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Life has only developed once in the history of the Earth.

  1. Aug 4, 2011 #1
    Wouldn't that suggest that life is a rare event in the universe? If it were common, shouldn't we expect it to have developed more than once on Earth?
  2. jcsd
  3. Aug 4, 2011 #2


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    Can you cite the scientific reference for your claim in the thread title?
  4. Aug 4, 2011 #3


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    If new life were to start to develop today, or anytime after life was well established here on Earth, it would almost certainly get eaten by the more advanced organisms which are laready here before it had a chance to develop further. So we really can't say that life has only developed once on Earth.
  5. Aug 4, 2011 #4

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    For starters, practically every biologist since Darwin, who wrote in Origin of Species that "all the organic beings which have ever lived on this earth have descended from some one primordial form."

    More recently, there is the May 13 2010 article in Nature by Douglas Theobald, A formal test of the theory of universal common ancestry. Abstract here: http://www.nature.com/nature/journal/v465/n7295/abs/nature09014.html. Theobald calculates that a universal common ancestor is at least 102,860 times more probable than having multiple ancestors. You can find full copies of the paper on the 'net, but I'm not publishing those links as I don't know if they are legit.

    That all existing life has a universal common ancestor does not necessarily mean that the origin of life is extremely rare. It could just mean that once life did originate, it locked out any independent subsequent formation of life. If life did formed independently after life originated, that new life may well have just been a new (but fleeting) food source to the already existing life.

    Another issue here is "what is life?" Viruses are currently not thought of as a form of "life".
  6. Aug 4, 2011 #5
    The assumption here seems to be that life originated on earth. Why? Is it not more probable that life originated much earlier on one of millions of planets in our galaxy, and spead through our galaxy over many more millions of years as a result of collisions that released little bits of resilient life into space?

    I actually don't know how to calculate the odds of this, but giving millions of planets instead of just one the chance to start life outwardly appears to improve the bet.
  7. Aug 4, 2011 #6


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    We don't yet have a comprehensive theory of abiogenesis but the understanding is good enough to suggest that life evolved here out of the chemical soup of the seas back then rather than landing in meteorites. I also don't really see how it's more likely life evolved elsewhere and got here rather than evolving here.
  8. Aug 4, 2011 #7


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    So do we expect then, that several lifeforms of the same phenotype were generated on Earth's surface around the same time?

    Or instead, that there one single organism that all life came from?

    To we expect that the first life form was a big soup that split into several life forms or that the the first life form was a single, self-contained, lipid-enclosed rybozyme reaction?

    Or that several lifeforms existed at abiogenesis but only one survived long enough to make genetic history?
  9. Aug 4, 2011 #8

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    This thread is not about astronomy. It is about life on Earth, and perhaps about astrobiology. I am moving this to biology where some more versed in biology will be more likely to answer the question.
  10. Aug 4, 2011 #9
    "Viruses are currently not thought of as a form of "life"."

    Well, some viruses have a genome bigger than some bacteria, there's currently controversy over nano-bacteria, and the 'protein folding' plaques called prions appear to reproduce and infect...
    Some interesting links at... http://en.wikipedia.org/wiki/Nanobacterium

    My opinion is that viruses are obligate parasites, hence the current definition of 'life' is inadequate...

    OT: It's a bit like the way the recent re-definition of 'planet' sadly excluded Pluto, but may yet need clarification to accommodate 'Trojan' objects, extended 'halo' orbits, resonance configurations and other outré members of our solar system's zoo...
  11. Aug 4, 2011 #10


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    There are some scientists in fact that are searching far and wide on Earth for current forms of life that may be not the same as what we evolved from.

    Their theory is that, while they may masquerade as current lifeforms (bacteria, algae), they won't be distinguished in any normal tests, since those tests are not looking for the right things.
  12. Aug 4, 2011 #11
    This is a legit propsal known as "cenancestor'. This only speaks of modern life forms though. Basically the suggestion is that it is much more probable that all life on earth has come from a single common ancestor, instead of from multiple. This does not mean that there was a single original life form. Life could have formed multiple times one after the other, all at the same time, all over the place. Until eventually one (our common ancestor) was able to somehow get a stronger foothold (probably some sort of special protein developed) and probably out competed other life.

    This is supported by many things which are compared between all current life forms on earth. (number of amino acids, usage of proteins, usage of DNA, double helix DNA, usage of RNA, etc. etc..)
  13. Aug 5, 2011 #12
    My understanding is that it is considered quite likely that life did arise on multiple occasions at the time when the circumstances were right for abiogenesis – and that is a key point, I think it is broadly accepted that the circumstances for abiogenesis do not exist now. It is just that all life surviving today is descended from just one of those occasions. Much in the same way that all non-indigenous African human beings alive today are descendants of a single migration out of Africa, traced to something like 70,000 years ago, but that does not mean that there was only one migration out of Africa. There were several migrations before that one, but there are no surviving descendants of the earlier migrations.
  14. Aug 5, 2011 #13


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    But was it a single organism or multiple instances of a single species that arose at abiogenesis?

    That is, were the conditions pervasive across Earth or only occured in "hot spots"?
  15. Aug 5, 2011 #14
    Would you like to define what you consider a species at the time of abiogenesis???

    This thread also still needs a working definition of life. Were the organisms created billions of years ago living? They depended more on sheer luck and physical forces than anything. (temperature, current, ph levels... etc.)
  16. Aug 5, 2011 #15

    As Ryan has said, the simple truth is that there are no certainties, nor even any particularly strong evidence to support one idea over another. So to some degree all of this remains speculative. Clearly, some ideas do have a stronger scientific basis than others, but it is also possible that the answers will never be known with any scientific certainty. There is a man whose ideas in this regard seem to be among the more scientifically respected, and he is called Tibor Ganti. Suffice to say, his writings do not make light reading.

    In any case, one of the ideas I recall hearing centred around – I’m not sure what the precisely correct term is but essentially something like a volcanic funnel in the deep ocean. I’m struggling to remember here, but there has been a relatively recent discovery around hot springs somewhere on the surface of some organism that is thought to bear some resemblance to early life forms. I think it is broadly accepted that prokaryotes came first and eukaryotes were an early piece of evolution.

    In The Blind Watchmaker, Dawkins describes a process by how crystalline mud could develop into life. Dawkins always made it clear that he was not proposing this as an actual explanation of abiogenesis, only as a demonstration of how it could happen unguided by an intelligent force.
  17. Aug 5, 2011 #16


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    Semantics are not particularly important as long as you can describe the process. I'll give my picture that has developed so far from lectures, skimming abstracts, and water-cooler talk. Maybe we can look work it from the other direction (correcting mistakes or unreasonable belief):

    Say we have a "soup" (i.e. in a Miller-Urey fashion, we have a self organized set of organic tools). There's a lot of crazy reactions going on Ribozymes and amino acids being key players:

    Q1: are these reactions already happening without lipid membranes containing them, just free-floating? Or do they require lipid membranes to have any kind of mentionable lifetime?

    At some point, a particular permutation of this particles/energy dance (one of the lipid-contained reactions) bifurcates into a set of rich replication dynamics (probably involving a high rate of mutation). Several such systems are doing this all from the same basic set of matter, but each has a slightly different permutation of the matter. As we know bacteria do, these primitive "lifeforms" (don't really care about the semantics) are consuming each other and mixing and matching this system of particle permutations; of course, some are just sputtering out without external influence because their dynamics have a short life time inherently.

    A particular permutation begins to dominate the others, perhaps consuming some, incorporating whole functional bits of others...

    Or perhaps, due to a lucky adaptation, one of the weaker configurations is the only one to survive Earth's early transitional period, and simply goes around, eating up and incorporating the remains of the failed system...

    Anyway, that's the intuition I've built for it, but I don't feel confident enough in the literature to say whether i'm very obviously missing some point or experiment.

    (i've tried to intentionally leave the words life, organism, DNA, etc, out of this in hopes of distancing us from the system, as if it were any other system)
  18. Aug 5, 2011 #17


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    I recognize there's a lot of speculation on the subject of abiogenesis; professional speculation is a good first step to new science, though!

    To go further, I favor endosymbiotic theory, which suggests that eukaryotes are actually instances of prokaryotes eating and retaining the functionality of other, smaller prokaryotes (of course, mitochondria having their own genetic makes them an interesting study for this theory).
  19. Aug 5, 2011 #18
    Pythagorean, may I just ask you, just out of interest… when you talk of lectures, abstracts and water-cooler talk, is this an area of actual professional (or perhaps academic) involvement for you, rather than just interest, however intense? I’m really not sure how much serious scientific research goes on into understanding the origin of life. If you are involved in serious work in this regard, have you not come across this guy Tibor Ganti.? His work, as I understand it and again, I have to acknowledge the limits to my understanding, centres around this whole question of at what point does a chemical process become ‘life’. I’m interested in what you know of this.

    Edit: Some links to demonstrate that I am not talking about some crackpot.




    Last edited: Aug 5, 2011
  20. Aug 5, 2011 #19


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    As we currently understand it lipid vesicles can spontaneously form under a variety of conditions. These lipids would be very good at keeping large molecules out but small molecules such as nucleotide monomers could diffuse in, these monomers can then polymerise which prevents them leaving. Lipid vesicles grow due to absorption of lipids from other vesicles what is very cool is that large polymers inside a vesicle will increase the membrane pressure which will increase the strength at which the membrane can absorb lipids from smaller vesicles! The larger the vesicles grow the more chance there is that they will split into smaller vesicles a bit like when a soap bubble get's to big. At that point polymers will be divided up into different vesicles, what is really cool is that now selection can occur because the polymers that polymerise faster (thanks to sequence, structure and enzymatic activity) will increase their size increase the membrane pressure increase the vesicle growth speed at the expense of others and increase it's replication rate. It's all very interesting stuff.

    EDIT: I managed to find a video that goes over most of this, it was made in response to creationism so the first 3 minutes are debunking creationist claims. Annoyingly someone has chosen to put an unnecessary classical track over the top so you may want to mute. But it is a good video!

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  21. Aug 5, 2011 #20


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    Mildly. My thesis (under construction) is computational neuroscience from the dynamical systems perspective; I work in a lab that studies evolutionary neuroscience, so I'm developing collaborative work (applying my models to their questions) to study the evolution and development of neural networks.

    I've started to become interested in more fundamental networks, the molecular networks of life, so I've generalized my biology study beyond neuroscience, but I am still lacking quite a bit of organic chemistry to really claim a solid academic foundation in abiogenesis.

    I haven't heard about it by name, but it seems very much up my alley and in my line of thinking.
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