So a couple of things i remember from back in the old structural bioinformatics days...

Firstly, there are naturally occurring mixed-chirality (alternating) peptides. They are usually circular iirc.

Secondly, no you can't really have larger proteins with both left and right (ignoring glycine). They would not fold into nice helix/sheet strucures and likely just be random coil.

For cells to have mixed populations of all-L and all-R proteins would mean doubling up all the machinery for creating them.

One theory that I thought was reasonable for why there's a monochiral world is that once the arbitrary choice is made (L or R) then that gets 'locked in' by all the machinery around that choice. As in, L 'won'.

It's a good question, but:

> might be tricky to distinguish chemically during the protein synthesis process

No, amino acids are bind to tRNA by special proteins that have handiness and can easily distinguish the L and R version. Most proteins can only operate on one handiness of the target molecule.

> making e.g. different codons for left and right-handed amino acids infeasible to implement

No, there are 64 codons and we are using them to map only 20 amino acids and a stop signal. So there is a lot of duplication. Some bacterias have one or two more amino acids or a small tweak in one or two of the conversion table, so it's possible to add more stuff there if necessary.

My guess is that mixing L and R amino acid would break ribosomes. The ribosomes read the mRNA and pick the correct tRNA and connect the amino acid that the tRNA has. I guess that the part that makes the connection assumes the correct handiness of the amino acids.

Going down the rabbit hole I found https://en.wikipedia.org/wiki/Nonribosomal_peptide that explains that some peptides (that are like small proteins) are formed by special enzymes instead of ribosomes, and some of them have D-amino acids or other weirs stuff.

fun fact some left handed amino acids are poisonous to most mammals

It's still not obvious how they could be separated at all by pre-biotic processes. You need to go from (in principle anyway) a pretty well-mixed 50-50 mixture to basically only lefties. I believe this is still one of the bigger problems for abiogenesis, and frankly I think you're being too glib about the antimatter problem too. I expect we're eventually going to find out about specific mechanisms that cause those.

That's a relevant paper, but this is the one which "deepened" the mystery: https://www.nature.com/articles/s41467-024-52362-x

It asserts:

> L-proteins need not emerge from a D-RNA World

So if more than one amino acid chirality could have emerged, why did we get the one we got and not several?

From the paper in the parent comment:

> Achiral linearly polarized light interacts with chiral objects and their enantiomers differently. An interesting example is a light-driven motor. Linearly polarized light can rotate a gammadion-shaped gold structure embedded in a silica block as a motor.

Imagine you were using some kind of optical tweezers to manipulate chiral molecules. I wonder if there's a reason that such a device would work better if you had a sample which had the same chirality. Suppose so...

If one of your samples made its way to Earth and replicated... Well that would be a reason for earth proteins to be biased in one direction, despite the laws of physics not prescribing such a bias.

Indeed I did miss that, what an incredible find, I can’t believe this never broke through into my routine news feeds!

https://www.hou.usra.edu/meetings/lpsc2024/pdf/1219.pdf

Well, that's the question isn't it? Is it just a frozen accident, or is there some nonarbitrary reason for the left-handed molecules to be favoured?

That assumption is even more mysterious.

Why one specific handedness "won"? What caused the other one to be food? How can we be sure it was by chance?

Lots of questions.

(Comsuming enantiomers and pooping out metabolic fragments in its native chirality)

Could these asteroids be from when the moon was created?

Right hand rule is just an arbitrary decision defining counterclockwise to be positive, but I guess it's true that it could be "less arbitrary" if certain things are more counterclockwise than clockwise

Though, a new and plausible terrestrial origin of life hypothesis:

Methane + Gamma radiation => Guanine && Earth thunderstorms => Gamma Radiation https://news.ycombinator.com/item?id=42131762#42157208 :

> A terrestrial life origin hypothesis: gamma radiation mutated methane (CH4) into Glycine (the G in ACGT) and then DNA and RNA.

I know I really shouldn’t take this bait, but…no one has proof either way. That said, we have a massive amount of scientific evidence that shows it could have naturally evolved and zero evidence that something created us. Finding something that we don’t understand doesn’t mean we have evidence of creation. Ancient civilizations believed that rain came from the gods because they were unaware of how weather combines with the phases of matter and creates atmospheric condensation.

That being the state of things at the moment, I lean towards the evidence. Also, this is a scientific oriented discussion forum, so you must expect that many people here are going to disagree with you. Could you be correct? Sure, but we just don’t have reason to believe that at this point.

Enough evidence to overcome the enormous pile of evidence that life evolved over billions of years. Often literal piles, in the case of geology, but there's a lot of different kinds of interlocking evidence that suggest a pretty clear picture, even if a few puzzle pieces are still missing.

Unless you're thinking of panspermia, in which case most any hard evidence would do. But that doesn't really sound like your thing.

- a former creationist

For better or worse the standard of evidence for almost everything is more like “smoking gun” than “I found a bullet”. In some cases this is bad, in others it is good. Just consider all the criminal matters where the crime is only a crime if you can additionally demonstrate intent, which is strange right, since it doesn’t change outcomes / injuries at all. Since sufficiently ancient guns won’t even be smoking anymore this will be problematic for creationists even if they are correct, so I think we’d need a new kind of burning bush.

I certainly am trending that way as I grow older. As I've recently started to re-dive into Christian theology, the fine-tuning argument seems more and more interesting, and it's pretty difficult to find "good" secular arguments against it.