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For some reasons, it seems that particular "handedness" are chosen by nature, because we have the prevalence of some enantiomer, that have different optical properties respect to polarization. In nature L-amino acids are prevalent respect to D-amino acids and D-glucose are prevalent respect to L-glucose. Is this breaking of symmetry a poor accident, in both distinct cases, or could there be a deep physical reason that caused it to evolve?

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    $\begingroup$ Possible duplicate: physics.stackexchange.com/q/404149/44126 $\endgroup$ – rob Apr 20 at 11:51
  • $\begingroup$ I VTC as duplicate, but I might have accidentally selected the wrong link. Oops $\endgroup$ – BioPhysicist Apr 20 at 14:27
  • $\begingroup$ Not completely. That question talks about a possible link between chirality and violation of parity in weak interaction. Because this link seems not true, there must be another reason or randomness. My question is in general if there are mechanisms that explain this asimmetry. Possible, something that has some possibility of experimental test. $\endgroup$ – Mark_Bell Apr 20 at 20:11
  • $\begingroup$ This question seems to me larger in scope than the questions&answers that it is lonked to. $\endgroup$ – Roger Vadim Apr 28 at 12:10
  • $\begingroup$ @Vadim Why? It's not general, maybe some implications can be general. But the question is focused on one thing. Even if having some gain or reference can be useful for further readings, if the answer is too broad $\endgroup$ – Mark_Bell Apr 29 at 10:12
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How do we know?
We cannot know for sure, since this stage of the evolution happened long time ago, leaving no fossil evidence. The main sources by which we can reconstruct the early evolution are the idiosyncratic features, like the molecular asymmetry, the sequence of steps in metabolic cycles, the protein configurations and random features of certain parts of DNA sequences that are common to all living organisms (which are not many).

It is likely has happened by an accident, although not necessarily a poor one. In fact, one should consider in this context two separate questions: the emergence of the first molecules and their subsequent evolution.

Early life
From chemistry we know that, when synthesized from achiral compounds, the molecules of alternative chiralities appear in approximately equal proportions (which poses a serious issue when producing medical drugs). Thus, it is quite possible that at the time the first self-replicating molecules emerged, there were ones of both polarities. We can say with high confidence that the chemistry (and thus also physics) prevented them from mixing, so the two forms of life likely existed in parallel.

Population genetics
Evolutionary theory tells us that a genetic diversity is removed from the population for hard probabilistic reasons, with one allele (in this case one polarization) becoming fixed. (See fixation.) Note that this does not require that one allele has an advantage over the other, in what is known as neutral evolution. Indeed, the need for a selective advantage was the early Darwinian belief, but the evolutionary theory has significantly evolved since then - notably in mathematical respect (see, e.g., Gellespie's little book). Now we have good reasons to believe that a significant part of evolution (although definitely not all) is a result of completely random events.

Moreover, modern evolutionary theory claims that all the currently living organisms have descended from a single organism, LUCA (lust universal common anscestor, see Common descent), likely inheriting its chirality.

Other idiosyncratic features
As another example of randomness easily understandable to physicists, one could mention the protein folding configurations. All the known proteins fold in about a thousand of possible structures... out of billions available in each protein's phase space. Moreover, these configurations are often identical between the proteins with completely different sequences of amino-acids.

Life as spontaneous symmetry breaking
A more physical (although less biologically concrete) way of looking at this problem is to view life as a result of spontaneous symmetry breaking in non-equilibrium systems (see the article by Anderson and Stein). In this sense the chirality of organic molecules is but one of the many components of the complex order parameter characterizing the "life phase". One could thus say that this breaking of symmetry is indeed an accident... which is a manifestation of the deep physical reason.

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  • $\begingroup$ Would be this as to literally assume one and only one simple organism to be ancestor of all living things? The asymmetry described is almost ubiquitous in the living realm (to my knowledge). Also, the fixation process you refer to seems to require an advantage, to happen. So it is hard to see what this advantage could have been in simple cells without previous symmetry breaking. I mean enantioners are identical in optically inactive surrounding. Could this have been a side effect (ie emerged because associated to some other advantage taking place independently of the surrounding)? $\endgroup$ – Alchimista Apr 20 at 10:59
  • $\begingroup$ @Alchimista I have expanded my answer, to address some of the questions that you posed above. $\endgroup$ – Roger Vadim Apr 20 at 11:32
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    $\begingroup$ Indeed, the modern evolutionary theory claims that all the living organisms descended from MRCA - the most recent common ancestor. (apart from viruses perhaps - but these are not counted as living) $\endgroup$ – Roger Vadim Apr 20 at 11:36
  • $\begingroup$ I don't get the last part about protein folding. I know that the space of possible structures is more big than that explored by nature, and maybe the evolution have created just tiny cluster of structures in this big space. Do you mean that the particular structures we have are random or at least some parts of them? The structures are not driven by advantages? Like the possibility of create sites for binding molecules, or other topologies that can accomplish specific task. These don't seems like a random choice $\endgroup$ – Mark_Bell Apr 20 at 18:12
  • $\begingroup$ @Mark_Bell Indeed, the nature didn't have time to explore all of the possible structures. The selection could act only on those structures which were explored (and are being explored). Again the evolution here can be neutral - if a new structure does not present a specific advantage or disadvantage, it may become the majority structure. Importantly, these structures are not necessarily the minimum energy configurations of the protein, i.e., they cannot be determined solely from the protein sequence (i.e., from the genetic code). $\endgroup$ – Roger Vadim May 12 at 10:18

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