Endosymbiosis shows how some cell transport evolved

My main goal in responding to Behe’s books is not to try to answer everything.  My primary aims are to show how inadequate his simplistic criticisms of evolution are, how he does not address the evidence that demonstrates evolution which does not betray the slightest bit of design nor deviance from the usual mechanisms of evolution, and of course, evidence of the evolution of complex pathways.  In line with the last bit, while I am neither knowledgeable enough to discuss the intricacies of cellular transport, nor aware of the evolutionary evidence behind most of it, I can point to the obvious example that endosymbiosis did occur, and it co-opted bacterial export mechanisms for at least some cellular transport.

This post is in response to Chapter 5, “From here to there” (pp. 98-116), of Darwin’s Black Box.  But this is not a direct response to his “challenge,” since of course he merely caricatures the problems, and fails to recognize that intracellular transport would likely evolve gradually as membrane-bounded cellular compartments themselves gained and lost functions.  Moreover, his response to the evidence for bacterial export systems evolving into intracellular transport would be to demand that we explain the bacterial export systems, ignoring the complexity of the system into which the latter had evolved (as well as how it almost certainly gradually evolved the “irreducibly complex” steps after endosymbiosis took place).  Always demand explanation of the hardest problems, and you have your excuse to deny evolution.  Never mind that, the point is what the evidence shows, not Behe’s endless attempts to deny the direction the evidence points.

Endosymbiosis itself is very interesting evolutionarily, because it not only supports the notion that evolution of biochemical pathways was not easy, it demonstrates that evolution had to deal with that fact.  Eukaryotes are complex organisms which do not seem to easily evolve new biochemical pathways (probably due in part to their specific complexity), and are generally (with some exceptions) not good at exchanging DNA with unrelated species.  While many biochemical pathways almost certainly were horizontally exchanged between all of the early organisms–the archaea, bacteria, and the precursors of the eukarya–when it came to aerobic respiration and photosynthesis, neither ability would be passed to eukaryotes via horizontal gene transfer.  Only endosymbiosis gave us and our nucleated relatives aerobic respiration, as bacteria(the α-subgroup of proteobacteria)-derived mitochondria, and photosynthesis came endosymbiotically to plants as cyanobacteria-derived chloroplasts.

Indeed, it appears that neither of those pathways do evolve readily, nor did design make up for the difficulty in eukaryotes.  Physical precursors, not conceptual precursors, are demanded from Darwinian evolution, as Behe pointed out (DBB 44-45), and of course that is exactly what we find.  With design not operating in the origin of biochemical pathways as we generally would expect it to act, evolution of photosynthesis and of aerobic respiration instead had to come another way, via endosymbiosis. 

But of course there were no transport mechanisms to deal with the now-intracellular output of chloroplasts once these were engulfed by a proto-eukaryotic (or a “eukaryotic”) cell, nor for the proteins needed by chloroplasts as genes were moved out of the chloroplasts and into the nucleus.  It is unlikely that early chloroplasts really needed specific intracellular transport mechanisms for the symbiotic relationship to be mutually beneficial, however there would not be much coordination of the needs and abilities of both the eukaryotic host and the endosymbiotic cyanobacterium/chloroplast.  So intracellular transport and communication would almost certainly be selected-for at once, with much more transport capability evolving as genes moved from the chloroplast genome to the nucleus of the eukaryotic cell.

I will not exhaustively discuss the transport system that evolved, for it has been done by more qualified individuals in a Nature article which happily is available without a fee.  Mostly that article is only detailing the transport mechanisms and processes, but it also addresses evolutionary issues to some degree. One window in particular addresses the evolution of intracellular transport with chloroplasts:

Endosymbiosis was accompanied by massive gene transfer from the endosymbiont to the host nucleus. However, before genes could be eliminated from the endosymbiont genome, a system to import the now nuclear-encoded gene products into the new organelle had to be established. Although the endosymbiotic bacterium had several systems to export (or secrete) proteins across the membranes, the organelle now had to import proteins (see figure).

Most striking is the homology of the translocon of the outer-chloroplast-envelope subunit TOC75 to bacterial outer-membrane proteins that are involved in the transport of polypeptides across the outer membrane of Gram-negative bacteria.  This conserved β-barrel ion channel has, in most cases, no strong preference for the direction of ion permeation and therefore represents an ideal starting point to build a translocon. Subunits that convey the specificity and directionality of transport are eukaryotic additions, for example, the TOC34 receptor and the TOC159 motor.

But, what formed the translocon of the inner chloroplast envelope (TIC)? There is no detectable homologue for the putative TIC110 channel, and the putative second channel subunit TIC20 shows only a low homology to bacterial proteins. Maybe the early endosymbiont continued to use bacterial export systems in reverse, such as the secretory pathway (SEC), the twin-arginine translocon (TAT) system or the albino3 (ALB3) homologue YIDC. Therefore, the TIC translocon — including the adaptation of chaperones in the stroma to provide the driving force for import — could have been an invention of the endosymbiont. Gram-negative bacteria, including cyanobacteria, use the Sec or the Tat system, YidC and an SRP (signal-recognition particle)-dependent pathway to translocate proteins into and across the plasma membrane and the thylakoid membranes. All these systems are still operational in chloroplasts today and are essential for thylakoid biogenesis.

While one might prefer recognizable homologies throughout, rather than a seemingly new TIC translocon, there is nothing implausible about the scenario given in the quote above.  Furthermore, homology indicates that TOC75 did evolve from gram-negative outer membrane transport protein. 

The fact is, then, that a highly specific intracellular transport system arose after endosymbiosis took place.  Behe simply complains in DBB that because gated transport requires “a minimum of three separate components to function, it is irreducibly complex”:

And for this reason the putative gradual, Darwinian evolution of gated transport in the cell faces massive problems.  If proteins contained no signal for transport, they would not be recognized.  If there were no receptor to recognize a signal or no channel to pass through, again transport would not take place.  And if the channel were open for all proteins, then the enclosed compartment would not be any different from the rest of the cell. DBB 109

All of that is hardly the case for the endosymbiotic cyanobacterium/chloroplast at the first, for signals could evolve as genes transferred into the nucleus (most certainly not all cyanobacteria proteins would already be recognized for export from the cyanobacterium).  And what I primarily wished to point out here is that a complex transport system (subsystem) can arise from endosymbiosis.

Also important, however, to the general case is that Behe gets things wrong once again in the case of non-endosymbiotic compartments.  If, as is likely, many membranes of eukaryotic compartments evolved from eukaryotic cell membranes (either primarily or secondarily), they would already be selective in ways which could be useful in a compartment, just as the cyanobacterium’s membrane was already selective in the endosymbiotic example.  No doubt adaptation would change transport across those membranes, much as the transport across the membranes of chloroplasts adapted to changing relationships between it and the nucleus.

Anyway, that’s it for the “irreducible complexity” of intracellular transport.  I did not touch upon many specifics in the chapter, like the fact that Behe seemed to anticipate the fact that such transport could evolve, and seemingly for that reason chose to bring in the “problem” of evolving transport across cell membranes, in a rather confusing manner.  But that’s a sideshow to the fact that intracellular transport can evolve gradually, as both the homological evidence demonstrates in the case of chloroplasts, and because of the reasons I brought up for both the specific case and in the general case.

The particular phenomenon of chloroplast evolution from the cyanobacterium points to an inconvenient fact that Behe would all too gladly ignore.  This being the fact that–almost certainly especially so in the later complex and more precisely controlled reproduction of later organisms, such as eukaryotes–biochemical pathways do not readily evolve, thus (there being no design evident in life) the basic pathways have had to be horizontally transferred either by sharing genes or through endosymbiotic means.  This is why chloroplast transport processes had to evolve later on, or rather, it had to evolve for that reason and because no designer steps in to recreate pathways in organisms which had been lacking these. 

The fact is that oxygenic photosynthesis only evolved once, which is likely also the case for photosynthesis itself.  Yet it is perhaps even more interesting that oxygenic photosynthesis evolved only once, because it has all of the earmarks of duplication of photosystem I, plus rather significant changes once that occurred (which almost certainly is the truly constraining part of the evolution of oxygenic photosynthesis), showing that it did evolve.  Nevertheless, this evolution was so difficult that it happened only once, and then it had to be endosymbiosed and extensively adapted to supply eukaryotes with solar energy. 

More on that story in a later post.

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.

Explore posts in the same categories: Darwin's Black Box

Tags: , ,

You can comment below, or link to this permanent URL from your own site.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: