6 ms·

I think it's very likely that life has originated on Earth many times. If it happened once, so early in the age of Earth - it should happen more than once. But

• gnulinux 7y ago
  It is possible abiogenesis is such a low probability event that it occurred once since the exact conditions weren't met. The earth is not very similar to early earth currently. Consider endosymbiotic hypothesis. The evidence is that it seems to have occurred at least multiple times (mitochondria, chloroplast, nucleus) in different geological eras. I would expect if life had a high enough probability to happen multiple times, it could occur in different geological eras.
• garmaine 7y ago
  Then why isn’t there ANY examples of life simpler than a bacterium, which itself is insanely complex? Why does all life share the same genetic and amino acid coding system, a relatively arbitrary choice? More likely, in all seriousness, that life originated in Mars and came here via panspermia during the early bombardment period. Mars was warm and wet while Earth was still suffering cataclysmic impacts and only relatively mature examples of Martian life (e.g. bacteria) would have survived the trip. If life originated multiple times on Earth there would be evidence for that. Heck if life originated on Earth there should be evidence for that, which there bizarrely isn’t.
  • fiblye 7y ago
    The theory I’ve seen is that anything simpler couldn’t defend itself and would instantly be eaten by something more complex.
  • ajuc 7y ago
    > Then why isn’t there ANY examples of life simpler than a bacterium, which itself is insanely complex? Exactly because they were outcompeted? If I understand correctly the early life would be living much slower and reproducing much less efficiently. So it would just get eaten by more efficient life that had millions years more to develop. How do you know life isn't arising every few million years somewhere on Earth and immediately being eaten by something before it could reproduce 10 times? What evidence would you expect to find that it happens?
    • garmaine 7y ago
      Fossils. Micro or nano scale tracks of chemotropic entities. Unusual small geological features indicative of anything other than normal random chemistry going on. We can see all this in the rocks around us. TFA is about finding such things in mines deep underground. Remarkably we can even find such evidence in fossils more than 3 billion years old. Yet.. the oldest things we find are microbial sedentary structures indicative of remarkably complex life: with genes, ribosomes, proteins, etc. We know this because those same life forms still exist today and leave pretty much exactly the same tell tale signs. We call them simple life, but that is just by comparison. They are insanely complex compared to the random geochemistry going on alongside them. Why don’t we find ANYTHING older than this which differs from expected geological features? Where are the fossils from the origin of life? Also, the oldest fossils we have are of an organism that still exists: simple life remarkably doesn’t get competed to extinction. It turns out that simpler life forms have some advantages from their simplicity allowing them to be more adaptive to micro niches. So why don’t we have extant examples of even simpler life forms, surviving where complex life doesn’t fare as well?
      • andrewflnr 7y ago
        Possible answer: there was a hard takeoff in complexity between the first thing you can call life with a straight face and some minimal level of complexity you need to stay alive and spread in the real world. That minimal level of complexity characterizes the first organisms that show up in fossils. If it happened fast enough (maybe up to a couple million years, I dunno), there would be an infinitesimal amount of evidence that could easily be missed forever. If it only happened once, we would need to look at exactly the right rock, across the whole planet, with a microscope. Or it could already be recycled into the mantle. Ocean crust doesn't have a long lifespan, IIRC. Dunno about the rest, but I do think that rapid advancement from proto-life to something pretty sophisticated is likely, wherever it starts.
  • dpark 7y ago
    > More likely, in all seriousness, that life originated in Mars and came here via panspermia during the early bombardment period. I am always surprised at the number of people willing to make assertions like this. Panspermia itself is a hypothesis. Panspermia from Mars specifically is a huge stretch. There is absolutely no basis for claiming that this is likely. > only relatively mature examples of Martian life (e.g. bacteria) would have survived the trip. Why do you make this assumption? Life complexity is not generally correlated with space survivability. At least not positively correlated.
    • partialrecall 7y ago
      > "Panspermia itself is a hypothesis. Panspermia from Mars specifically is a huge stretch. There is absolutely no basis for claiming that this is likely." Perhaps not 'likely', but there is reason to believe it's plausible. Extremophile organisms plausibly capable of surviving in space for the requisite timespans are certainly known to exist, and there is also known to be small amounts of material transfer between earth and mars. The part I wonder about is whether any extremophile organism could survive the doubtlessly violent circumstances that are involved in ejecting rocks from either the earth or mars gravity well.
    • garmaine 7y ago
      I’ve actually worked in astrobiology research :) I would point you towards the research that went into ALH-84001. Even if you don’t buy that it was an example of fossilized Martian life, analysis of the thermal stresses it suffered from ejection from Mars, time in flight, to impact on Earth showed conclusively that it was survivable. Had it been the other way around (Earth to Mars) it would have seeded Mars with life. We’ve shown that rock which could be carrying microbes can and has been exchanged between earth and Mars. Lots of such specimens land each and every year. You can quibble that we haven’t found actual life yet, but (1) lots of people think that’s exactly what ALH-84001 was. (2) we don’t have that many samples of Martian meteorites. We know lots of it falls every year, but most is either not found/collected/identified or contaminated on impact. (3) Mars meteorites falling now, ejected from a dry and sterile surface, would not be representative of what was falling in the early earth that was thrown from a wet, habitable Mars. To be clear, from an astrobiology perspective a negative result from the search for life on (or from) Mars is harder to explain than finding it. Both panspermia and second abiogenesis are simpler explanations.
  • chongli 7y ago
    ANY examples of life simpler than a bacterium Viruses are much simpler, for one. Or perhaps you don't consider them to be life forms. Either way, if you understand how bacteria operate at even the most crude level, you'll realize that they (all bacteria, collectively; individually, they're specialized, of course) eat basically anything and everything in terms of organic molecules. What are the ramifications for this? It means any sort of abiogenesis that takes place today needs to somehow avoid being immediately eaten by bacteria and then continue to survive for potentially millions of years in order to evolve into an advanced life form. Needless to say, this seems to be vanishingly unlikely these days.
    • garmaine 7y ago
      Viruses are life, but they depend on the machinery of other, more complex life to reproduce. Life doesn't start with viruses, for if it did, how would they reproduce?
      • chongli 7y ago
        You asked about examples of life simpler than bacteria. You didn't stipulate that they had to be ancestors. How would that even work, anyway? An organism would have to be extremely long-lived to be living concurrently with its direct, more complicated descendent.
        • TeMPOraL 7y ago
          > How would that even work, anyway? Think in terms of lines, not individuals. Mutations are acquired on individual level, so out of a whole family of bacteria, one individual may reproduce into something slightly different, while the rest of the family continue unchanged. (Modulo fast mutation rate in bacteria, and modulo the wide use of horizontal gene transfer. The more I learn about bacteria and archaea, the more I feel there's little point in trying to find the tree of life.)
    • mirimir 7y ago
      As I vaguely understand it, both combination and separation have played important roles in evolution. Combination, for example, in that the nuclei, mitochondria and chloroplasts of modern eukaryotes derive from initially separate organisms. Many eukaryotes carry transposable elements, which are DNA sequences that exploit DNA and RNA machinery to create numerous copies of themselves in the genome. That is, they're like viruses, except that for the most part they don't get transmitted, except through cell replication. But I believe that there are examples where transposable elements do get transmitted like viruses. Or sometimes as parasites on actual viruses. So that's separation. And there are also examples, where viruses become integrated into the genome. Sometimes reversibly, as with HSV, which is why you can never really cure an HSV infection. Anyway, it seems most likely that simple stuff evolved first, and then combined into more complicated stuff. But the simple stuff has survived, to some extent, as transposable elements and viruses.
  • zaarn 7y ago
    Mitochondria carry their own genome, which is substantially different to bacteria. Although nowadays a core part of eukaryotic cells, it's not hard to believe that they were once a separate form of life much simpler than a bacteria. Considering the age of life on this planet, panspermia from Mars is unlikely; in the early life of the solar system, the sun was much dimmer and Mars was significantly colder than today. Any liquid water on the surface, on which life as we know it could have developed, would only be present in fairly recent history (no longer than a billion years really).
• partialrecall 7y ago
  I personally doubt it. While abiogenesis is in progress but before it has produced life, the intermediate products are probably eaten by existing life. New forms of life are probably 'out-competed' before they even become forms of life. The conditions for the formation of life might be relatively trivial, but after it succeeds once it's unlikely to ever succeed again. Of course, sterile experiments in a laboratory might give us a lower bound on just how trivial the requirements for abiogenesis are. So far nobody has noticed it happening in a lab, despite more than a few trying.
• SomeOldThrow 7y ago
  > If it happened once, so early in the age of Earth - it should happen more than once. You can’t extract a (meaningful) probability from a single sample no matter how intuitive it seems.
  • perl4ever 7y ago
    People calculate probabilities with a sample size of zero, even. For instance, people frequently talk about the probability a given voter will cast the deciding vote in an election.
    • ajmurmann 7y ago
      But we use proxy effects like polls to simulate the actual event. On this case we haven't even been able to do that.
  • hhs 7y ago
    > You can’t extract a (meaningful) probability from a single sample no matter how intuitive it seems. It's possible to get insights from a single sample. Science journals, especially in fields like medicine, do it to talk about atypical case studies. It's a good learning experience, too, when reflecting on the structure and context of those edge cases.
    • SomeOldThrow 7y ago
      The terms “atypical” and “edge case” are not meaningful with a single sample size that is also the only known observation. There is only the one sample and that’s all you can observe. There’s nothing to compare it against to give “atypical” and “edge case” meaning. Individual samples can be meaningful in a larger distribution, but you still need multiple samples to establish any distribution at all. I think you are referring to outliers within the distribution.
  • ACow_Adonis 7y ago
    Ah, but is it a single sample, or multiple repeated samples at times t subscript n, with several negative observations and (at least) one positive? Indeed, posit n repeated trials over a given time line, it seems equally implausible that an observation of a positive outcome earlier in the timeline contains no information on implied probabilities relative to a positive observation occurring later in the time lines.
• EdwardDiego 7y ago
  > I think it's very likely that life has originated on Earth many times. How would you prove that though?
  • TheSpiceIsLife 7y ago
    How would you disprove it?
    • ajmurmann 7y ago
      Asking for a way to falsify a theory is crucial. On this case I think we need to look for falsifiable predictions the theory makes that then can be disproven. If a theory makes no falsifiable predictions it's useless. So slightly backwards, it seems in this case, asking for how to proof it is closer to asking "what predictions does this theory make?". A theory that's not disprovable isn't valuable. See Russel's tea pot. Edit: phone auto typo.
  • simonh 7y ago
    Find life that doesn’t use DNA, or uses a radically different form of DNA.