10 ms·

Why complex life probably evolved only once

113 points by hartror 16y ago

eof 16y ago
This is crap. It doesn't say anything. Article tl;dr: Ignoring the inconceivably remote chance of life forming from non-life, it's very very rare for simple life to evolve into more complicated life. Also, we just realized that cells need to get bigger to get more complex, and that somehow isn't just 'another' mutation. And then the title? There is nothing about how or why complex life only evolved once. Just that it was unlikely. Chance of life forming from non life * age of the universe * number of stars * the chance a star has an earth like planet, is far from zero. I wouldn't even bother ranting but this was number 3 on the front page.
- earnubs 16y ago
  Isn't the optimistic guess at Drake 2 or something (correct me if I'm wrong)? That's not /very/ far from zero, and if you consider that it's the number in the Milky Way I'd say that's pretty very much close to zero.
- danteembermage 16y ago
  I upvote based on content not title. Perhaps a mod can change it to "New theory suggests Mitochondria precede complexity in early life" I definitely would classify engulfing mitochondria as not just another mutation, it's an entirely different thing.
  - eof 16y ago
    Then everything is an entirely different thing. The simple act of a single cell engulfing mitochondria is not the birth of a species. Even on content I would bury this if I had the button for it, it makes pretty heavy claims without any evidence and necessarily brushing aside fundamental problems with the overall conclusion. I am not bashing the scientific article that this editorial is 'about'; but the article itself, besides a tiny nugget about the necessity of mitochondria being perhaps otherwise overlooked, it is just drivel.
- pygy_ 16y ago
  The original article [1] is titled "The energetics of genome complexity", which is much more reasonable. It argues that the energy processing of mitochondria was necessary in order for eukaryotes to expand in complexity. The article is behind a paywall, so I can't read the details. Anecdotically, there exist anaerobic eukariot cells that live on hydrogenosomes [2]. I don't know if they were originally aerobic and lost their mitochondria, or if they evolved in parallel. There also exist sea slugs that live in symbiosis with the chloroplasts of the algae they eat [3], so this kind of symbiosis has happened more than once. 1. http://www.nature.com/nature/journal/v467/n7318/full/nature09486.html http://www.nature.com/nature/journal/v467/n7318/full/nature0... 2. http://en.wikipedia.org/wiki/Hydrogenosome http://en.wikipedia.org/wiki/Hydrogenosome 3. http://www.plantphysiol.org/cgi/content/full/123/1/29 http://www.plantphysiol.org/cgi/content/full/123/1/29
  - nollidge 16y ago
    One of the perks of working for a school... I'll try to summarize as best I can: The main calculation in the Nature paper is the energy budget of a cell. They give the example of E. coli: "to raise gene number tenfold, E. coli must also increase its energy budget by close to tenfold; and therein lies the problem." The reason so much more energy is required is due to protein synthesis (DNA codes to RNA which codes proteins, if you're unfamiliar with genetics). So obviously with a larger genome, there's more proteins to synthesize, and this cannot be accomplished by simply making fewer copies of the old proteins. If anything, you need more of some of those original proteins as infrastructure support for this new Genome 2.0. So (and this is my wording) prokaryotes are at a local maximum for genome and cell size. Chucking mitochondria into the picture provides the wattage (literally) for them to reach a much greater genome and cell size, and therefore organismic complexity.
    - anigbrowl 16y ago
      I am unclear about why the acquisition of mitochondria by a cell is considered to be such an unlikely event, as I lean towards the view that given enough cells/space/time, unlikely short term possibilities become distinct probabilities eventually. Also, I can't help wondering if the reason that simple cells are not observed to absorb free mitochondria is that eukaryotic cells and the organisms they evolved into have somehow poisoned the well, or attacked newly-complex competitors so aggressively that they've suppressed the population of omnivorous simple cells. But I'm no biologist. Am I missing something obvious, or is there a particular paper explaining why this mitochondrial upgrade is considered such an unlikely event?
- zeteo 16y ago
  "Chance of life forming from non life * age of the universe * number of stars * the chance a star has an earth like planet, is far from zero" You have no proof for this, and things can well be the other way round. Let n = age of the universe * number of stars * the chance a star has an Earth like planet n is a number that we currently have a pretty good guess at. Let p = chance of life forming from non life on Earth-like planet within a given year. We are basically clueless as to the value of p. p may well be much less than 1/n, which would make our biosphere very special indeed.
  - eof 16y ago
    I have no proof for it, but I don't need it. The burden of proof is not on me. They are making the claim that 'complex life only happened once'. Life certainly does exist so p > 0. Since our understanding (or I should say, my under standing of 'our' understanding) of n is approaching infinity, it stands to reason that p x n is > 0.
    - zeteo 16y ago
      You've made a statement that a certain quantity is far larger than zero, so you do incur a burden of proof for that statement. p * n > 0 doesn't imply p * n >> 0, not even for large values of n (and "approaching infinity" doesn't apply to constants).
      - eof 16y ago
        Sorry, 'far from zero' is really meaningless and I shouldn't have said it that way. I don't actually mean to make a positive statement about the existence of complex life elsewhere in the universe, though intuitively I think it's absurd to believe our little planet is the only one with life on it. I mean, why would we believe that other than residual religious dogma? Our understanding of n is not a constant.
        jerf 16y ago
        "I mean, why would we believe that other than residual religious dogma?" Because when we look out into the universe, it looks like we expect it to look when it is dead, not full of life, some significant percentage of which becomes intelligent. Nothing that looks like Dyson spheres of any form (recalling that the "solid shell" is actually only a special case), nothing that looks like curated stellar evolution (very high-end stuff but a truly advanced civilization will not want to leave stars just randomly flying around a galaxy and exploding), very very little that could even conceivably be a ship traveling between the stars (which turns out with any physically-plausible propulsion mechanism to be visible from a very long ways away and would show very clear blue/red shift changes), nothing that doesn't appear entirely natural. This isn't the whole story (maybe they're hiding, maybe our idea of the top end of tech is wrong, maybe they're all virtual, maybe they escaped into another, friendlier dimension, maybe they all kill themselves, maybe the universe is soaked with simple life but this article is essentially correct and complex intelligent life really is a 10^-1000 event, etc.), but it is not true that the claim that complex life must be hard is some sort of stupid idea in light of the size of the universe. Arguably it's actually the claim best supported by the evidence at this time. People are really bad at thinking about small probabilities even on the scale of one out of a million, and it's not impossible that complex intelligent life turns out to be one out of 10^1000 or worse; we simply don't know. It is not hard to come up with calculations that plausibly set the odds of advanced life occurring even once in the universe as less than 50/50 without having to make stuff up. Our ignorance dominates our knowledge here.
- jokermatt999 16y ago
  The last part of your post is known as the Drake Equation (http://en.wikipedia.org/wiki/Drake_equation http://en.wikipedia.org/wiki/Drake_equation). I'm guessing you've probably heard of it, but it could be of interest to other people, so I figured I'd link it.
  - Roboprog 16y ago
    Tangent: the Drake Equation seems to be the template for a class of "Google interview" questions: estimate something you can't really know, at least off the top of your head, in which you then start to identify factors, and start to guess at upper and lower bounds, to come back with a plausible answer, or at least a range. I suppose a lot of research is like that, but the Drake just happens to deal with factors that are harder to measure.
hartror 16y ago
Still a sample size of one, my the drake equation is still looking rather full of letters. Though it is looking like we may be able to make a decent crack at fp (fraction of stars that have planets) soon given the success of keplar!
- nooneelse 16y ago
  In a TED talk, Dimitar Sasselov, a co-investigator on the Kepler mission, said their current best guess for the number of Earth-ish, habitable, planets in the galaxy is looking to be like 100 million (at about the t=9:20 mark in the talk). That is, naturally, an early guess from extrapolations and such, but still it is more based on something than the based-on-nothing and intuition guesses that sometimes float around in these discussions.
CapitalistCartr 16y ago
We know the odds of evolution are low; the point is that the number of stars in the Universe is astoundingly huge. With at least 10^20 stars in the Universe, those low odds get much more reasonable, especially now it's starting to look like planets are common.
- Dove 16y ago
  Do you know the actual odds, though? I mean, there's low and then there's low. If evolution on a habitable planet has a 10^-4 chance of happening, then the universe is almost certainly teeming with life. If it's 10^-18, the size of the universe is the difference between believing and doubting the idea. If it's 10^-200, the universe barely helps at all, and you need to start talking about billions upon billions of barren universes and we just happen to live in this one. Do you know what the number is? Do you know where it lies between 0.9 and 10^-(10^10^10)? Do you know whether the 20 orders of magnitude you get from the stars is overwhelming, decisive, or not even enough to make a dent?
  - hugh3 16y ago
    To my way of thinking, the fact that life showed up on Earth relatively quickly, within half a billion years of the planet's formation, suggests that life is relatively easy. The fact that complex life took another few billion years after that suggests that simple to complex is the difficult step. But the fact that we nonetheless find ourselves here around a live-fast-die-young yellow star when the universe is only 14 billion years old rather than in close orbit around a dim red star a hundred billion years later suggests to me that it's probably not a one-in-every-zillion universes sort of event.
    - petervandijck 16y ago
      That sounds reasonable enough to me. [big bang --(we're here-ish)------------------------------------------------------- end of universe] So the facts that we're that far to the left of that line, and that the universe(s) is/are really really big, suggest that life isn't that rare.
billswift 16y ago
If you are really interested in this, read "Rare Earth", http://www.amazon.com/Rare-Earth-Complex-Uncommon-Universe/dp/0387952896/ref=sr_1_1?ie=UTF8&qid=1287666028&sr=8-1 http://www.amazon.com/Rare-Earth-Complex-Uncommon-Universe/d..., it goes through an incredible mass of astronomical, geological, and biological information. It concludes that, because so many things had to go just right, and so many others were completely random, primitive life (bacterial-level) is likely to be more common than previously thought, while advanced life (multicellular) is likely to be much less common. Because of the sheer size of the galaxy, it is probable that other intelligences evolved elsewhere, but they are probably going to be too far away for any sort of contact, even receipt of potentially meaningful radio. It is a well-written book, despite the amount of information it is very readable.
erikstarck 16y ago
The number of bacteria on earth is estimated to be 5000000000000000000000000000000. This is five million trillion trillion. All of them are eating. Some of them are mutating. Let them do this for a few billions of years and what seems to be a very unlikely event suddenly becomes almost certain to occur. Complex life FTW!
- Dove 16y ago
  You've got what there, 10^16? That is certainly a very big number. But then, the article is claiming that the chance of becoming complex is a very small number. Is it less than 10^-16? (Is it even knowable? At a minimum, the author thinks he can say something about it.) You'd be surprised how fast these numbers get small if you need to brute force yourself away from a local maximum and over a valley. I don't care how big your bignum is compared to everyday experience. Bignum * Smallnum does not automatically equal certainty, impossibility, or feasibility. It means do the math.
jacquesm 16y ago
One of the best reasons I ever read about why it happened only once is that if it had happened twice the newcomer would have been called 'food' at a stage too early to get much further up the ladder. For it to happen twice the 'new' trail would have to be totally non-nutritious to the established kind. I wished I remembered where I read it.
- eru 16y ago
  Yes, though you can also be non-edible by being out of the way.
- ced 16y ago
  Was it John Maynard Smith - The Major Transitions in Evolution? It's a great book, that goes deeper than Dawkins' vulgarizations.
  - jacquesm 16y ago
    I wouldn't call any of Dawkins' work a 'vulgarization'.
    - ced 16y ago
      Uh, yeah, I was wondering if that word was appropriate in English. I meant it in the sense of: to make (a technical or abstruse work) easier to understand and more widely known; popularize. (from dictionary.com)
      - jokermatt999 16y ago
        Vulgar has a very negative connotation to it. The main thing I associate with "vulgar" would be swearing, crassness, or uncouth behavior. I haven't seen the word vulgarization used really, but it inherits that contexts from its root word to me.
        ced 16y ago
        Likewise in French! But we still use the word "vulgarisation" to describe popular science books, in a good way.
GavinB 16y ago
Image that there are several different ways to meet these energy requirements, and mitochondria just happened to occur first. Mitochondrial life was first and so it had a chance to evolve and optimized itself. Any new systems that arose through random mutation would not have had a chance to evolve into a high-efficiency configuration and would not be able to compete. Other potential systems may exist and simply never got the foothold necessary to evolve into an efficient competitor. I would guess that it's quite likely we're at a local maximum, not the single and only possible way to organize cellular life.
- jimbokun 16y ago
  "Imagine that there are several different ways to meet these energy requirements" OK, what are they?
  - farnsworth 16y ago
    "Imagine..." It isn't necessary to know what they are. GavinB is just saying that there could be others.
    - shadowfox 16y ago
      That comment would be more worthwhile if there were some examples though
      - ovi256 16y ago
        He's in management dude, he has the grand vision, boring details are for Engineering.
      - InclinedPlane 16y ago
        He's explaining why we wouldn't see examples of such, even if they did exist. Whether they could exist or not is another point.
  - stcredzero 16y ago
    Remember your philosophy of science? Homework: is it possible to prove that something doesn't exist?
pavel_lishin 16y ago
So, we go from "Simple cells hardly ever engulf other cells" to "it has/will never happen anywhere in the universe"?
- hugh3 16y ago
  Blame the headline writer, but the actual article is still pretty interesting.
- stcredzero 16y ago
  What about the odds of one computer virus in the wild partially overwriting another one in memory and creating a novel strain? That one has happened.
Vivtek 16y ago
Mitochondrial symbiosis may only have happened once, but I'm not sure I buy that it's such a leap - mitochondria aren't the only symbiotic organelles. There are also chloroplasts. So unless chloroplasts are modified mitochondria, it seems that the eukaryotic leap happened at least twice. That tells me it's not all that improbable, or so I imagine.
- acqq 16y ago
  Exactly, the existence of chloroplasts (in the language of the the article: the "power generators" in every plant cell) is an obvious proof that "complex life probably evolved only once" can't be worse title.
- noglorp 16y ago
  Also, there are NON-symbiotic organelles as well! Why can't life elsewhere have created such organelles with identical function to mitochondria?
boredguy8 16y ago
The two key arguments in the paper are: 1) Cell membrane invagination isn't sufficient to overcome the energy well of prokaryotic cells; 2) Endosymbiosis is the only mechanism to provide the energy necessary to give cells a chance at complexity. http://www.nature.com/nature/journal/v467/n7318/full/nature09486.html http://www.nature.com/nature/journal/v467/n7318/full/nature0... In re: 1, "Mitochondria must respond quickly to changes in membrane potential and the penalty for any failure to do so is serious. The electron and proton transfers of chemiosmotic energy coupling generate a transmembrane potential of 150–200 mV over the membrane (~5 nm across), giving a field strength of about 30 million volt per metre, equal to that discharged by a bolt of lightning. This high membrane potential sets the inner membrane of bioenergetic organelles (mitochondria and chloroplasts) apart from all other eukaryotic membrane systems. Failure to maintain the mitochondrial membrane potential is penalized by a collapse in energy charge, blocking active transport across the cell membrane, and a rise in free-radical leak, which in eukaryotes and many prokaryotes leads directly to programmed cell death. .... "This requirement for physical association of genes with bioenergetic membranes to maintain ATP synthesis constrains both the genomes and the complexity of prokaryotes. If some genes for oxidative phosphorylation must be physically associated with a certain unit area of bioenergetic membranes, then beyond that threshold prokaryotes could not maintain membrane potential homeostasis unless additional genomes are co-localized with the membranes." In re: 2, "The main difference between endosymbiosis and polyploidy relates to the size and distribution of genomes over evolutionary time. In endosymbiosis, surplus organelle genes are lost or transferred to the host’s chromosomes, streamlining endosymbiont replication via cytoplasmic inheritance. The outcome is a massive reduction in genome size, both in prokaryotic endosymbionts and organelles, with a reciprocal relocation of genes in low copy number to nuclear chromosomes in the latter. By contrast, in giant polyploid prokaryotes, all genomes are essentially the same. Without cytoplasmic inheritance, no genomic specialization ensues. "In principle, prokaryotes could control respiration using specialized, membrane-associated plasmids that emulate organelle genomes in gene content and function. In practice, such plasmids are not found. Bacteria usually have small, high-copy-number plasmids that segregate randomly at cell division, or very few giant plasmids that co-segregate with chromosomes on filaments from midpoint. For plasmids in a prokaryote to support electron flux as organelle genomes do, high-copy-number giant plasmids encoding components of the electron-transport chain would need to associate with the plasma membrane, and evolve counter to the tendency to segregate with size rather than function. That no mtDNA-like plasmids are known indicates that high energetic barriers preclude their evolution: unlike organelles, which pay back energetically from the start, substantial energetic costs must be paid up front (high copy number of the correct plasmids, and the machinery to associate them with the membrane at regular intervals) before any energetic advantage can accrue. .... "Thus, being large and having masses of DNA is not enough to attain complexity: cells need to control energy coupling across a wide area of membranes using small, high copy, bioenergetically specialized genomes like mtDNA. Segregating the genes relinquished by the endosymbiont (mtDNA) into low copy number in the host’s chromosomes, specialization of the endosymbiont into an ATP-generating organelle and increasing organelle copy number provides sufficient energy per gene to support the evolution, maintenance and expression of some 10^5 more host genes, affording the cell the chance—but not the necessity—of becoming complex."
VladRussian 16y ago
there is some fallacy in such discussing of the precise state of chaotic dynamic systems. It is not possible to exactly reproduce the state we have with life on Earth, like it isn't possible to get precisely the same weather in 2 different days - both are results of unimaginably many values behaving and interacting in unique way and even minor change will change the resulting outcome . Yet there are a lot of days that look and feel very similar and there is 99.9999999% chances that tomorrow, for example, we wouldn't have 150 F in San-Francisco while we can't be sure what precisely it will be - 60 or 70 or 50? Does the life evolution system have such bound-ness and attractor-ness properties - well, as of now we can only guess. My guess is "yes". Btw. Uniqueness because of complexity, in the sense of "evolved only once", stinks like Creationism/ID
sovande 16y ago
This correlate fine with what we observe and with the Fermi paradox. That is, there are no indications of intelligent life elsewhere in the universe. When Frank Drake first turned his radio telescope to the sky and started to listen, he expected the ether to be full of radio signals from other civilizations. To his astonishment it was silent. Obviously there is a "great filter" in place which prevent intelligent life to evolve in abundance. And this article seems to point to such a filter. Nick Bostrom has an interesting philosophical take on this http://www.nickbostrom.com/papers/fermi.pdf http://www.nickbostrom.com/papers/fermi.pdf
msg 16y ago
Or, as Lane puts it: "The underlying principles are universal. Even aliens need mitochondria." Right, midichlorians are in all living things of sufficient complexity. Episode 1 was onto something? Kill me quickly.
dnautics 16y ago
I'm sorry, but the authors of this paper are totally clueless. "whenever simple cells start to become more complex" The definition of complexity here is irrigorous to the point of laughability. At one point the definition makes a call about "size" - " if a bacterium grew to the size of a complex cell, it would run out of juice" Nonsense. There was recently discovered a macroscopic bacterium - http://en.wikipedia.org/wiki/Thiomargarita_namibiensis http://en.wikipedia.org/wiki/Thiomargarita_namibiensis Yes, you can see them with your own eyes. "If the energy-producing machinery straddling the membrane is not constantly fine-tuned, it produces highly reactive molecules that can destroy cells. Yet fine-tuning a larger membrane is problematic because detecting and fixing problems takes longer." The solution, then is to create short-length response pathways. Presumably the "reactive molecule" they're talking about is superoxide - Life has evolved an incredibly awesome enzyme that takes care of it - superoxide dismutase - and even a slightly defective SOD doesn't lead to inviability. Certain types of photosynthetic bacteria, which need to "carefully manage reactivity" have solved this problem by creating 'hydrogenases' which bleed off excess energy in the form of hydrogen molecules (these may become useful in the alternative fuel industry). "Acquiring mitochondria, it seems, was a one-off event." Well, no. Plants later acquired chloroplasts. Some organisms have hydrogenosomes (which is inetresting, because we now have discovered vertebrates that seem to be able to survive off their hydrogenosomes, mitochondria be damned). Some bacteria have evolved anammoxasomes. "cells capable of complex functions – such as communicating with each other and having specialised jobs – could evolve" Clearly the author has not heard of quorum sensing. The notion that bacteria are hyperindividualistic robinson crusoes adrift in their environment is generally not believed anymore. http://en.wikipedia.org/wiki/Quorum_Sensing http://en.wikipedia.org/wiki/Quorum_Sensing Then the author redefines complexity to be a 'large genome' thing. Of course there really isn't anything stopping bacteria from having ploidy. Deincococcus Radiotolerans, for example, has 4-10 copies of its genome inside itself (for reasons that you might be able to guess from its name). http://en.wikipedia.org/wiki/Deinococcus_Radiotolerans http://en.wikipedia.org/wiki/Deinococcus_Radiotolerans And the whole complexity thing is bunk anyways. The actinomyces are such complex bacteria that for many many years scientists thought they were fungi. http://en.wikipedia.org/wiki/Actinomyces http://en.wikipedia.org/wiki/Actinomyces It's also worth noting, that there certainly are eukarya that are plenty 'complex' that simply don't have mitochondria. http://en.wikipedia.org/wiki/Giardia http://en.wikipedia.org/wiki/Giardia Better explanation: Eukarya were enabled because the Great Oxygen Catastrophe suddenly made being a paired oxidizer (mitochondrion)-reducer (host cell) beneficial. Prior to the oxygen catastrophe, mitochondrial cells would have only been able to survive in homeostasis with a cell that was happy to have its poop (oxygen) taken care of locally... Not to mention that eukarya are generally considered to have branched off of archae... The specialness of eukarya is that they were a unification of two highly divergent branches of the phylogenetic tree.

113 points by hartror 16y ago

eof 16y ago

This is crap. It doesn't say anything. Article tl;dr: Ignoring the inconceivably remote chance of life forming from non-life, it's very very rare for simple life to evolve into more complicated life. Also, we just realized that cells need to get bigger to get more complex, and that somehow isn't just 'another' mutation. And then the title? There is nothing about how or why complex life only evolved once. Just that it was unlikely. Chance of life forming from non life * age of the universe * number of stars * the chance a star has an earth like planet, is far from zero. I wouldn't even bother ranting but this was number 3 on the front page.

earnubs 16y ago
Isn't the optimistic guess at Drake 2 or something (correct me if I'm wrong)? That's not /very/ far from zero, and if you consider that it's the number in the Milky Way I'd say that's pretty very much close to zero.
danteembermage 16y ago
I upvote based on content not title. Perhaps a mod can change it to "New theory suggests Mitochondria precede complexity in early life" I definitely would classify engulfing mitochondria as not just another mutation, it's an entirely different thing.
- eof 16y ago
  Then everything is an entirely different thing. The simple act of a single cell engulfing mitochondria is not the birth of a species. Even on content I would bury this if I had the button for it, it makes pretty heavy claims without any evidence and necessarily brushing aside fundamental problems with the overall conclusion. I am not bashing the scientific article that this editorial is 'about'; but the article itself, besides a tiny nugget about the necessity of mitochondria being perhaps otherwise overlooked, it is just drivel.
pygy_ 16y ago
The original article [1] is titled "The energetics of genome complexity", which is much more reasonable. It argues that the energy processing of mitochondria was necessary in order for eukaryotes to expand in complexity. The article is behind a paywall, so I can't read the details. Anecdotically, there exist anaerobic eukariot cells that live on hydrogenosomes [2]. I don't know if they were originally aerobic and lost their mitochondria, or if they evolved in parallel. There also exist sea slugs that live in symbiosis with the chloroplasts of the algae they eat [3], so this kind of symbiosis has happened more than once. 1. http://www.nature.com/nature/journal/v467/n7318/full/nature09486.html http://www.nature.com/nature/journal/v467/n7318/full/nature0... 2. http://en.wikipedia.org/wiki/Hydrogenosome http://en.wikipedia.org/wiki/Hydrogenosome 3. http://www.plantphysiol.org/cgi/content/full/123/1/29 http://www.plantphysiol.org/cgi/content/full/123/1/29
- nollidge 16y ago
  One of the perks of working for a school... I'll try to summarize as best I can: The main calculation in the Nature paper is the energy budget of a cell. They give the example of E. coli: "to raise gene number tenfold, E. coli must also increase its energy budget by close to tenfold; and therein lies the problem." The reason so much more energy is required is due to protein synthesis (DNA codes to RNA which codes proteins, if you're unfamiliar with genetics). So obviously with a larger genome, there's more proteins to synthesize, and this cannot be accomplished by simply making fewer copies of the old proteins. If anything, you need more of some of those original proteins as infrastructure support for this new Genome 2.0. So (and this is my wording) prokaryotes are at a local maximum for genome and cell size. Chucking mitochondria into the picture provides the wattage (literally) for them to reach a much greater genome and cell size, and therefore organismic complexity.
  - anigbrowl 16y ago
    I am unclear about why the acquisition of mitochondria by a cell is considered to be such an unlikely event, as I lean towards the view that given enough cells/space/time, unlikely short term possibilities become distinct probabilities eventually. Also, I can't help wondering if the reason that simple cells are not observed to absorb free mitochondria is that eukaryotic cells and the organisms they evolved into have somehow poisoned the well, or attacked newly-complex competitors so aggressively that they've suppressed the population of omnivorous simple cells. But I'm no biologist. Am I missing something obvious, or is there a particular paper explaining why this mitochondrial upgrade is considered such an unlikely event?
zeteo 16y ago
"Chance of life forming from non life * age of the universe * number of stars * the chance a star has an earth like planet, is far from zero" You have no proof for this, and things can well be the other way round. Let n = age of the universe * number of stars * the chance a star has an Earth like planet n is a number that we currently have a pretty good guess at. Let p = chance of life forming from non life on Earth-like planet within a given year. We are basically clueless as to the value of p. p may well be much less than 1/n, which would make our biosphere very special indeed.
- eof 16y ago
  I have no proof for it, but I don't need it. The burden of proof is not on me. They are making the claim that 'complex life only happened once'. Life certainly does exist so p > 0. Since our understanding (or I should say, my under standing of 'our' understanding) of n is approaching infinity, it stands to reason that p x n is > 0.
  - zeteo 16y ago
    You've made a statement that a certain quantity is far larger than zero, so you do incur a burden of proof for that statement. p * n > 0 doesn't imply p * n >> 0, not even for large values of n (and "approaching infinity" doesn't apply to constants).
    - eof 16y ago
      Sorry, 'far from zero' is really meaningless and I shouldn't have said it that way. I don't actually mean to make a positive statement about the existence of complex life elsewhere in the universe, though intuitively I think it's absurd to believe our little planet is the only one with life on it. I mean, why would we believe that other than residual religious dogma? Our understanding of n is not a constant.
      - jerf 16y ago
        "I mean, why would we believe that other than residual religious dogma?" Because when we look out into the universe, it looks like we expect it to look when it is dead, not full of life, some significant percentage of which becomes intelligent. Nothing that looks like Dyson spheres of any form (recalling that the "solid shell" is actually only a special case), nothing that looks like curated stellar evolution (very high-end stuff but a truly advanced civilization will not want to leave stars just randomly flying around a galaxy and exploding), very very little that could even conceivably be a ship traveling between the stars (which turns out with any physically-plausible propulsion mechanism to be visible from a very long ways away and would show very clear blue/red shift changes), nothing that doesn't appear entirely natural. This isn't the whole story (maybe they're hiding, maybe our idea of the top end of tech is wrong, maybe they're all virtual, maybe they escaped into another, friendlier dimension, maybe they all kill themselves, maybe the universe is soaked with simple life but this article is essentially correct and complex intelligent life really is a 10^-1000 event, etc.), but it is not true that the claim that complex life must be hard is some sort of stupid idea in light of the size of the universe. Arguably it's actually the claim best supported by the evidence at this time. People are really bad at thinking about small probabilities even on the scale of one out of a million, and it's not impossible that complex intelligent life turns out to be one out of 10^1000 or worse; we simply don't know. It is not hard to come up with calculations that plausibly set the odds of advanced life occurring even once in the universe as less than 50/50 without having to make stuff up. Our ignorance dominates our knowledge here.
jokermatt999 16y ago
The last part of your post is known as the Drake Equation (http://en.wikipedia.org/wiki/Drake_equation http://en.wikipedia.org/wiki/Drake_equation). I'm guessing you've probably heard of it, but it could be of interest to other people, so I figured I'd link it.
- Roboprog 16y ago
  Tangent: the Drake Equation seems to be the template for a class of "Google interview" questions: estimate something you can't really know, at least off the top of your head, in which you then start to identify factors, and start to guess at upper and lower bounds, to come back with a plausible answer, or at least a range. I suppose a lot of research is like that, but the Drake just happens to deal with factors that are harder to measure.

hartror 16y ago

Still a sample size of one, my the drake equation is still looking rather full of letters. Though it is looking like we may be able to make a decent crack at fp (fraction of stars that have planets) soon given the success of keplar!

nooneelse 16y ago
In a TED talk, Dimitar Sasselov, a co-investigator on the Kepler mission, said their current best guess for the number of Earth-ish, habitable, planets in the galaxy is looking to be like 100 million (at about the t=9:20 mark in the talk). That is, naturally, an early guess from extrapolations and such, but still it is more based on something than the based-on-nothing and intuition guesses that sometimes float around in these discussions.

CapitalistCartr 16y ago

We know the odds of evolution are low; the point is that the number of stars in the Universe is astoundingly huge. With at least 10^20 stars in the Universe, those low odds get much more reasonable, especially now it's starting to look like planets are common.

Dove 16y ago
Do you know the actual odds, though? I mean, there's low and then there's low. If evolution on a habitable planet has a 10^-4 chance of happening, then the universe is almost certainly teeming with life. If it's 10^-18, the size of the universe is the difference between believing and doubting the idea. If it's 10^-200, the universe barely helps at all, and you need to start talking about billions upon billions of barren universes and we just happen to live in this one. Do you know what the number is? Do you know where it lies between 0.9 and 10^-(10^10^10)? Do you know whether the 20 orders of magnitude you get from the stars is overwhelming, decisive, or not even enough to make a dent?
- hugh3 16y ago
  To my way of thinking, the fact that life showed up on Earth relatively quickly, within half a billion years of the planet's formation, suggests that life is relatively easy. The fact that complex life took another few billion years after that suggests that simple to complex is the difficult step. But the fact that we nonetheless find ourselves here around a live-fast-die-young yellow star when the universe is only 14 billion years old rather than in close orbit around a dim red star a hundred billion years later suggests to me that it's probably not a one-in-every-zillion universes sort of event.
  - petervandijck 16y ago
    That sounds reasonable enough to me. [big bang --(we're here-ish)------------------------------------------------------- end of universe] So the facts that we're that far to the left of that line, and that the universe(s) is/are really really big, suggest that life isn't that rare.

billswift 16y ago

If you are really interested in this, read "Rare Earth", http://www.amazon.com/Rare-Earth-Complex-Uncommon-Universe/dp/0387952896/ref=sr_1_1?ie=UTF8&qid=1287666028&sr=8-1 http://www.amazon.com/Rare-Earth-Complex-Uncommon-Universe/d..., it goes through an incredible mass of astronomical, geological, and biological information. It concludes that, because so many things had to go just right, and so many others were completely random, primitive life (bacterial-level) is likely to be more common than previously thought, while advanced life (multicellular) is likely to be much less common. Because of the sheer size of the galaxy, it is probable that other intelligences evolved elsewhere, but they are probably going to be too far away for any sort of contact, even receipt of potentially meaningful radio. It is a well-written book, despite the amount of information it is very readable.

erikstarck 16y ago

The number of bacteria on earth is estimated to be 5000000000000000000000000000000. This is five million trillion trillion. All of them are eating. Some of them are mutating. Let them do this for a few billions of years and what seems to be a very unlikely event suddenly becomes almost certain to occur. Complex life FTW!

Dove 16y ago
You've got what there, 10^16? That is certainly a very big number. But then, the article is claiming that the chance of becoming complex is a very small number. Is it less than 10^-16? (Is it even knowable? At a minimum, the author thinks he can say something about it.) You'd be surprised how fast these numbers get small if you need to brute force yourself away from a local maximum and over a valley. I don't care how big your bignum is compared to everyday experience. Bignum * Smallnum does not automatically equal certainty, impossibility, or feasibility. It means do the math.

jacquesm 16y ago

One of the best reasons I ever read about why it happened only once is that if it had happened twice the newcomer would have been called 'food' at a stage too early to get much further up the ladder. For it to happen twice the 'new' trail would have to be totally non-nutritious to the established kind. I wished I remembered where I read it.

eru 16y ago
Yes, though you can also be non-edible by being out of the way.
ced 16y ago
Was it John Maynard Smith - The Major Transitions in Evolution? It's a great book, that goes deeper than Dawkins' vulgarizations.
- jacquesm 16y ago
  I wouldn't call any of Dawkins' work a 'vulgarization'.
  - ced 16y ago
    Uh, yeah, I was wondering if that word was appropriate in English. I meant it in the sense of: to make (a technical or abstruse work) easier to understand and more widely known; popularize. (from dictionary.com)
    - jokermatt999 16y ago
      Vulgar has a very negative connotation to it. The main thing I associate with "vulgar" would be swearing, crassness, or uncouth behavior. I haven't seen the word vulgarization used really, but it inherits that contexts from its root word to me.
      - ced 16y ago
        Likewise in French! But we still use the word "vulgarisation" to describe popular science books, in a good way.

GavinB 16y ago

Image that there are several different ways to meet these energy requirements, and mitochondria just happened to occur first. Mitochondrial life was first and so it had a chance to evolve and optimized itself. Any new systems that arose through random mutation would not have had a chance to evolve into a high-efficiency configuration and would not be able to compete. Other potential systems may exist and simply never got the foothold necessary to evolve into an efficient competitor. I would guess that it's quite likely we're at a local maximum, not the single and only possible way to organize cellular life.

jimbokun 16y ago
"Imagine that there are several different ways to meet these energy requirements" OK, what are they?
- farnsworth 16y ago
  "Imagine..." It isn't necessary to know what they are. GavinB is just saying that there could be others.
  - shadowfox 16y ago
    That comment would be more worthwhile if there were some examples though
    - ovi256 16y ago
      He's in management dude, he has the grand vision, boring details are for Engineering.
    - InclinedPlane 16y ago
      He's explaining why we wouldn't see examples of such, even if they did exist. Whether they could exist or not is another point.
- stcredzero 16y ago
  Remember your philosophy of science? Homework: is it possible to prove that something doesn't exist?

pavel_lishin 16y ago

So, we go from "Simple cells hardly ever engulf other cells" to "it has/will never happen anywhere in the universe"?

hugh3 16y ago
Blame the headline writer, but the actual article is still pretty interesting.
stcredzero 16y ago
What about the odds of one computer virus in the wild partially overwriting another one in memory and creating a novel strain? That one has happened.

Vivtek 16y ago

Mitochondrial symbiosis may only have happened once, but I'm not sure I buy that it's such a leap - mitochondria aren't the only symbiotic organelles. There are also chloroplasts. So unless chloroplasts are modified mitochondria, it seems that the eukaryotic leap happened at least twice. That tells me it's not all that improbable, or so I imagine.

acqq 16y ago
Exactly, the existence of chloroplasts (in the language of the the article: the "power generators" in every plant cell) is an obvious proof that "complex life probably evolved only once" can't be worse title.
noglorp 16y ago
Also, there are NON-symbiotic organelles as well! Why can't life elsewhere have created such organelles with identical function to mitochondria?

boredguy8 16y ago

The two key arguments in the paper are: 1) Cell membrane invagination isn't sufficient to overcome the energy well of prokaryotic cells; 2) Endosymbiosis is the only mechanism to provide the energy necessary to give cells a chance at complexity. http://www.nature.com/nature/journal/v467/n7318/full/nature09486.html http://www.nature.com/nature/journal/v467/n7318/full/nature0... In re: 1, "Mitochondria must respond quickly to changes in membrane potential and the penalty for any failure to do so is serious. The electron and proton transfers of chemiosmotic energy coupling generate a transmembrane potential of 150–200 mV over the membrane (~5 nm across), giving a field strength of about 30 million volt per metre, equal to that discharged by a bolt of lightning. This high membrane potential sets the inner membrane of bioenergetic organelles (mitochondria and chloroplasts) apart from all other eukaryotic membrane systems. Failure to maintain the mitochondrial membrane potential is penalized by a collapse in energy charge, blocking active transport across the cell membrane, and a rise in free-radical leak, which in eukaryotes and many prokaryotes leads directly to programmed cell death. .... "This requirement for physical association of genes with bioenergetic membranes to maintain ATP synthesis constrains both the genomes and the complexity of prokaryotes. If some genes for oxidative phosphorylation must be physically associated with a certain unit area of bioenergetic membranes, then beyond that threshold prokaryotes could not maintain membrane potential homeostasis unless additional genomes are co-localized with the membranes." In re: 2, "The main difference between endosymbiosis and polyploidy relates to the size and distribution of genomes over evolutionary time. In endosymbiosis, surplus organelle genes are lost or transferred to the host’s chromosomes, streamlining endosymbiont replication via cytoplasmic inheritance. The outcome is a massive reduction in genome size, both in prokaryotic endosymbionts and organelles, with a reciprocal relocation of genes in low copy number to nuclear chromosomes in the latter. By contrast, in giant polyploid prokaryotes, all genomes are essentially the same. Without cytoplasmic inheritance, no genomic specialization ensues. "In principle, prokaryotes could control respiration using specialized, membrane-associated plasmids that emulate organelle genomes in gene content and function. In practice, such plasmids are not found. Bacteria usually have small, high-copy-number plasmids that segregate randomly at cell division, or very few giant plasmids that co-segregate with chromosomes on filaments from midpoint. For plasmids in a prokaryote to support electron flux as organelle genomes do, high-copy-number giant plasmids encoding components of the electron-transport chain would need to associate with the plasma membrane, and evolve counter to the tendency to segregate with size rather than function. That no mtDNA-like plasmids are known indicates that high energetic barriers preclude their evolution: unlike organelles, which pay back energetically from the start, substantial energetic costs must be paid up front (high copy number of the correct plasmids, and the machinery to associate them with the membrane at regular intervals) before any energetic advantage can accrue. .... "Thus, being large and having masses of DNA is not enough to attain complexity: cells need to control energy coupling across a wide area of membranes using small, high copy, bioenergetically specialized genomes like mtDNA. Segregating the genes relinquished by the endosymbiont (mtDNA) into low copy number in the host’s chromosomes, specialization of the endosymbiont into an ATP-generating organelle and increasing organelle copy number provides sufficient energy per gene to support the evolution, maintenance and expression of some 10^5 more host genes, affording the cell the chance—but not the necessity—of becoming complex."

VladRussian 16y ago

there is some fallacy in such discussing of the precise state of chaotic dynamic systems. It is not possible to exactly reproduce the state we have with life on Earth, like it isn't possible to get precisely the same weather in 2 different days - both are results of unimaginably many values behaving and interacting in unique way and even minor change will change the resulting outcome . Yet there are a lot of days that look and feel very similar and there is 99.9999999% chances that tomorrow, for example, we wouldn't have 150 F in San-Francisco while we can't be sure what precisely it will be - 60 or 70 or 50? Does the life evolution system have such bound-ness and attractor-ness properties - well, as of now we can only guess. My guess is "yes". Btw. Uniqueness because of complexity, in the sense of "evolved only once", stinks like Creationism/ID

sovande 16y ago

This correlate fine with what we observe and with the Fermi paradox. That is, there are no indications of intelligent life elsewhere in the universe. When Frank Drake first turned his radio telescope to the sky and started to listen, he expected the ether to be full of radio signals from other civilizations. To his astonishment it was silent. Obviously there is a "great filter" in place which prevent intelligent life to evolve in abundance. And this article seems to point to such a filter. Nick Bostrom has an interesting philosophical take on this http://www.nickbostrom.com/papers/fermi.pdf http://www.nickbostrom.com/papers/fermi.pdf

msg 16y ago

Or, as Lane puts it: "The underlying principles are universal. Even aliens need mitochondria." Right, midichlorians are in all living things of sufficient complexity. Episode 1 was onto something? Kill me quickly.

dnautics 16y ago

I'm sorry, but the authors of this paper are totally clueless. "whenever simple cells start to become more complex" The definition of complexity here is irrigorous to the point of laughability. At one point the definition makes a call about "size" - " if a bacterium grew to the size of a complex cell, it would run out of juice" Nonsense. There was recently discovered a macroscopic bacterium - http://en.wikipedia.org/wiki/Thiomargarita_namibiensis http://en.wikipedia.org/wiki/Thiomargarita_namibiensis Yes, you can see them with your own eyes. "If the energy-producing machinery straddling the membrane is not constantly fine-tuned, it produces highly reactive molecules that can destroy cells. Yet fine-tuning a larger membrane is problematic because detecting and fixing problems takes longer." The solution, then is to create short-length response pathways. Presumably the "reactive molecule" they're talking about is superoxide - Life has evolved an incredibly awesome enzyme that takes care of it - superoxide dismutase - and even a slightly defective SOD doesn't lead to inviability. Certain types of photosynthetic bacteria, which need to "carefully manage reactivity" have solved this problem by creating 'hydrogenases' which bleed off excess energy in the form of hydrogen molecules (these may become useful in the alternative fuel industry). "Acquiring mitochondria, it seems, was a one-off event." Well, no. Plants later acquired chloroplasts. Some organisms have hydrogenosomes (which is inetresting, because we now have discovered vertebrates that seem to be able to survive off their hydrogenosomes, mitochondria be damned). Some bacteria have evolved anammoxasomes. "cells capable of complex functions – such as communicating with each other and having specialised jobs – could evolve" Clearly the author has not heard of quorum sensing. The notion that bacteria are hyperindividualistic robinson crusoes adrift in their environment is generally not believed anymore. http://en.wikipedia.org/wiki/Quorum_Sensing http://en.wikipedia.org/wiki/Quorum_Sensing Then the author redefines complexity to be a 'large genome' thing. Of course there really isn't anything stopping bacteria from having ploidy. Deincococcus Radiotolerans, for example, has 4-10 copies of its genome inside itself (for reasons that you might be able to guess from its name). http://en.wikipedia.org/wiki/Deinococcus_Radiotolerans http://en.wikipedia.org/wiki/Deinococcus_Radiotolerans And the whole complexity thing is bunk anyways. The actinomyces are such complex bacteria that for many many years scientists thought they were fungi. http://en.wikipedia.org/wiki/Actinomyces http://en.wikipedia.org/wiki/Actinomyces It's also worth noting, that there certainly are eukarya that are plenty 'complex' that simply don't have mitochondria. http://en.wikipedia.org/wiki/Giardia http://en.wikipedia.org/wiki/Giardia Better explanation: Eukarya were enabled because the Great Oxygen Catastrophe suddenly made being a paired oxidizer (mitochondrion)-reducer (host cell) beneficial. Prior to the oxygen catastrophe, mitochondrial cells would have only been able to survive in homeostasis with a cell that was happy to have its poop (oxygen) taken care of locally... Not to mention that eukarya are generally considered to have branched off of archae... The specialness of eukarya is that they were a unification of two highly divergent branches of the phylogenetic tree.