[HN Gopher] Cosmic rays may explain life's bias for right-handed...
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Cosmic rays may explain life's bias for right-handed DNA (2020)
Author : deesep
Score : 78 points
Date : 2023-02-26 11:32 UTC (11 hours ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| bmitc wrote:
| 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?
| photochemsyn wrote:
| The linked paper has some clues:
|
| > "Muller (1927) that showed that the mutation rate is
| proportional to the radiation dose, much of it attributable to
| ionization by cosmic rays. The muon component dominates the
| flux of particles on the ground at energies above 100 MeV,
| contributing 85% of the radiation dose from cosmic rays (Atri &
| Melott 2011). Muons have an energy sufficient to penetrate
| considerable depths, and they are, on average, spin-polarized.
| _Ionization by spin-polarized radiation could be
| enantioselective_ (Zel 'dovich et al. 1977). Therefore, we
| argue that the mutation rate of live and evil organisms would
| be different. As there could be billions or even trillions of
| generation of the earliest and simplest life forms, a small
| difference in the mutation rate could easily sustain one of the
| two early, chiral choices."
|
| https://iopscience.iop.org/article/10.3847/2041-8213/ab8dc6
|
| The actual models and calculations they employ to get an
| estimate of bias are really complicated, but if I read
| correctly, it's only a diffence of +/- 10^-7 relative to 50:50
| equality:
|
| https://iopscience.iop.org/article/10.3847/2041-8213/ab8dc6#...
|
| They suggest this is somehow amplified over time, but it's
| equally likely life just settled on one or the other handed
| version randomly.
| platz wrote:
| you appear to have failed to understand the question that the
| questioner was asking.
| photochemsyn wrote:
| There are three models in the second link describing the
| possible mechanisms, none of them seem all that
| condensable. E.g. this seems to be at the center of one of
| their approaches:
|
| > "If we only consider second-order perturbations,
| involving two atomic sites in addition to O, the
| interaction cannot be geometrically chiral. So, if the
| fount is completely isotropic, we need to consider third-
| order perturbations, involving three distinct atomic sites,
| in addition to O, to have the possibility of a chiral
| coupling. Furthermore, it is apparent that the strength of
| the perturbations associated with each site is proportional
| to the scalar charges Q, and a third-order perturbation to
| the mutation rate will be simply proportional to the
| product of these charges, which is unchanged on inversion.
| It is only their relative locations that matter."
|
| > "There are several types of third-order perturbation. For
| example, we can use the first-order displacement due to the
| first charge to evaluate the electric field due to the
| second charge along the perturbed trajectory and compute a
| second-order velocity perturbation to calculate the third-
| order magnetic displacement at O. Alternatively, we can
| take the second-order displacement at O and combine this
| with the electric field due to a third charge to calculate
| a third-order change in the kinetic energy of the cosmic
| ray at O. We must then sum over all permutations of
| charge..."
|
| See also Fig B3:
|
| > "Figure B3. Example of electric chirality (barber pole
| model). The electric charge distribution of two biopolymers
| of opposite chirality projected onto a cylinder is shown,
| together with the unperturbed vs. perturbed trajectory of a
| magnetically polarized cosmic ray interacting with the
| molecule."
| [deleted]
| phyzome wrote:
| 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.
| SideburnsOfDoom wrote:
| > 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.
|
| You could, and it's going to be one effect or the other that is
| stronger - One of the chiralities will have an advantage due to
| this effect, relative to the other. And now we know which one.
| yyyk wrote:
| >You could, and it's going to be one effect or the other that
| is stronger... And now we know which one.
|
| If the result would have been the other way around, we could
| have had here on NH the same story with the same argument
| with about the same conclusion. That's not very helpful.
|
| Note also the null hypothesis (none of these 'effects'
| matter, and choice is rather random) has the same non-
| explanatory potential as the story does.
| thomastjeffery wrote:
| We already know the conclusion of this story: right-handed DNA
| is what exists today.
|
| The effort here is not to create a story _blindly_ from known
| fundamentals into wherever that story might lead. The effort is
| to _fill the gap_ in story from known fundamentals to a known
| conclusion.
|
| We don't know if other circumstances could have lead to another
| conclusion, and that would indeed be an interesting question to
| answer. But we do know that the conclusion _we are living in_
| somehow favored right-handed DNA.
|
| If this backstory leads to two potential conclusions - either
| left-handed or right-handed DNA, and not both - and we are
| already experiencing one of those conclusions (right-handed
| DNA), then it is OK for that backstory to be ambiguous. We
| don't need to prove the impossibility of the left-handed
| result. We only need to illustrate the impossibility (or
| unlikelihood) of both conclusions existing together at the end
| of the story.
| iForgotMyPW wrote:
| [flagged]
| supernova87a wrote:
| 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?
| iForgotMyPW wrote:
| [flagged]
| dang wrote:
| 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)
| __MatrixMan__ wrote:
| 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.
| tough wrote:
| As a leftie I wonder what cosmic rays did to me
| Lapsa wrote:
| yeah, article initially reads somewhat confusing
| garbagecoder wrote:
| Lots of conjectures these days.
| platz wrote:
| I finally know what the definition of "evil" life is. Thank you.
| fractallyte wrote:
| The origin of the word _sinister_...
|
| https://www.merriam-webster.com/words-at-play/sinister-left-...
| xwdv wrote:
| When you look at the DNA of evil beings, such as the kinds you
| might pull through a portal to hell, you would find their DNA
| spirals to the left instead of the right.
|
| It implies hell is bathed in high energy rays with opposite
| polarization from the cosmic rays in our own universe.
| BuyMyBitcoins wrote:
| If true, Hell is indeed a sinister place.
| idlewords wrote:
| The problem with this explanation is that the high
| temperature of hell would tend to racemize enantiomeric
| mixtures, and this process would easily dominate the very low
| cosmic ray effect.
| nextaccountic wrote:
| This also explains why left-handed people are identified with
| the devil. The whole universe is biased against being left-
| handed
|
| https://en.wikipedia.org/wiki/Bias_against_left-
| handed_peopl...
| h2odragon wrote:
| It doesn't explain why the people on the other side of the
| portal favor goatee styled facial hair, tho.
|
| That is a deeper mystery.
| photochemsyn wrote:
| 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. ???
| idlewords wrote:
| This argument depends on whether there is a 'natural'
| biochemical scaffolding for life that resembles what we have on
| Earth. There's no reason in principle that a totally different
| biochemistry that made use of racemic mixtures couldn't work,
| but here we run into the wall of being unable to imagine forms
| of life substantially from our own. It's very easy to get into
| a loop of circular reasoning when answering questions like why
| life on Earth has a preferred handedness.
| Garlef wrote:
| > This argument depends on whether there is a 'natural'
| biochemical scaffolding for life that resembles what we have
| on Earth.
|
| I don't think the argument hinges on a notion of
| 'naturality'.
|
| Instead it seems to be more of an statistical/occams razor
| kind of argument: Mixed chiralities would introduce a lot of
| incompatabilities among the building blocks, making the
| emergence of a stable form/process more unlikely.
|
| And also the argument (as far as I understand it) is in no
| contradiction to your caveat: Of course, a stable
| form/process might have emerged with mixed chirality. But it
| did not and was never very likely to begin with.
|
| [Note: This is not a statement about assigning truth values
| to these arguments but rather I hope it clearifies the
| argument itself.]
| idlewords wrote:
| What I'm saying is that the statistical argument already
| bakes in assumptions, since we don't know anything about
| the underlying distribution.
| CyanBird wrote:
| OP already stated that the initial distribution would
| ~seem to be ~50:50 per winchcombe meteorite
| idlewords wrote:
| I'm sorry; I meant the set of all possible
| biochemistries.
| lumost wrote:
| Is this question substantially different from "why don't
| animals have four legs and 2 hands?"
|
| It strikes me that there could have been dozens of "genesis"
| events early in earth's history which evolution aligned into
| a few winning blueprints.
| danuker wrote:
| > With a series of toy models, the researchers calculated that
| the biased cosmic ray particles were ever-so-slightly more
| likely to knock an electron loose from a "live" helix than from
| an "evil" one, an event that theoretically causes mutations.
|
| With a dataset of n=1 (a single example of abiogenesis), there
| is quite limited evidence for the claim. But you don't need
| much of an effect to influence something that could go either
| way.
| Aardwolf wrote:
| 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.
| SideburnsOfDoom wrote:
| The mention the asymetry is here:
|
| "When the high energy protons in cosmic rays slam into the
| atmosphere, they produce particles called pions, and the rapid
| decay of pions is governed by the weak force -- the only
| fundamental force with a known mirror asymmetry."
|
| Pion decay is apparently not symmetric. I don't know much about
| the topic, maybe start here:
| https://en.wikipedia.org/wiki/Weak_interaction#Violation_of_...
|
| It doesn't seem to be the case that "we don't have _any_ cosmic
| rays with inverted polarization " - just that there is more in
| one direction than in the other. Even a slight bias would add
| up over time.
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