As far as I'm aware, only one abiogenesis event has happened on earth- a place with copious amounts of the exact right molecules in cosmically unique quantity, density, and environment to form self-replicating RNA. If abiogenesis has happened anywhere else, it hasn't evolved into intelligent life whose work has intercepted our light cone. My current model for how abiogenesis went down in our world begins with the semi-random coalescence of more than a thousand nucleotides- for reference, the smallest known self-replicating bacterium has a roughly 580,000 base-pair genome.  Assuming each additional required nucleotide introduces a 1/2 chance of failure, each random coalescence event has a less than (1/2)^1000 ~= 10^-300 chance of forming a viable self-replicator. This estimate is completely off the scale with respect to events that you can expect to occur in a 100 billion l.y. radius sphere.

This is my current idea, at least. Do you know of a more persuasive argument?

EDIT: For future readers, I now consider the Grabby Aliens paper to present a compelling alternate model.

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You can do something similar to the Drake equation:

where Nlife is how many stars with life there are in the Milky Way and it is assumed that a) once self-replicating molecule is evolved it produces life with 100% probability; b) there is an infinite supply of RNA monomers, and c) lifetime of RNA does not depend on its length. In addition:

  • Nstars - how many stars capable of supporting life there are (between 100 and 400 billion),
  • Fplanet - Number of planets and moons capable of supporting life per star - between 0.0006 (which is 0.2 of Earth-size planets per G2 star) and 20 (upper bound on planets, each having Enceladus/Europe-like moon)
  • Tplanet - mean age of a planet capable of sustaining life (5-10 Gy)
  • Splanet - typical surface area of a planet capable of sustaining life (can be obtained from radii of between 252 km for Enceladus and 2Rearth for Super Earths)
  • Fsurface - fraction of surface where life can originate (between tectonically-active area fraction of about 0.3, and total area 1.0)
  • D - typical depth of a layer above surface where life can originate (between 1m for surface-catalyzed RNA synthesis and 50 km for ocean depth on Enceladus or Europa)
  • TRNA - typical time required to synthesize RNA molecule of typical size for replication, between 1s (from replication rate of 1000 nucleotides per second for RNA polymerases) and 30 min, a replication rate of E.coli
  • VRNA - minimal volume where RNA synthesis can take place, between volume of a ribosome (20 nm in diameter) and size of eukaryotic cell (100 um in diameter)
  • Rvolume - dilution of RNA replicators - between 1 (for tightly packed replicating units) and 10 million (which is calculated from a typical cell density for Earth' ocean of 5*10^4 cells/ml and a typical diameter of prokaryotic cell of 1.5 um)
  • Nbase - number of bases in genetic code, equals to 4
  • LRNA - minimal length of self-replicating RNA molecule.

You can combine everything except Nbase and LRNA into one factor Pabio, which would give you an approximation of "sampling power" of the galaxy: how many base pairs could have been sampled. If you take assumption that parameters are distributed log-normally with lower estimated range corresponding to mean minus 2 standard deviations and upper range to mean plus 2 standard deviations (and converting all to the same units), you will get the approximate sampling power of Milky Way of 

Using this approximation you can see how long an RNA molecule should be to be found if you take top 5% of Pabio distribution: 102 bases.  Sequence of 122 bases could be found in at least one galaxy in the observable universe (with 5% probability).

In 2009 article the sequence of the RNA on the Fig. 1B contained 63 bases. Given the assumptions above, such an RNA molecule could have evolved 0.3 times - 300 trillion times per planet (for comparison, abiogenesis event on Earth' could have occurred 6-17 times in Earth's history, as calculated from the date of earliest evidence of life).

Small 16S ribosomal subunit of prokaryotes contains ~1500 nucleotides, there is no way such a complex machinery could have evolved in the observable universe by pure chance.

I read The Vital Question by Nick Lane a while ago and it was the most persuasive argument I've seen on abiogenesis. May be of interest to you. The argument made was that it could be fairly common based on proton gradients.

I'm not aware of an argument that there was only on abiogenesis event on Earth, just the observation that all known surviving lineages come from a universal common ancestor fairly early on. In principle that would be compatible with any number of initial events. It's just that once a given lineage evolved enough adaptions/improvements, it would spread and take over, and then no new lineage would be able to compete/get started.

Also, your scale for probability seems to be starting from assuming a single long self-replicating genome, but that isn't strictly necessary to bootstrap the evolution of a basic self-replicating metabolism. There are much shorter RNA strands (<200 base pairs) that have some catalytic activity including synthesizing additional RNA (though not copying themselves, AFAIK). Something like that could locally generate large numbers of shorter RNA strands, many with some form of catalytic activity of their own, collectively comprising some form of catalytic cycle that includes making more of all of them. Such a system would also be better able to cope with low copying fidelity b/c the individual strands that need to be copied correctly are shorter.

As far as going from bare RNA to a bacterium, I admit I don't know how this happen(ed? happens?). My naive initial thought is some RNA arising in this environment that could produce fatty acids, which could form a lipid layer around a cluster of RNA molecules spontaneously. Repeat and replicate enough times, and add in some endosymbiosis events and you're not too far off?

I'm not aware of an argument that there was only on abiogenesis event on Earth, just the observation that all known surviving lineages come from a universal common ancestor fairly early on. In principle that would be compatible with any number of initial events. It's just that once a given lineage evolved enough adaptions/improvements, it would spread and take over, and then no new lineage would be able to compete/get started.

Your observation is an argument for only one abiogenesis event, and your claim that one would spread and take over and no new lineag... (read more)

That's fair, and I genuinely wasn't trying to nitpick, it is a very good question. If I try to answer that question as written, I'd say that any time I see a probability estimate with on-the-order-of-hundreds of zeroes, when I know that event actually happened (at least) once in Earth's past light cone, I'm going to assume there is an error in the model that generated the estimate, whether I know what it is or not. So what I way trying to point to is that if a catalytic cycle of many (much smaller) RNA strands was sufficient for an abiogenesis event, that could lower the probability estimate enough to make such events more likely by enough that there could have been multiple even just on Earth without straining credulity, and the world today would likely look basically the same either way since the more-competitive biochemistry would have long since reach fixation (and/or the lineages could have merged in some analog of later endosymbiosis events).
I would agree for pretty much any other topic. This is an event required for people to be around to observe it. Imagine a universe in which abiogenesis events really were absurdly rare- unlikely to ever occur in a given observable universe sized area. Every observer in this universe would still look back and see an abiogenesis event occurring in their past! Having observed exactly one event is completely required and provides no evidence. This is essentially the weak anthropic principle. The fact that we observe only exactly one event is thus bayesian evidence in the direction of rare abiogenesis. You bring up the point that the fact we observe only exactly one event in earth's history isn't that strong of evidence because e.g. events being concealed by significantly more advanced competitors. I certainly don't disagree- that was just unfortunate overemphasis on my part. I was thinking of systems more complex than the random coalescence of nucleotides when I wrote my post, but I didn't know how to productively model that.  Someone else had a similar argument and I responded that I slightly shifted away from rare-abiogenesis because I wasn't thinking of complicated groups of cross-catalyzing RNA in particular, which are dumb enough to have no modern analogues but not so dumb as to not be competitive against single-strand solutions which require the random coalescence of another 100 base pairs, or whatever it is. It's unclear to what degree this should affect my model, given a lack of understanding of how such systems work.

"Now, a pair of Scripps Research Institute scientists has taken a significant step toward answering that question. The scientists have synthesized for the first time RNA enzymes that can replicate themselves without the help of any proteins or other cellular components, and the process proceeds indefinitely." From

OK, but how does this evolve into a bacterium? Won't it evolve into a local maximum of RNA enzyme replication efficiency and stay there?

RNA alone is a good enough self replicating life form that arose first before we the rest of the cell machinery. This would be the most extreme form of the RNA world hypothesis. There is no need for anything else to evolve to claim that life has arisen.

While cool, I didn't expect indefinite self-replication to be hard under these circumstances. The enzymes work by combining two halves of the other enzyme- i.e. they are not self-replicating using materials we would expect to ever naturally occur, they are self-replicating using bisected versions of themselves.

I've slightly downgraded my estimate for the minimum viable genome size for self-replicating RNA because I wasn't thinking about complicated groups of cross-catalyzing RNA.

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The currently smallest self-replicating life form is a battle-hardened descendant of billions of years of attack by hostile lifeforms that have been actively trying to eat its ancestors. I would expect the task of surviving that (and the corresponding structure) to be much more complex than anything that developed during the process of abiogenesis.