from Hacker News

Cosmic rays may explain life’s bias for right-handed DNA (2020)

by deesep on 2/26/23, 11:32 AM with 35 comments

• by photochemsyn on 2/26/23, 3:58 PM

  Note the recently discovered Winchcombe meteorite had equal amounts of L- and R- amino acids, suggesting that non-biological processes produce both at the same rate, which is perhaps not what you'd expect if these cosmic ray or other physical processes were influencing L- vs R- handed molecule production/breakdown rates.

  https://news.ycombinator.com/item?id=33680493

  https://www.science.org/doi/10.1126/sciadv.abq3925

  It's likely life had to choose one or the other early on; imagine the mess it would be if you tried to build proteins or nucleic acids with a random assortment of left and right handed molecules, it'd be hard to get consistent structures. You'd have to have specific codons, the three-letter gene sequences that convert the 4-letter DNA code to the amino-acid code (22 for terrestrial life), for each handed version of the amino acids, so you'd need 44 different codons (out of 63 possible 3-letter combinations of ATCG) for the complete set, with no [little] overlap.

  [edit: it's 64, not 63! Today's life uses 61 for amino acids and 3 are reserved as stop signals. To get the full set in Python: set(list(itertools.permutations('AAATTTCCCGGG', 3))) ]

  I'd guess the early self-replicating systems (likely something like a RNA-protein ribosome complex capabale of self-replication using abiotically produced amino and nucleic acids) were only able to consistently replicate if they only used one enantiomer consistently, and it was just a roll of the die as to which one they settled on. There might even have been mixed populations of L-handed and R-handled life at the beginning, with only one branch surviving.

  Of course, that's the claim here, that of these two branches, one was more impacted by cosmic rays than the other. ???

• by Aardwolf on 2/26/23, 2:59 PM

  The only thing that actually showed something asymmetric was, afaik, the image showing cosmic rays coming towards you with a certain magnetic polarization of muons. So if cosmic rays themselves would have had the other polarity (coming with N towards you and S away from you), then instead left handed would have been more advantageous? But, then, what's the reason why we don't have any cosmic rays with inverted polarization?

  Also, I think that even if the cosmic rays were not an issue and there was no advantage between left and right, only one of them would eventually survive: if there were both lifeforms with the left and the right orientation, they'd be competing for the same resources (while only being able to mutate/copy/evolve within their own type). Eventually one branch is bound to die out while the other remains, even if due to sheer luck rather than large advantage, and from then on the opposite orientation can't really appear/evolve anymore since it can't compete against the existing much more complex life for those resources.

• by phyzome on 2/26/23, 2:34 PM

  You could just as well say that the cosmic ray pions are more likely to destroy right-handed DNA, which would have the opposite effect. And I don't think essential mutations were particularly hard to come by in the primordial soup. UV light works fine, as do many, many chemicals. Or just more-primitive copying machinery.

  I don't think this weighs very heavily against the random symmetry-breaking hypothesis.

• by bmitc on 2/26/23, 2:31 PM

  The article never explains why right-handed polarization knocks electrons off right-handed helixes more often than left-handed ones. What's the mechanism there?

• by platz on 2/26/23, 1:32 PM

  I finally know what the definition of “evil” life is. Thank you.

• by supernova87a on 2/26/23, 6:01 PM

  Is it me, or does the actual referenced ApJL paper, while having all the trappings of a scientific argument and an almost diarrheal collection of assorted physics references, basically boil down to a very extensively wrapped plausibility argument?

• by jacknews on 2/27/23, 1:38 AM

  It's interesting, but I don't think it answers the question.

  I believe life probably started as an autocatalytic set of chemicals which then 'discovered' they could use rna as a backbone/memory. So the reason for the chirality is at a lower level than discussed here.

  All just a hunch and IMHO of course.

• by dang on 2/26/23, 7:46 PM

  Discussed at the time:

  Cosmic Rays May Explain Life’s Bias for Right-Handed DNA - https://news.ycombinator.com/item?id=23688535 - June 2020 (56 comments)

• by __MatrixMan__ on 2/26/23, 3:45 PM

  Would this affect be the same in both hemispheres? Or would only one side get that handedness bias?

  My knowledge of the standard model isn't good enough to be able to puzzle it out.

• by tough on 2/26/23, 2:39 PM

  As a leftie I wonder what cosmic rays did to me

• by garbagecoder on 2/26/23, 6:05 PM

  Lots of conjectures these days.