Evolution of the eukaryote flagellum

There is not much truly known about how the eukaryotic flagellum evolved, other than that it did and from what cellular components most of it evolved.  That is considerable, of course, and is infinitely more than the facts that ID explains (zero).  Yet the one good thing about ID and Behe is that they point out how much is not known, which is often not well discussed in scientific circles–although occasionally journals will have articles discussing what is so little known in many areas, including evolution.  So there is some point to agreeing that much remains in question, so long as it noted that it is not as little as Behe claims, nor does the evidence indicate anything that differs from the expectations of unguided processes.

There is an online paper that makes some educated guesses about how the eukaryote’s flagella/cilia evolved, which is worth a look.  It is: Speculations on the evolution of 9+2 organelles and the role of central pair microtubules, by David R. Mitchell. It elaborates on the obvious point that cilia are made from part of the framework of the eukaryotic cell and some of the machines that provide intracellular transport.

Soon it gets to the most interesting, and probably the most questionable, speculation, which is that eukaryotes used microtubules radiating from a centrosome in order to partition duplicated chromosomes. Apparently this idea is patterned on the microtubule-based spindle used to separate chromosomes in present-day animal cells. And from the early proto-cilium that was used to separate the chromosomes evolved the cilium, by asymmetrically growing into a bundle (actually, only two at first) of microtubules that extended the cell outward to create gliding motors that acted both to move the cell and to provide sensory activities.

After that the beating cila/flagella would evolve for swimming, and the numbers of microtubules bundled together would increase dramatically, at least in some lines. From that the 9+2 arrangement would evolve because it is fairly good for the sake of controlling the beat of flagella and cilia. A fair bit of the paper is taken up discussing the evolution of the ancestral 9+2 configuration of the cilium and flagellum.  Anyhow, I do not intend to try to re-write what is written in that short paper, particularly not the 9+2 configuration, which seems least relevant to the evolvability of the eukarote flagellum.

What is important is that there does exist at least a somewhat reasonable path toward evolution of the flagellum in the literature, and there seems to be nothing to preclude our cilia and spermatic flagella from evolving in that manner.  Surely it could happen.

I have to wonder about the step in which the cilium evolves from microtubules involved in mitosis, however, which is credited to earlier authors by Mitchell.  One problem is that in present-day mitosis the spindle only operates during mitosis, and is dissassembled rather quickly afterward.  Why would an early eukaryote keep its spindle-equivalent around, when it would just get in the way?  Furthermore, the operation and set-up of microtubules during mitosis have relatively little to do with the operation of a cilium.  Combining mitotic processes with even the processes occurring in a gliding cilium (gliding cilia, by the way, are found in a few modern eukaryotes) does not appear to be a successful evolutionary “strategy”.

Evidently part of the reason various authors want to understand the cilium as coming from microtubules involved in mitosis is that it provides an explanation for how eukaryotic cells become polar.  Yet that, too, seems to be insufficient to combine mitotic and ciliary microtubules, because the cells only need to use the polarity provided by mitosis to place a flagellum at one pole.  It seems far more likely to me that intracellular transport mechanisms might evolve into gliding motors (for movement, chemosensation, or both), which evolved to be more efficient by protruding from one pole of the cell.  This could be somewhat similar to the movements that amoebae have evolved, it’s just that in this case the movement became compartmentalized and specialized to a far greater degree.

Like I implied above, this is all just speculation without a great deal of constraint.  Take it or leave it, whether it is Mitchell’s paper, or my criticisms of the hypothetical mitosis-flagellum link.  They are included because, once again, these are examples of how caring about observable causes can lead to thought, as poof “explanations” cannot.  What really matters is all of the evidence that the eukaryotic flagellum did evolve, and the hope that this might eventually help us to tease apart the mystery of how our cilia/flagella evolved–though it is not certain that all distant evolutionary processes will be knowable, including this one. 

The evidence that it evolved will soon be the subject of another post.  The fact is that we have enough reason to believe that the evolution of the eukaryotic flagellum is possible, but more importantly, we have good evidence that it did.  Real science proceeds from the evidence that, say, language evolution, or biological evolution, happened to the actual causes behind it.  And of course “actual causes” means “observable causes” (at least in principle), if we take epistemology at all seriously.

This is part of a series of posts that I am combining into one long post, which may be found at Darwin’s Black Box.

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