Why Complex Life Is Rare

I’ve said before that we have pretty good evidence for off-Earth bacteria life, suggesting that such life is common in the nearby universe. However, bacterial life might be common, yet complex multi-cellular life very rare. Here’s a plausible detailed theory about why:

Under conditions typical of alkaline hydrothermal vents, the combining of H2 and CO2 to produce the molecules found in living cells – amino acids, lipids, sugars and nucleobases – actually releases energy. … Life … is an inevitable consequence of a planetary imbalance, in which electron-rich rocks are separated from electron-poor, acidic oceans by a thin crust, perforated by vent systems that focus this electrochemical driving force into cell-like systems. The planet can be seen as a giant battery; the cell is a tiny battery built on basically the same principles. … The origin of life needs a very short shopping list: rock, water and CO2. … The universe should be teeming with simple cells. …

The problem that simple cells face is this. To grow larger and more complex, they have to generate more energy. The only way they can do this is to expand the area of the membrane they use to harvest energy. To maintain control of the membrane potential as the area of the membrane expands, though, they have to make extra copies of their entire genome – which means they don’t actually gain any energy per gene copy. …

Eukaryotes get around this problem by acquiring mitochondria, … containing both the membrane needed to make ATP and the genome needed to control membrane potential. … They were stripped down to a bare minimum. … Mitochondria originally had a genome of perhaps 3000 genes; nowadays they have just 40 or so genes left. For the host cell, it was a different matter. As the mitochondrial genome shrank, the amount of energy available per host-gene copy increased and its genome could expand. …

We know it happened just once on Earth because all eukaryotes descend from a common ancestor. The emergence of complex life, then, seems to hinge on a single fluke event – the acquisition of one simple cell by another. … The outcome was by no means certain: the two intimate partners went through a lot of difficult co-adaptation before their descendants could flourish. This does not bode well for the prospects of finding intelligent aliens. (more)

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  • Briancpotter

    “The problem that simple cells face is this. To grow larger and more complex, they have to generate more energy. The only way they can do this is to expand the area of the membrane they use to harvest energy. To maintain control of the membrane potential as the area of the membrane expands, though, they have to make extra copies of their entire genome – which means they don’t actually gain any energy per gene copy.”

    I don’t understand this portion – why should a cell ‘care’ how much energy per gene copy it has?

    • V V

       Indeed, it doesn’t.

      The issue is actually a classical square-cube relation between the cell surface and its volume (and mass), which limits the maximum cell size.

      Some bacteria do form simple colonial structures, however, only eukaryotes have the surface signalling mechanisms that allow the degree of coordination needed to form the complex anatomical features of a truly multicellular organism.

    • Dave 944

       Who’d a thunk it! Forget explanatory modesty. Those darned membranes are why Martians an never exist except as germs. 

  • V V

    Actually, the embedding of an endosymbiotic bacterium happened at least two other times:

    The ancestor of all plants and various eukaryotic algae (archaeplastida) included free-living cyanobacteria, which become chloroplasts.
    Another inclusion of cyanobacteria by an eurkaryote happened more recently in the ameboid paulinella chromatophora.

    • http://overcomingbias.com RobinHanson

      He didn’t claim that was the only embedding. A fuller quote is:
      “These huge genomes provided the genetic raw material that led to the evolution of complex life. Mitochondria did not prescribe complexity, but they permitted it. It’s hard to imagine any other way of getting around the energy problem – and we know it happened just once on Earth because all eukaryotes descend from a common ancestor.”

      • Carl Shulman

        You, however, claimed that it was a “plausible” explanation for complex life being “very rare.” If such endosymbiosis happens multiple times (and perhaps even more often in lineages that have since gone extinct, perhaps losing to the ancestors of today’s eukaryotes) then the base rate for mitochondria formation looks much too high to explain very much of the Great Filter. 

      • http://overcomingbias.com RobinHanson

        It seems to be a certain sort of embedding that he claims is rare. This isn’t my theory it is his – read him for more details.

  • http://newstechnica.com David Gerard

    Your first link is to a blog post about the Journal of Cosmology, a crank journal of breathtakingly low quality. Your second link is to a New Scientist article about the same article in the Journal of Cosmology. We do not in fact have good evidence for panspermia.

    • http://www.uweb.ucsb.edu/~criedel/ Jess Riedel

      Yep.  After re-reading those two posts, I am just blown away that Robin Hanson can (correctly) criticize people for refusing to accept the overwhelming expert consensus on various topics, and then come back and say that there’s good evidence for panspermia based on this one silly article (the non-acceptance of which he attributes, of course, to status reasons).

      • http://newstechnica.com David Gerard

        I commend to all readers the Wikipedia article on the Journal and on the paper in question which is the reason for its fame. It’s remarkably pointed for a Wikipedia article.

        Or, as PZ Myers described it: “the ginned-up website of a small group of crank academics obsessed with the idea of Hoyle and Wickramasinghe that life originated in outer space and simply rained down on Earth.”

        Consider this a cautionary example of the non-transferability of expertise and of ability to assess quality in fields not one’s own.

      • http://entitledtoanopinion.wordpress.com TGGP

        I was disappointed in the section on the Hoover paper. It simply said there was a lot of criticism (and had citations as evidence), but what in particular was the reason for the criticism was never stated. Could be wikipedia deletionists, could be the nature of the controversy would be over the heads of most readers.

      • http://newstechnica.com David Gerard

        TGPP – Wikipedia bends over backwards to appear neutral. Saying outright “this is gibberish from a ridiculous pseudojournal” is not likely to last very long without reprocessing and a string of blue numbers. I think you can still see it there, though.

      • http://overcomingbias.com RobinHanson

        I read the article itself, and found it reasonable. As TGGP indicates, the wikipedia article contains no substantive criticisms.

      • dmytryl

        It so happens that the way human brain works, the correctness requires more scepticism (especially towards one’s own conclusions), not less, but the rationalism is the belief in the greater power of reason than customarily assumed, and calls for less scepticism towards ‘reason'; the reason unfortunately being largely subjective in any topics that are too big to be tractable. Consequently the tendency towards such failure mode.

        When you read atrociously bad biological article and it asserts that something only happens once, you can’t subject this to scrutiny unless you already know it happened more than once; internally you can’t evaluate it’s validity, but it feels intuitively that you can as you often evaluate validity of less bad articles that very rarely get their assertions wrong, and which can be evaluated on the purely logical merits alone.

  • daedalus2u

    Intracellular infection is not rare.
    Malaria infects red blood cells, TB is an intracellular infection, so
    is Lyme, and Q fever.

    Rickettsiae
    are mostly intracellular, and Wolbachia
    infects tens of percent of insects where it modifies reproduction by
    allowing or causing parthenogenesis.

    It
    may be that there were many different intracellular acquisitions of
    mitochondria-like organisms. That all eukaryotes are descended from
    a single instance does not mean there were not others which have
    since died out.

    • Mark M

      Our eukaryotes ate all the others.  We win!

  • SurfSmurf

    The fact that it occurred just once is not an argument on how likely it is. Once it occurred, the available niches would likely be filled with those getting the breakthrough. These would crowd out new discoveries.

    • Ilya Shpitser

      If so, why is that heterotrophic amoeba well on its way to acquiring a photosynthetic organelle (apparently the original event was about 60 mya?).  The question is, why did we only observe this happening only once since the first time (maybe we aren’t looking in the right place, most single celled life is unknown and in the oceans).

  • Kurt9

    The New Scientist article is written by Nick Lane, who has written a book about Mitochondria and the emergence of the Eularyote. And, yes, I think he is correct and that complex life is really, really rare.

  • Mark M

    It took us billions of years to get where we are today.  Given the length of this time frame and the number of planets in the universe, you would expect that our improbable eukaryotic fluke to happen over and over and over again.

    There are lots of reasons why our prospects of finding intelligent aliens are low that have nothing to do with this lame evolutionary excuse.

    There must first be the raw materials, the building blocks of life, and then the environment conducive to creating and then supporting some sort of life.  Whatever emerges must survive several mass-extinction events while developing into intelligent creatures.  Then they must avoid destroying themselves.  Then they can tackle the possibly insurmountable problems of interstellar space flight.  Then, if they happen to be close enough and look in our direction at the right time, they might find yet another planet to add to their long list of planets to investigate.

    In the end, creatures that have technology fantastic enough to find and visit us may not even think we’re very interesting.  Instead of the classic “mostly harmless” description, after observing our political landscape it seems more likely our planet would be considered “not worth the trouble.”

    • http://twitter.com/omslin m n

      > It
      took us billions of years to get where we are today.  Given the length
      of this time frame and the number of planets in the universe, you would
      expect that our improbable eukaryotic fluke to happen over and over and
      over again.Nope, does not follow.

      • Mark M

        Eukarites developed on Earth so we know that it’s not impossible, however improbable.

        Planets are not rare.  The building blocks of life are not rare.  They’ve had billions of years to interact and evolve.

        It seems unimaginably unlikely that any naturally occurring mix of chemicals and conditions occurred exactly once on exactly one planet over the life of the universe.  (Hey!  If I stop here, I could be a creationist!)

        I’m not saying it’s common – I’m saying a Eukarite barrier is unlikely to be the reason we haven’t heard from aliens.  It’s far more likely that mass extinction events and the vast distances involved have made it physically impossible or at least very difficult for aliens to contact us.

  • Sigivald

    “Why Complex Life Is Rare” vs. “However, bacterial life might be common, yet complex multi-cellular life very rare”.

    Is vs. Might is rather important, isn’t it?

    (Plus what Mark M. said – all we really know about life in the universe is that none of it’s said hello yet.

    We can’t tell from that that “complex multi-cellular life is very rare” or “intelligence is very rare”.

    Only that “intelligence sufficient to communicate with us hasn’t successfully done so within our light-cone*”, a radically more restrained fact.)

    (* Might not be any in our light-cone that has sent a signal in a band we watch in the time we’ve been able to watch. Or might be far enough away that they don’t have enough power for us to detect it.

    Or might not want to signal – ability is not the same as desire. Or might not be any anywhere in the entire universe apart from us, ever before or after.

    See also Lem’s Fiasco.)

    • V V

       We have good reasons to hypothesise that interstellar travel is essentially technologically impossible, or at least uneconomical, for pretty much any type of civilization we can imagine.
      So we don’t expect direct contact.

      Remote detection is impossible with our current detection technology, unless the aliens happened to be on a close star (something we can already rule out, IIUC) or used a technology that radiated many orders of magnitude more power than ours. Similarly, aliens with a detection technology similar to ours, wouldn’t be able to detect us.

      Detection technology can improve with time, but there are physical limits.

      • Brian Huntington

         Interstellar travel is certainly not technologically impossible.  It is extremely unlikely to be viable for complex organisms a la “Star Trek”, but reproducing probes powered by nuclear engines should definitely be viable. Even our feeble technology circa the 1970s was able to build the voyager probe currently on the edge of the solar system. Technology centuries hence will certainly be able to reach other star systems with some form of reproducing entity.

      • V V

         

        reproducing probes powered by nuclear engines should definitely be viable.

        That’s dubious. A probe would need to function continuously for thousands of years during the transit between one star and another, periodically performing course correction (hence expending energy and reaction mass), in an environment with cosmic rays and ablative interstellar dust.
        While most stars are hypothesised to have planets, they might not have easly retrievable uranium and thorium.

        Even our feeble technology circa the 1970s was able to build the voyager probe currently on the edge of the solar system.

        The Voyager 1 will stop working in a few years or decades at most, then it will just drift away. It’s not going anywhere.

        Technology centuries hence will certainly be able to reach other star systems with some form of reproducing entity.

        Technology is not magic.
        A few centuries from now, they will possibly have discovered the “theory of everything”, and solved various current open problems, but with high probability, most of their physics, in particular the physics relevant to space travel, will be pretty much the same as our physics.

        Our space technology quickly reached maturity in just a few decades, how much room for improvement might there be? We don’t expect future probes to be orders of magnitude faster or more energy efficient than the current ones.

        There might be significant improvements in automation technology, but in a barren, hostile environment with extremely diluited resources, even being super-smart and flexible isn’t going to help you very much.

      • Brian Huntington

        V V: We are still using chemical rockets, even though we already know how to make vastly more powerful nuclear rockets. We’ve plateaued where we are in space travel for the time being, but that is not unusual even when there is great progress to be made. Sailboats were the “mature” ocean travel tech for a long time, until they weren’t and vastly more powerful motor-tech came a long. 

        You just seem to have a provincial belief that we are near the peak tech. level for space travel. There is no real justification for this. And yes, future probes will be vastly faster. We currently aren’t even using the fission rockets engines we already know how to build.

      • V V

         

        We are still using chemical rockets, even though we already know how to make vastly more powerful nuclear rockets.

        This claim is too bold.
        NASA conducted preliminary experiments on nuclear rockets, then politicians lost interest and the project was abandoned. Initial results were encouraging, but this still remains a speculative and largely unproven technology.

        Sailboats were the “mature” ocean travel tech for a long time, until
        they weren’t and vastly more powerful motor-tech came a long.

        Yes, but this required harnessing a new energy source. Maybe they’ll invent a fusion-antimatter-green rocks engine in the future, but I’m not holding my breath.

        You just seem to have a provincial belief that we are near the peak
        tech. level for space travel.

        Calling a belief “provincial” doesn’t diminish its likelihood.

        And yes, future probes will be vastly faster. We currently aren’t even
        using the fission rockets engines we already know how to build.

        According to Wikipedia ( http://en.wikipedia.org/wiki/Nuclear_thermal_rocket#Nuclear_vs._chemical ) the nuclear rocket design NASA was working on was expected to yield relatively moderate (same order of magnitude) performance improvements with respect to the Saturn V.

  • http://www.facebook.com/troy.camplin Troy Camplin

    Complex life is in fact highly probable. Once the environment at a particular level is filled, the only way, in a real sense, is “up,” as in up in complexity. Evolution drives complexity.

    • http://twitter.com/omslin m n

      That’s nonsensical.

  • daedalus2u

     We can make estimates about how easy it is for life to evolve by looking at how long it took for life to evolve on Earth.  

    Prokaryotes evolved just about as soon as the Earth cooled below the boiling point, a few hundred million years.  Eukaryotes evolved just about as soon as the Earth had an atmosphere containing more than a few percent O2.  If we consider the appearance of life to be stochastic and depend upon the volume of suitable habitat and the length of time that the habitat exists.  

    For an Earth surface area times a biosphere depth (couple km) and 0.2 billion years, even if we assign a 10% likelihood, the chances of life emerging on another planet in 2 billion years becomes quite high (1- (1-0.1)^10 = ~65%).  If this calculation is correct, then the likelihood of bacterial life on Mars is pretty high.  There is still a very large habitable zone on Mars, beneath the surface where it is warm enough for liquid water to circulate but not so hot that proteins can’t form.  The volume of habitable zone on Mars might even be larger than the volume on Earth because Mars is cooler.  Mars cooled faster, and probably did not experience the kind of complete melting that happened during moon formation. 

    • http://profiles.google.com/daedalus4u David Whitlock

      If this analysis is correct, the likelihood of life on Mars now is pretty high.  Mars has had a habitable zone for 4.5 billion years.  If there is a 10% chance of life developing every 200 million years (the way it did on Earth), then the likelihood of life on Mars is about 1- (1-0.1)^(4.5/0.2) = 90%.