Bacteria are tiny. Compared to our cells, they can seem insignificant. There are about ten times more bacteria cells in your gut *right now* than there are human cells in your entire body, but they only make up about 5% of your mass. They're tiny, but they're successful - they live in places we can't, they can metabolize things we can't, and they're everywhere. Despite this success, there's some things they don't do, like multicellularity, but why?
PZ has a great review of a recent paper in Nature that tries to answer that question, so I don't need to recapitulate it, but I have just a couple of (minor) quibbles with this analysis.
First off, many prokaryotes (which includes bacteria and archaea - all others are called "eukaryotes") do achieve something like multicellularity. Biofilms are communities of bacteria that communicate with and help each other. There are even examples of something that looks like differentiation in a community of the same (or different) species in a biofilm.
That said, there's clearly a profound difference in the level and complexity of eukaryotic multicellularity. But I'm not sure I fully buy these authors' conclusions (as expressed by PZ):
Eukaryotic power production per gram isn't any better than what prokaryotes do, all they've done is made their cells bigger, and there's nothing to stop prokaryotes from growing large and doing the same thing. In fact, they do: the largest known bacterium, Thiomargarita, can reach a diameter of a half-millimeter. It gets more metabolic power in a similar way to how eukaryotes do it: we eukaryotes carry a population of mitochondria with convoluted membranes and a dedicated strand of DNA, all to produce energy, and the larger the cell, the more mitochondria are present. Thiomargarita doesn't have mitochondria, but it instead duplicates its own genome many times over, with 6,000-17,000 nucleoids distributed around the cell, each regulating its own patch of energy-producing membrane. It's functionally equivalent to the eukaryotic mitochondrial array then, right?
Wrong. There's a catch. Mitochondria have grossly stripped down genomes, carrying just a small cluster of genes essential for ATP production. One hypothesis for why this mitochondrial genome is maintained is that it acts as a local control module, rapidly responding to changes in the local membrane to regulate the structure.
At first glance, this conclusion seems plausible, but there's no reason in principal that bacteria could not compartmentalize the necessary genes for metabolism. Bacteria routinely hold bits of genetic material on plasmids that can replicate and express proteins independently of the main chromosome. If Thiomargarita learned to replicate only metabolic genes on a plasmid instead of it's whole genome, that would be the functional equivalent of mitochondria. The authors' conclusion is that mitochondria are "required," but bacteria can achieve all the features of mitochondria (compartmentalization, aerobic metabolism, independent genes). There's no reason in principal that bacteria could not evolve a mitochondria-like organelle, complete with an independent genome.
I'm not saying the authors are wrong here - they're math seems totally reasonable and the idea that energy production might be limiting makes perfect sense. Even the notion that independent metabolic genes would be required to scale up energy production is legit. But the idea that prokaryotes are incapable of these inovations is suspect. There must be something else going on.
Lane N, & Martin W (2010). The energetics of genome complexity. Nature, 467 (7318), 929-34 PMID: 20962839
Perhaps bacteria could do it, but eukaryotes have become much, much better at it. Thus, any bacteria that might evolve the necessary adaptations for multicellularity would be at a competitive disadvantage, versus both existing eukaryotes and their bacterial relatives. Just a thought.
The idea that bacteria in biofilms help each other rather than exploit each other is supported more by our social-animal biases than by solid evidence of selection for cooperation.
@ qetzal - that may be the case today, but it doesn't explain why prokaryotes never got there before. And besides, there are both prokaryotes and eukaryotes filling different single-cell niches, I can't imagine why the same wouldn't be true for at least small multicellular niches.
@ Dr Redfield - I'm not quite sure I understand your point. We see behavior in biofilms (like quorum sensing) that look like cooperation. You could frame it in terms of exploitation instead, but I feel like you could frame social animal behavior in the same manor. Certainly,the idea carries with it all the same apparent problems as altruism in the animal world. But you think it's inherently different?
I'm way behind the ball on this one. But, I can tell you, that Nick Lane addresses the issue of prokaryote v. eukaryote size and size potential in his book, "Power, Sex, Suicide: Mitochondria and the Meaning of Life." I can only follow the argument as I read it, I've never had serious discussions of it, nor do I get the opportunity to really explore it. I mention it, very humbly, as your knowledge is most likely past it...but thought perhaps it could add to the discussion, if examined. I also recall that a big impediment was that in the mitochondria, the genome is stripped down and this means less energy must be expended to reproduce the entire organism because of this: bacteria which have larger genomes must use greater time and energy to reproduce...thus bacteria lose genes as soon as they no longer serve a purpose. Again, it's been awhile and I maybe way, way, way off base.
Okay, then! I'm feeling like an idiot! My apologies, I peruse other forums and have other interests. I generally don't read PZ because most times when I have it ventures into the political or social commentary...an area I've grown tired of hearing debate over, for the most part. At any rate, I obviously did not realize whose paper he was discussing, and only read your comment about his comment...which begins a bizarre circle: my comment about your comment about his comment about a paper by an author I then more or less cited, failing to realize it was his worked that sparked your comment. I'll be leaving now with a promise to back check the comment upon the comment and attempt to re-familiarize myself with the work by the author who created all the comments....a lot of work....I'll finish reading PZ's commentary and then go fly an R/C heli...and post on forums where debate rages as to the strengths and weaknesses of Blades and Trex450's.....
Mike - you need to stop apologizing for continuing to be engaged in a subject you're not an expert in. Your comments have always been welcome, and demonstrate a lot more understanding than I would expect from most casual readers. It's no surprise that you occasionally miss something (I miss stuff even in my own field all the time), and your willingness to correct your mistakes (though your own reading and in response to others) is quite refreshing.
If eukaryotes got there first, even if only by chance, then that might preclude prokaryotes making a similar transition. Much like there's probably no chance for abiogenesis to occur today, because existing life would eat it and out-compete it before it could ever get going.
I'm not persuaded that the persistence of unicellular eukaryotes is a strong counter-argument either. Perhaps having mitochondria (or something functionally equivalent) is critical to competing in multicellular niches, but doesn't present any significant disadvantage in many unicellular ones.
I've also seen another argument why mitochondria are important for multicellularity. Namely, that mitochondria allow compartmentalization of mutagenic radicals generated during oxidative respiration. This minimizes oxidative damage to nuclear DNA, while still allowing a high metabolic rate. (Presumably, segregating the DNA inside a nucleus helps as well.) All of this allows for the larger, more complex genomes supposedly necessary to support multicellularity.
I don't have a citation for the above argument handy, and I'm not necessarily persuaded by it. Nor do I think the 'eukaryotes got there first' hypothesis is necessarily all that strong. But I haven't seen anything to rule it out. This question has interested me for a while, so I'm glad to see new ideas being tested.
Here's a related question that I find interesting: Why don't unicellular organisms evolve 'naturally' from multicellular ones? Tumor cells grow essentially as unicellular organisms in the 'artificial' environment of a lab, so it's clealy possible. Again, my guess is that once an organism is specialized for multicellularity, its individual cells can't readily compete with existing unicellular organisms.
1 - Yes, if Eukaryotes got there first it might preclude prokaryotes from getting to the same niche. I think the argument is plausible, but not probable for 2 reasons. First, prokaryotes had a lot of time to try this before eukaryotes showed up. If they could have managed it, I think they would have. Second, I have a bit of a bias that goes (and imagine the music here) "Anything we can do/ microbes do better." Clearly, these are merely speculative, and we have empirical evidence that there are complex multicellular eukaryotes and not prokaryotes, so I'm arguing here from a position of weakness. Still, I think that all the features that make mitochondria useful are present in prokaryotes, so this argument alone is not sufficient to explain the difference.
2 - You're right, it's not a terribly persuasive argument. Your point is entirely valid, something like a mitochondria may be necessary, but again, the trouble is that I don't see why prokaryotes wouldn't evolve something functionally equivalent to mitochondria to compete in the multicellular niche if that was the only thing stopping them. All the components are there.
3 - Bacteria are fully capable of (and quite good at) compartmentalizing toxic byproducts of metabolism. As I mentioned, many bacteria already use oxidative metabolism. As you said, maybe the double segregation of DNA in the nuclease and extra segregation of metabolism is required. Still, some bacteria also segregate their genomic DNA (nothings so complex as a nucleus, but...).
Here's a related question that I find interesting: Why don't unicellular organisms evolve 'naturally' from multicellular ones?
I'm not sure that this doesn't happen. Consider the contagious cancer that's spreading around the Tasmanian devil population. That could almost be what you describe. But you're right, the differentiation of multicellular organisms would make it a bit tough to go back to unicellular existence.
How about if Darwins 'Evolution by natural selection' isn't quite right, its missing the key ingredient; namely that all life forms are optimized to capture energy in specific contexts?
The idea that our cells help each other rather than exploit each other is also supported more by our desire to see ourselves as one entity rather than as teaming collection of bugs tenuously held together by self-glue.
@Qetzal: don't (some) yeast form an example of unicellular life evolving from multicellular life?
Back to the post: doesn't this provide a proximate rather than ultimate explanation? It provides a reasonable candidate difference for answering the question, but doesn't really explain why that difference is stable.
"But I'm not sure I fully buy these authors' conclusions (as expressed by PZ):"
And why do you think PZ accurately reported Lane's and Martin's work accurately?
Read the comments of the article and you'll see why that might be erroneous.
You'll be better off reading either the original paper in Nature or "Power, Sex, Suicide: Mitochondria and the Meaning of Life", or the fourth chapter of "Life Ascending: The Ten Great Inventions of Evolution". The latter really did deserve the Royal Society prize. Anyway, the scientific paper published by Lane and Martin is a more rigorous treatment of a topic Lane has already written about.
And the original thought that mitochondria made eukaryotes what they are came from either Martin and/or Miklos Muller, co-authors of the original Hydrogen Hypothesis paper.
P.S. Actually, Lynn Margulis is the the one that pointed the way to the origin of eukaryotic organelles and endosymbiotic theory, but Martin and Muller are the first to come up for a compelling explanation of how the relationship of mitochondria and hydrogenosomes matches a probable origin of eukaryotes.