Disclaimer: this is an edited version of a much harsher review I wrote at first. I have no connection to the authors of the study or to their fields of expertise, but am someone who enjoyed the paper here critiqued and in fact think it very nice and very conservative in terms of its numbers (the current post claims the opposite). I disagree with this post and think it is wrong in an obvious and fundamental way, and therefore should not be in decade review in the interest of not posting wrong science. At the same time it is well-written and exhibits a good understanding of most of the parts of the relevant model, and a less extreme (and less wrong :) version of this post would pass my muster. In particular I think that the criticism
However, since this parameter is capped at 1, while there is no lower limit to the long tail of very low estimates for fl, in practise this primarily has the effect of reducing the estimated probability of life emerging spontaneously, even though it represents an additional pathway by which this could occur.
is very valid, and a model taking this into account would have a correspondingly higher credence for “life is common” scenarios. However the authors of the paper being criticized are explicitly thinking about the likelihood of “life is not common” scenarios (which a very naive interpretation of the Drake equation would claim are all but impossible) and here this post is deeply flawed.
The essential beef of the author of the post (henceforth the OP) with the authors of the paper (henceforth, Sandberg et al) concerns their value fl, which is the “log standard deviation in the log uncertainty of abiogenesis” (abiogenesis is the event wherein random and non-replicating chemical processes create the first replicating life). A very rough explanation of this parameter (in the log uncertainty model which Sandberg et al use and OP subscribes to) is the probability of the best currently known model for abiogenesis occuring on a given habitable planet. Note that this is very much not the probability of abiogenesis itself, since there can be many other methods which produce abiogenesis a lot more frequently than the best currently known model. The beautiful conceit of this paper (and the field it belongs to) is the idea that, absent a model for a potentially very large or very small number (in this case, the probability of abiogenesis, or, in the larger paper, the probability of the emergence of life on a given paper), our best rough estimate for our uncertainty it is more or less log uniformly distributed between the largest and smallest “theoretically possible” values (so a number between 10^-30 and 10^-40 is roughly as likely as a value between 10^-40 and 10^-50, provided these numbers are within the “theoretically possible” range. The difference between “log uniform” and “log normal” is irrelevant to a first approximation). The exact definition of “theoretically possible” is complicated, but in the case of abiogenesis the largest theoretically possible value of fl (as of any other probability measure) is 1 while the smallest possible value is the probability of abiogenesis given the best currently known methods. The model is not perfect, but by far the best we have for predicting the lower tail of such distributions, i.e., in this case, the likelihood of the cosmos being mostly devoid of intelligent life. (Note that the model doesn’t tell us this probability is close to 1! Just that it isn’t close to 0.)
Now the best theoretically feasible model for abiogenesis currently known is the so-called RNA world model, which is analyzed in supplement 1 of Sandberg et al. Essentially, the only sure-fire way we know of abiogenesis is spontaneously generating the genome of an archaeobacterium, which has hundreds of thousands of base pairs, and would put the probability of abiogenesis at under 10^-100,000 (insanely small). However, we are fairly confident both that much smaller self-replicating RNA sequence would be possible in certain conducive chemical environments (the putative RNA world), and that there is some redundancy in how to generate a near minimal self-replicating RNA sequence (so you don’t have to get every base pair right). The issue is that we don’t know how small the smallest genome is and how much redundancy there is in choosing it. By the nature of log uncertainty, if we want to get the lowest value in the range of uncertainties (what OP and Sandberg et al call log standard deviation) we should take the most pessimistic reasonable estimates. These are attempted in the previously mentioned supplement, though rather than actually taking pessimistic values, Sandberg et al rather liberally assume a very general model of self-replicating RNA formation, with their lower bound based on assumptions about protein folding (rather than a more restrictive model based on assuming low levels of redundancy, which I would have chosen, and which would have put the value of fl significantly lower even than the Sandberg et al paper: they explicitly say that they are trying to be conservative). Still, they estimate a value of fl equal or lower than 10^-30 with the current best model. In order to argue for a 10^-2 result while staying within the log normal model, OP would have to convince me of some drastic additional knowledge. Either that they have a proof, beyond all reasonable doubt, that either an RNA chain shorter than the average protein is capable of self-replicating, or that there is a lot of redundance in how self-replicating RNA can form, and a chemical “RNA soup” would naturally tend to self-replication under certain conditions. Both of these are plausible theories, but as such methods for abiogenesis are not currently known to exist, assuming they work for your lower bounds on log probability is precisely not how log uncertainty works. In this way OP is, quite simply, wrong. Therefore, as incorrect science, I do not recommend this post for the decade review.
Disclaimer: this is an edited version of a much harsher review I wrote at first. I have no connection to the authors of the study or to their fields of expertise, but am someone who enjoyed the paper here critiqued and in fact think it very nice and very conservative in terms of its numbers (the current post claims the opposite). I disagree with this post and think it is wrong in an obvious and fundamental way, and therefore should not be in decade review in the interest of not posting wrong science. At the same time it is well-written and exhibits a good understanding of most of the parts of the relevant model, and a less extreme (and less wrong :) version of this post would pass my muster. In particular I think that the criticism
is very valid, and a model taking this into account would have a correspondingly higher credence for “life is common” scenarios. However the authors of the paper being criticized are explicitly thinking about the likelihood of “life is not common” scenarios (which a very naive interpretation of the Drake equation would claim are all but impossible) and here this post is deeply flawed.
The essential beef of the author of the post (henceforth the OP) with the authors of the paper (henceforth, Sandberg et al) concerns their value fl, which is the “log standard deviation in the log uncertainty of abiogenesis” (abiogenesis is the event wherein random and non-replicating chemical processes create the first replicating life). A very rough explanation of this parameter (in the log uncertainty model which Sandberg et al use and OP subscribes to) is the probability of the best currently known model for abiogenesis occuring on a given habitable planet. Note that this is very much not the probability of abiogenesis itself, since there can be many other methods which produce abiogenesis a lot more frequently than the best currently known model. The beautiful conceit of this paper (and the field it belongs to) is the idea that, absent a model for a potentially very large or very small number (in this case, the probability of abiogenesis, or, in the larger paper, the probability of the emergence of life on a given paper), our best rough estimate for our uncertainty it is more or less log uniformly distributed between the largest and smallest “theoretically possible” values (so a number between 10^-30 and 10^-40 is roughly as likely as a value between 10^-40 and 10^-50, provided these numbers are within the “theoretically possible” range. The difference between “log uniform” and “log normal” is irrelevant to a first approximation). The exact definition of “theoretically possible” is complicated, but in the case of abiogenesis the largest theoretically possible value of fl (as of any other probability measure) is 1 while the smallest possible value is the probability of abiogenesis given the best currently known methods. The model is not perfect, but by far the best we have for predicting the lower tail of such distributions, i.e., in this case, the likelihood of the cosmos being mostly devoid of intelligent life. (Note that the model doesn’t tell us this probability is close to 1! Just that it isn’t close to 0.)
Now the best theoretically feasible model for abiogenesis currently known is the so-called RNA world model, which is analyzed in supplement 1 of Sandberg et al. Essentially, the only sure-fire way we know of abiogenesis is spontaneously generating the genome of an archaeobacterium, which has hundreds of thousands of base pairs, and would put the probability of abiogenesis at under 10^-100,000 (insanely small). However, we are fairly confident both that much smaller self-replicating RNA sequence would be possible in certain conducive chemical environments (the putative RNA world), and that there is some redundancy in how to generate a near minimal self-replicating RNA sequence (so you don’t have to get every base pair right). The issue is that we don’t know how small the smallest genome is and how much redundancy there is in choosing it. By the nature of log uncertainty, if we want to get the lowest value in the range of uncertainties (what OP and Sandberg et al call log standard deviation) we should take the most pessimistic reasonable estimates. These are attempted in the previously mentioned supplement, though rather than actually taking pessimistic values, Sandberg et al rather liberally assume a very general model of self-replicating RNA formation, with their lower bound based on assumptions about protein folding (rather than a more restrictive model based on assuming low levels of redundancy, which I would have chosen, and which would have put the value of fl significantly lower even than the Sandberg et al paper: they explicitly say that they are trying to be conservative). Still, they estimate a value of fl equal or lower than 10^-30 with the current best model. In order to argue for a 10^-2 result while staying within the log normal model, OP would have to convince me of some drastic additional knowledge. Either that they have a proof, beyond all reasonable doubt, that either an RNA chain shorter than the average protein is capable of self-replicating, or that there is a lot of redundance in how self-replicating RNA can form, and a chemical “RNA soup” would naturally tend to self-replication under certain conditions. Both of these are plausible theories, but as such methods for abiogenesis are not currently known to exist, assuming they work for your lower bounds on log probability is precisely not how log uncertainty works. In this way OP is, quite simply, wrong. Therefore, as incorrect science, I do not recommend this post for the decade review.