Reviewer 3: Sergei Maslov (Brookhaven National Laboratory)
Any manuscript invoking the anthropic selection principle to explain anything is bound to raise controversy. Let alone if what is being explained is the origin of Life itself.
I personally liked the style and the clear language of the manuscript. I also appreciated an honest back of the envelope estimate of the probability of primitive replication+translation machinery arising by pure chance:
Author also clearly indicates how his theory could be falsified or modified by future discoveries. For example, the discovery of independently evolved life in our cosmic neighborhood would obviously put an end to any anthropic selection-based models of the origin of Life. On the other hand, a discovery of an evolutionary plausible path to the appearance of translational machinery in a full-fledged RNA world might simply reduce the severity of the anthropic selection needed to explain the origin of Life without completely eliminating the need for it.
The manuscript also postpones the discussion of plausibility of a "reactor" which uses raw materials to generate polynucleotides in sufficient quantity/density. It is simply stated that while "such "reactors" are not known", "networks of inorganic compartments existing at hydrothermal vents might be plausible candidates". It might turn out that the problem of lack or rarity of such reactors might dwarf the problem of low likelihood of the appearance of replication+translation machinery in one such reactor.
Author response: It is curiously serendipitous that, while this manuscript was under review, new data have been published indicating that, given that monomers (nucleotides) are synthesized at any appreciable rate, the emergence of a "reactor" producing polymers (RNA molecules) in the vicinity of a hydrothermal vent, is not unlikely. These new findings are quoted in the revised paper (Refs. 56, 57).
While personally I hope we would find a way to explain the origin of biocomplexity without invoking the anthropic selection (perhaps, by some yet unknown mechanism of self-organization), one cannot deny that at the present state of affairs in understanding the origin of Life the anthropic selection at the very least provides a viable alternative.
Author response: Actually, I think a rather common misunderstanding is involved here. I am convinced that the anthropic principle is unavoidable as part and parcel of any scenario for the origin of life, whether or not some still unknown principles of self-organization exist (they very well might). Plenty of anthropic selection is required to account for the formation galaxies and earth-like planets, prebiological organic syntheses etc etc. The real question is not whether or not anthropic selection is important (to me, there is no doubt whatever) but where is the transition between it and biological evolution, the threshold of complexity where Darwinian selection becomes possible (see Fig. 1 in this paper). In this regard, one certainly may "hope" that the threshold is (considerably?) below the level of complexity associated with a coupled system of translation and replication (again, see Fig. 1) but so far there is no strong evidence or even a compelling model of biological evolution occurring at this stage.
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