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Comment by geysersam | Hacker News Reader

If this is such a powerful niche for an organism to be in, why haven't they already emerged naturally over the 3 billion year history of life on earth?

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I think that's a great question!

But I also think it's similar in a way to 'why matter and not antimatter' or 'why ~3d space rather than no structure' (much less obvious the second one!).

But I think the real answer here is touched-on by nmstoker in a sibling comment: We're so far down the energy-gradient of the current-default-chirality on earth, that it would take exploration of an insanely-deep valley for all molecules in a cell (there are zillions even in a bacteria!) to flip the the whole thing to the alternate chirality.

It's a bit like the idea that a good chunk of matter (like even a whole amino acid) could all spontaneously-convert to antimatter at once, sure it's possible in theory but the chances are so low we might as well say it's impossible.

But then this is also an argument against spontaneous emergence of life, right? Sicne mirror-life could just emerge on its own, instead of evolving from regular life.

Even if the timescales and environments conducive to the emergence of basic life exist on Earth today, I would expect any spontaneously-emerged proto-cell to almost immediately get eaten by normal microorganisms without any meaningful chance to proliferate. Reverse-chirality or no, it'd be a glorified bag of nutrients compared to the more evolved flora surrounding it.

Not necessarily. A pre-condition for emergence and evolution towards more complex life forms can be removed from the system by the existing complex life forms.

At the end of the day, all life is competing for energy. Spontaneous self replicators would be uncompetitive with existing biology and would never make it to more complex stages.

Life forms are effectively locked into the current set of chiral molecules. For a cell to flip, it would need all the corresponding mirror changes to happen as well, since the behaviours are interlinked - this makes it something that would be highly unlikely to occur due to evolution and why it's only realistic if carried out by people all at once.

That argument would be a good one if the time scale was shorter.

But over the course of the entire evolution of life on earth...

It's hard to argue that the step from chirality A to B is larger than all the other insanely unlikely steps evolution has taken. From earliest proto-life to complex multicellularity and beyond.

The issue is not that one chirality has to flip and you can accumulate change over long time periods, it's that every occurrence of chirality had to flip at once or the things won't work together: so the DNA has to mysteriously all get reversed (v difficult when all the DNA/RNA machinery is handed in the other direction), plus all the supporting items need to flip at that same point for all sorts of other cell functions that interlink (energy supply, waste handling etc etc).

But if your argument is given the extraordinary timeframes then why didn't life evolve again separately in the other chirality, so it wasn't descended from the earlier cells but was just a new unrelated line, then that is harder to generalise about but I recall hearing that there are sometimes slight advantages to one chirality over the opposite in certain cases as the reaction rates can differ (this is the same concept behind kinetic resolution). Perhaps life based on the less effective form would get crowded out by the more effective form, but I would defer to experts here as it's not my area.

The problem for new life with a different chirality would be that it would have to compete with the existing organisms. And the existing ones are already far, far more optimized than any new kind of life would be. So this is not comparable to the way life originally developed, where no competition existed and the entire environment was very different.

I remember learning about chirality from an Isaac Asimov article about “left-handed” sugar which, while it tastes the same as normal sugar, is unable to be metabolized by our digestive systems. I would imagine that something similar would come into play with a left-handed organism: it would be unable to consume right-handed nutrients so would starve to death. Overall, my assumption is that mirror bacteria would be unable to interact in any significant way with non-mirror organisms, but being somebody whose expertise begins and ends with reading an Isaac Asimov article on the subject as a 10-year-old, I’m very open to the possibility that I may be wrong.

The article mentions that such organisms may be able to subsist on nutrients which lack chirality.

its not a powerful niche. theres just no food available. even autotrophs and prototrophs scavenge for fully formed chiral building blocks if possible because building ftom scratch is more energy intensive than recycling

You go past all the path dependence with a external attempt at it.

https://www.science.org/doi/10.1126/science.ads9158#sec-1

I'm not particularly happy about that argument. It perfectly explains why complex organisms like bacteria don't suddenly evolve to its mirror image. But it doesn't explain why this hasn't happened at a much earlier stage in the evolutionary history.

We don't have multiple lineages of life in general. Everything is using 4 base pairs, roughly the same set of amino acids and some universally preserved genes. So there was a bottleneck where everything else was outcompeted.

So even if some mirror life chemistry was going on at some point, it just didn't make it along with all the other things that didn't.

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