Welcome to the Party

For several years now, I have been asking “where are the prokaryotic mice?”  Given that the prokaryotic cell plan is the most ancient, abundant, and successful cell plan on earth, why hasn’t the blind watchmaker been able to craft together the convergent equivalent of some metazoan?  In fact, as I noted, “had the eukaryotic cell design failed to emerge, the Earth would contain nothing more complex than any extant bacteria in existence today.”   Then, back in October 2010, a paper from Lane and Martin was published that supports my contention:

Our considerations reveal why the exploration of protein sequence space en route to eukaryotic complexity required mitochondria. Without mitochondria, prokaryotes—even giant polyploids—cannot pay the energetic price of complexity; the lack of true intermediates in the prokaryote-to-eukaryote transition has a bioenergetic cause.

A few days ago, biologist PZ Myers recently helped to popularize Lane and Martin’s paper and begins by essentially asking…you guessed it…”where are the prokaryotic mice?”

Myers writes:

I had to wonder: why have eukaryotes grown so large relative to their prokaryotic cousins, and why haven’t any prokaryotes followed the strategy of multicellularity to build even bigger assemblages? There is a pat answer, of course: it’s because prokaryotes already have the most successful evolutionary strategy of them all and are busily being the best microorganisms they can be. Evolving into a worm would be a step down for them.

That answer doesn’t work, though. Prokaryotes are the most numerous, most diverse, most widely successful organisms on the planet: in all those teeming swarms and multitudinous opportunities, none have exploited this path? I can understand that they’d be rare, but nonexistent? The only big multicellular organisms are all eukaryotic? Why?

So Myers is asking, “Why are there no prokaryotic, big multicellular organisms.  In other words, “Where are the prokaryotic mice?” (or better yet, “Where are the prokaryotic squids?”)

Myers also adds:

Eukaryotes have a key innovation that permits the expansion of genome complexity, and it’s the mitochondrion. Without that big powerplant, and most importantly, a dedicated control mechanism, prokaryotes can’t afford to become more complex, so they’ve instead evolved to dominate the small, fast, efficient niche, leaving the eukaryotes to occupy the grandly inefficient, elaborate sloppy niche.

Another way of saying this is that the prokaryotic cell plan constitutes an “edge” to evolution, guiding prokaryotes to the small, fast, efficient niche.  In contrast, the eukaryotic cell plan allows these cells to circumvent this edge, guiding them to terrain where the emergence of metazoan-type complexity is not only possible, but is just a matter of time. I explained the significance of this crucial step before:

This is not about whether prokaryotes could ultimately spawn eukaryotes, as I accept that.  As Steve notes, there is a solid hypothesis about that transition and I have previously explored some of the ways this symbiotic union may have been front-loaded. This is about whether the cell design – the composition and architecture of the prokaryotic cell – is capable of generating something as structurally complex as a mouse (for a mouse, like all animals, is an assembly of cells).  Seen from this angle, the endosymbiotic hypothesis supports my position.  That is, in order for prokaryotes to ultimately spawn eukaryotes, they first had to go through a radical re-design of cell structure.

Prokaryotic cells can be viewed as the highest expression of mutation and selection, for there is no better cellular candidate for a “self-replicator.”  Yet after billions of years, the prokaryotic cell plan has failed to achieve anything near the level of structural complexity as exhibited by the eukaryotic cell plan.  To reach such structural complexity, the cell design had to be radically retooled, partly through endosymbiotic union, a one-time event given the widely accepted monophyly of eukaryotes.  Once the eukaryotic cell design was established, prior to the radiation of all extant eukaryotes, the basic cell design was now capable of supporting the emergence of complex, metazoan life.  The evolution of metazoa did not require further extensive retooling of the eukaryotic cell plan, given that metazoan cells are so similar to protozoan cells; it was more like the natural outflow of the potential inherent in the eukaryotic cell plan.

Myers also adds:

So, what Lane and Martin argue is that the segregation of energy production into functional modules with an independent and minimal genetic control mechanism, mitochondria with mitochondrial DNA, was the essential precursor to the evolution of multicellular complexity — it’s what gave the cell the energy surplus to expand the genome and explore large-scale innovation.

In other words, had the eukaryotic cell design failed to emerge, the Earth would contain nothing more complex than any extant bacteria in existence today.

So this all leads to the next level of analysis – why haven’t any bacteria evolved a mitochondria-like structure to open up this niche to them?  In the next posting, we’ll build on some of Myers explanation, but take it a bit deeper.

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16 responses to “Welcome to the Party

  1. I found your blog on google and read a few of your other posts. I just added you to my Google News Reader. Keep up the good work Look forward to reading more from you in the future.

  2. Hi Mike,

    Would this support the view that the mitochondria was in fact an optimal design for eukaryotes? I’m thinking of …darn, I can’t remember his name at the moment…what’s his name’s argument that mitochondria are a suboptimal design.

  3. Hi Bilbo,

    I think you are thinking of John Avise and his argument that that mitochondria couldn’t be designed because they contain DNA. I don’t think Lane and Martin make the case that mitochondria are optimal for eukaryotes, but the mitochondria could be optimal for facilitating the emergence of metazoa. In other words, maybe there is not a better way to bring about their emergence.

  4. Mike: “To reach such structural complexity, the cell design had to be radically retooled, partly through endosymbiotic union, a one-time event given the widely accepted monophyly of eukaryotes.

    I think I just discovered a problem with your hypothesis, Mike. If the original cells were pre-adapted for endosymbiotic events, then shouldn’t there be multiple endosymibiotic unions, instead of one? I think this would favor the hypothesis of additional intelligent intervention, after the origin of life.

  5. Very nice. I think it breaks down to three possibilities.

    1. Front-loading need not entail that there be multiple endosymibiotic unions. It simply means that given the preadaptations, and with sufficient time, it would happen. After all, to spawn metazoa, we only needed the eukaryotic cell to emerge at some point, not at many points. Here we would argue that without the preadaptations, no eukaryotes would have emerged and the earth would still be covered purely with bacteria.

    2. As far as we know, eukaryotes could indeed by polyphyletic. But because of the effects of rampant convergence and lateral transfer (both appearing to be far more common than originally assumed), this signal is lost.

    3. There is true discontinuity here as evidenced by the one time appearance of eukaryotes that were radically re-tooled prior to branching into the various groups that exist today.

  6. I think the problem with (1) is that if it only happened once in natural history, then pre-adaptation didn’t do much to increase the odds of it happening at all. (2) looks like a better option. I’m wondering if there’s a fourth option: are we certain that eukaryotic cells weren’t part of the original design event?

  7. I think the problem with (1) is that if it only happened once in natural history, then pre-adaptation didn’t do much to increase the odds of it happening at all.

    But if it wouldn’t happen at all without the preadaptations, it did increase the odds of it happening. I don’t think increasing the odds by front-loading through preadaptations necessarily means the front-loaded event should happen multiple times. For example, as far as we know, metaozoa appeared once, as sponges, cnidarians, and placozoans appear to have a common ancestor. To trigger the appearance of eukarya or metazoa, it may have been an issue of bringing together a critical mass of preadaptations in the right place at the right time. A designer may not be able to predict exactly when, where, and what, but the designer may have been able to predict that sooner or later, it will happen. It was just a matter of time. What’s more, to add another twist, the whole system may have been rigged to increase the odds it would likely happen once and only once.

  8. The problem with (1) is that the designer must also reckon with the contingencies of history. Suppose “the whole system” was rigged to happen once and only once, but some unforeseen event spoiled it all. What then? All that work for nothing.

    So let me repeat my question: Are we certain that eukaryotes were not designed at the origin of life, along with prokaryotes?

  9. The problem with (1) is that the designer must also reckon with the contingencies of history. Suppose “the whole system” was rigged to happen once and only once, but some unforeseen event spoiled it all. What then? All that work for nothing.

    What this would mean is that front-loading should be robust in regard to “unforeseen events.” And one way to do this is to use highly adaptable cells that can share their genetic information with each other. The only unforeseen event that could spoil this front-loading would be global sterilization and that would thwart all design strategies to secure an objective across time.

    BTW, I should make it clear that I am not proposing front-loading “once and only once.” That is a possibility to consider. My hypothesis would simply be more modest – front-loaded to happen at least once.

    So let me repeat my question: Are we certain that eukaryotes were not designed at the origin of life, along with prokaryotes?

    No, I am not certain. But if this was the case, then why do bacteria and archaea have so many features that can be easily viewed as preadaptations for eukaryotic, even metazoan, life? You would hardly need to endow such cells with preadaptations to facilitate the emergence of eukarya if eukarya were there from the start. And then there is the more conventional evidence of a) scores of homologous genes shared by archaea/bacteria and eukaryotes and b) the fact that fossil eukaryotes do not appear until much later. Yet there is the possibility that some form of proto-eukaryotic cell might have been present from the start.

  10. Thinking it over, Mike, I would say that monophyly wouldn’t support your hypothesis as well as polyphyly would. If convergent evolution — many occurrences of the same feature evolving independently — supports FLE, then only occurrence of evolution, especially of something crucial to evolution, such as the endosymbiosis of mitochondria, should count against FLE.

  11. Oops, I saw your reply after I made my last comment [BTW, should be “…only one occurrence…”].

  12. Thinking it over, Mike, I would say that monophyly wouldn’t support your hypothesis as well as polyphyly would.

    I would agree with that. I have never argued that front-loading was suggested by monophyly.

    If convergent evolution — many occurrences of the same feature evolving independently — supports FLE, then only occurrence of evolution, especially of something crucial to evolution, such as the endosymbiosis of mitochondria, should count against FLE.

    I can agree with that also. If, with more and more higher resolution genomes analyses, we find that some of the major groups of protozoa arose independently (option 2 above), then the case for FLE at this point would become even stronger. Right now, the front-loaded origin of eukarya is supported mainly by the vast array of predadaptations among the bacteria and archaea.

  13. Mike: “If, with more and more higher resolution genomes analyses, we find that some of the major groups of protozoa arose independently (option 2 above), then the case for FLE at this point would become even stronger.

    I agree. On the other hand, if at higher resolutions we continue to find that there have been only one-time events at crucial junctures of evolution, I think FLE would be weakened.

    I think there might be a way to fix it, though. If we revise the hypothesis to “a designer who knew the future”, one time crucial events wouldn’t weaken FLE. But they would weaken the “human-like intelligence” part, since we don’t know the future.

  14. Mike Gene:

    But if this was the case, then why do bacteria and archaea have so many features that can be easily viewed as preadaptations for eukaryotic, even metazoan, life?

    One group of designers got the job for eukaryotes. Another group took their power plants- the mitochondria and chloroplasts- and modified them to be able to fend for themselves-> power plants plus, ie prokaryotes.

    These went to work first to terraform. That explains their position in the fossil record along with the genetic similarities.

  15. Joe, are you suggesting two different design events at two different times?

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