with light-sensitive chromophore molecules in the photoreceptor cells. 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.

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.

This is the beginning of vision. And while there are variations on this remarkable sequence, it is found throughout the wide variety of vision systems found in biology. It is even found in the relatively simple, non image forming, third-eye. You can read more about this here.

Phylogenetic duplicity


Because this signal transduction cascade is so widespread, evolutionists must envision it to be present in the earliest organisms with vision capabilities. This need for evolution to have created unimaginable complexity early on is a consistent theme in evolutionary theory. Over and over, the fascinating designs found in biology must have, according to evolution, appeared early on, even before any need for such marvels.

For vision, this theme of early complexity is repeated at the cellular level, where two distinct photoreceptor cell morphologies—rhabdomeric and ciliary—are found. These two morphologies have different membrane folding strategies as well as biochemical pathways. But their widespread presence in organisms forces evolutionists to conclude they both must have been present in the last common bilaterian ancestor. Rhabdomeric photoreceptor cells are often associated with invertebrates and ciliary photoreceptor cells with vertebrates, but both invertebrates and vertebrates have cells with both morphologies.

And so both morphologies must trace back to that last common bilaterian ancestor. While it may stretch common sense for early evolution to create such complexity in duplicate, if you believe it can perform the feat once, then why not twice?

But there is more to the story. In invertebrates the ciliary morphology plays a lesser role. It is not found to provide directional light detection, but in more rudimentary light detection roles. So how then does it emerge as the chief architecture in vertebrate vision systems? The evolutionary narrative calls for a migration of the ciliary photoreceptor cells to the retina where they overtake the rhabdomeric photoreceptor cells while attaining new visionary skills. Why (and how) this would happen is anyone’s guess.

Only a few years ago evolutionists were confident of this narrative. It was, according to evolutionists, a compelling story that reaffirmed the truth of evolution. These amazing claims were yet another demonstration of how evolutionists interpret unlikely data into a favorable apologetic.

But now the story has become even more unlikely. The new research has indeed found ciliary photoreceptor cells providing directional light detection in Terebratalia transversa, an invertebrate. The narrative of ciliary photoreceptor cells migrating to the retina in vertebrates suddenly makes little sense. Evolution needs a new narrative, and as usual it is more complex:

The presence of ciliary photoreceptor-based eyes in protostomes suggests that the transition between non-visual and visual functions of photoreceptors has been more evolutionarily labile than previously recognized, and that co-option of ciliary and rhabdomeric photoreceptor cell types for directional light detection has occurred multiple times during animal evolution.

In other words, yes that evolutionary scenario we were so confident of must be discarded, but so what? We can always add more drama to the plot line. Whatever biology reveals, it must have evolved—theory respectability is not important. Religion drives science, and it matters.

12 comments:

  1. You're getting better Cornelius. At least you didn't confuse a peptide with a protein this time.

    Now if only you could stop confusing your pathetic anti-science drivel with informed commentary.

    ReplyDelete
  2. Thorton,

    "Now if only you could stop confusing your pathetic anti-science drivel with informed commentary."

    If you don't understand CH's blog then you should take your comments somewhere else. You never add anything to the dialog. You have no credibility. You waste time and space.

    .

    ReplyDelete
  3. Dr Hunter,

    May we have a link to the original report of this new finding?

    ReplyDelete
  4. Peter Wadeck said...
    Waaah! waaah waah!


    If you don't understand honest criticism of the religiously driven anti-science rhetorical nonsense CH pushes here then you should take your comments somewhere else. You never add anything to the dialog. You have no credibility. You waste time and space.

    ReplyDelete
  5. Pedant said...

    Dr Hunter,

    May we have a link to the original report of this new finding?


    The paper is open access

    Ciliary photoreceptors in the cerebral eyes of a protostome larva

    ReplyDelete
  6. Thanks, Thorton. Your Google skills are impressive.

    ReplyDelete
  7. Pedant said...

    Thanks, Thorton. Your Google skills are impressive.


    Not Google. This paper was already being discussed by actual scientists (not wanna-be ones like CH) over at TalkRational.

    ReplyDelete
  8. Thornton, as always, utterly misses the point and as always uses inane drone pseudo-arguments given that she has no other.

    "religiously driven anti-science rhetorical nonsense"

    Wow, we have a new definition of Darwinian "theory" offered here.

    Thanks it fits perfectly!

    "Your Google skills are impressive."
    Possibly, but your reasoning skills are still pathetic.

    ReplyDelete
  9. Ah, it's always funny to watch Gary the lap dog piddle on the carpet then try to hump CH's leg!

    Hey Gary, how's your disproof of evolution by statistical mechanics coming? It's been more than half a year since you ran your mouth about having it, but no one's seen it yet!

    ReplyDelete
  10. Hunter:

    In other words, yes that evolutionary scenario we were so confident of must be discarded, but so what?

    As he does repeatedly, Hunter misrepresents, for rhetorical effect, the published evidence. There appear to have been several possible scenarios (interpretations of the data) based upon the data that were available at the times that their authors published those interpretations. (And Hunter certainly doesn't know the level of confidence of any of those authors in their opinions. From the paper by Passamaneck et al. that was quote-mined in Hunter's OP:

    "The diversity of eyes throughout the bilaterians has led to an ongoing debate regarding their evolution. Based upon the observation that photoreceptive structures generally have either ciliary or rhabdomeric morphology, Eakin [1, 9] proposed two lines of photoreceptor evolution, with the rhabdomeric type having evolved from the ciliary type early in the protostome lineage. Vanfleteren and Coomans [17] concluded that rhabdomeric photoreceptors were a subset of ciliary photoreceptors, and thus photoreceptors are homologous across bilaterians. In contrast to both these interpretations, Salvini-Plawen and Mayr [10, 18] interpreted the diversity of photoreceptors as evidence of polyphyletic origins, postulating that photoreceptors have evolved many times independently in bilaterians."

    Interpretations of data rest upon the data at hand. When new data are discovered, they sometimes provoke new interpretations. Eventually, the weight of data may lead to a completely new paradigm. Such has been the history of science.

    Are we at the tipping point for a new paradigm? If so, bring it on!

    ReplyDelete
  11. Thorton,

    "If you don't understand honest criticism of the religiously driven anti-science rhetorical nonsense CH pushes here then you should take your comments somewhere else. You never add anything to the dialog. You have no credibility. You waste time and space."

    It is you who are anti-science. Ever heard of Isaac Newton, the religious scientist who was the greatest scientist of all time. Of course, not, because you don't have a clue of what you are talking about, and it shows. That is why your posts are vacuous. Nobody is persuaded by your worthless statements. You are just wasting your time and proving CH's point. You are a joke.

    It is obvious that CH's religious views do not affect his scientific analysis. The same can not be said for you. But you are afraid to admit your obvious bias. Most people in the world believe in God. What is your bias? Are you part of the small minority of brain dead atheists? Do you have the honesty to admit your bias? I doubt you do.

    .

    ReplyDelete
  12. Peter said...

    It is you who are anti-science. Ever heard of Isaac Newton, the religious scientist who was the greatest scientist of all time. Of course, not, because you don't have a clue of what you are talking about, and it shows. That is why your posts are vacuous.


    What in the world does Isaac Newton have to do with a discussion of photoreceptors? You're floundering badly here Peter. Maybe you should read a beginning science text sometime.

    Nobody is persuaded by your worthless statements. You are just wasting your time and proving CH's point. You are a joke

    Gee, I didn't know you held some position of authority among the IDiots that allows you to speak for everybody. Were you elected? Or just self-appointed?

    It is obvious that CH's religious views do not affect his scientific analysis.

    LOL! Sure Peter. That's why CH is a member of the Discovery Institute, so his strong religious biases don't affect his rhetoric. If you tried I bet you could get that other foot into your mouth too.

    Most people in the world believe in God.

    But not your God, and not your narrow literal views of special creation. What makes your particular creation story so special?

    ReplyDelete