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Comment by disqard | Hacker News Reader

> Not only does Bennu contain all 5 of the nucleobases that form DNA and RNA on Earth and 14 of the 20 amino acids found in known proteins, the asteroid’s amino acids hold a surprise.

> On Earth, amino acids in living organisms predominantly have a ‘left-handed’ chemical structure. Bennu, however, contains nearly equal amounts of these structures and their ‘right-handed’, mirror-image forms, calling into question scientists’ hypothesis that asteroids similar to this one might have seeded life on Earth.

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Hm, why would chirality need to be a consequence of the panspermia hypothesis? I thought the mission defined "necessary ingredients for life" as any bio markers that might have seeded a primordial soup on Earth.

One way to interpret the results here is that all the building blocks of life can naturally be formed right here on earth or anywhere that has conditions similar to where Bennu came from.

The fact that the components on the asteroid is racemic meant or heavily suggested that they were formed using non-biogenic means. And if so, it also means that Earth could have had the same thing happened a long time ago, leading to the seeds of life.

tl,dr: this discovery weaken the panspermia hypothesis.

I don’t see how it weakens the hypothesis. We are making the assumption that life must be chiral. Maybe chirality is something more or less inherited. So it is a coin flip whether your last ancestor was left handed or right handed. So a racemic mix doesn’t disprove that because the population could be heterogeneous for this phenotype and not yet fixed. There are many heterogeneous phenotypes on earth.

It seems unlikely for such a hard thing as spontaneous creation of biomachinery from amino-acids happened at the same time for both left-handed and right-handed versions. And once you got machinery that uses, for example, left-handed version, it's pretty much game over for the alternative because life grows, evolves and colonizes at insane rates compared to random chemical processes. So it's very likely that wherever carbon based life exists it's chiral rather than racemic.

If life is chiral it weakens the hypothesis. If life is not chiral it changes nothing.

Nonzero-weighted sum of something and nothing is something.

Abiogenic synthesis of "primordial soup" is fairly straight forward, see the Miller-Urey Experiment. A soup of random amino acids and whatnot might be created in a number of ways on Earth, so it's a stretch and not very interesting to suppose that such soups didn't exist on Earth and were brought in by space. The interesting panspermia hypothesis is that actual life came down from space. Finding amino acids in space could be evidence for that, but if the stuff found in space is mixed chirality that undermines those chemicals as evidence of life in space. Mixed chirality implies a mundane abiogenic origin.

Couldn’t there exist a chirality filter on earth?

The question is "how?" though. The chirality filter on Earth is basically Earth-life: it almost ubiquitously creates left-handed amino acids. But all known chemical ways of doing this produce racemic mixtures of both types.

If we found say, a high pressure synthesis method which was heavily biased chirally, then that would be good evidence but none has been discovered.

Is there any evidence of one or such a theory?

I am a total noob in chemistry and Claude said that it was a great question, but the answer were quite unclear about the feasibility. Mainly theoretical filters on lab scale. So that led me to put forward the thought.

Nobody has found or even so much as proposed a very good one. There's a bare handful of reactions that can tend to amplify chirality, but most uncontrolled chemistry (including things like Miller-Urey) produce a biologically-useless random high-entropy mix, and it is fairly unclear that these handful of reactions, which aren't even necessarily obviously useful ones and require some fairly special conditions, could possibly overwhelm the entropy of all the other reactions blasting out random combinations of things.

AFAIK there isn't even a known way to start with "normal chemicals" and produce highly-chiral reactions reliably in the lab. We get all our chiral molecules by extracting them from biology.

I think it should be possible to separate stereoisomers by crystallization, at least in some cases. You then have to sort through the crystals.

These findings show that biochemical compounds, once thought to be fragile and denaturate quickly in non-earth conditions, can apparently survive space conditions for a long time, even in universe metrics, so they could travel far. If they can, maybe cells can too. But that doesn‘t mean it‘s likely to find actual life on a random asteroid. Of course there is equal chirality in lifeless conditions. The ultimate question is: How did the first cell came to be? Everything after that seems explainable, if not predetermined. But that the first cell just randomly happened in the primordial soup - that looks extremely unlikely, yet it‘s the best explanation so far. If cells could travel on asteroids, it’s (equally? Less? Who could tell?) likely that the first cell just dropped here, intentionally or otherwise. Which would put the question of creation just to a different time and place. Somewhere in the universe, billions of years ago, life happened. Maybe it spread through the galaxy. Maybe sometimes a life carrying planet explodes, spreading asteroids with cells. Maybe some of them drop on planets with the right conditions. Given enough time, how unlikely is that as compared to random creation here?

> But that the first cell just randomly happened in the primordial soup - that looks extremely unlikely,

I don't understand this. Aren't cells just spherical structures that would form naturally from hydrophobic molecules suspended in water/tide pools? That seems likely to me, but I have no background in chemistry/biology.

Nick Lane argues in "The Vital Question" [0] that simple cell membranes are not enough for complex life and complex membranes may have evolved in matrixes around hydrothermal vents.

[0] https://en.wikipedia.org/wiki/The_Vital_Question

No! A cell is an incredibly complex machine and we have just begun to understand how it works! Admittedly, the wikipedia page for cells don‘t transfer that fact well (https://en.m.wikipedia.org/wiki/Cell_%28biology%29). I can‘t explain it all here, but yes, cells are essentially made of proteins and other organic compounds which have been found on that asteroid (and other asteroids before). But that is similar to the fact that a human is made of 14 chemical elements - it’s not telling you at all how the machine came to be and works. The Miller-Urey Experiment showed that those organic compounds could be created from anorganic material, which reversed the old belief that only organic matter can create organic compounds. But from there to a working, functional cell is still a long, looong way. Like, an aminoacid/ protein is a nail (and a plate, and a valve, and a million other components), a cell is a spaceship. That can harvest it’s environment and build new spaceships. Every living thing consists of cells, and we all go back to one first cell, that‘s the obvious conclusion when studying the system of life. But that first cell must have been incredibly complex, alone for the fact that it could take surrounding matter and build a new cell out of that.

There is an enormous amount of functional structure in even the simplest known cell. Cut down as much as possible, it's still billions of atoms.

There's a weak connection, but to me it reads like clever marketing.

Actually finding chirality in space would be extremely cool in that it would mean one of two things:

- the panspermia hypothisis is correct, or

- some non-biological process creates chiral molecules.

Physics itself has plenty of chiral processes, but they only show up in the weak interaction. As the name implies, the weak interaction is really really weak and essentially doesn't exist on the energy scales of molecular interactions. So chirality would be a bit of a smoking gun for panspermia.

On the other hand, not finding chirality just means we don't have a smoking gun. There might be another asteroid flying around that is 100% chiral, or maybe 50% of them and we were just unlucky.

The briefing joins the point about chirality with evidence against panspermia, but really that might miss the point. Chiral or not, abundant amino acids in space means that one of the many steps to create life is relatively simple. If we could show that every subsequent step is simple that would be a big blow against panspermia. But in that case ruling out panspermia would be pretty cool, since it would suggest that life exists everywhere.

> the panspermia hypothisis is correct

What I don't understand is why would chirality and panspermia be so tightly linked.

The data right now still leaves every option on the table just because having any ratio of chiral molecules doesn't have to define how life evolves. It can't answer whether those molecules formed on Earth or hitched a ride on an asteroid, or life itself formed here or was brought here.

We can assume that in a soup with balanced proportions of each chirality, the left handed molecules created a self replicating mechanism (some definition of "life") first or faster than right handed molecules, either accidentally or because some yet undiscovered advantage. Whether this happened on Earth, or was brought to Earth by one or more of the millions of asteroids is hard to prove.

That's a good point. You also need consistency in the chirality on asteroids (also with earth).

If we find one asteroid with chirality that doesn't match earth, it's good evidence that self replication just happens spontaneously rather than being seeded.

If we find only one asteroid with one chiral molicule that does match earth, that supports panspermia very weakly: it still might just be random chance. But multiple matching molecules, on multiple asteroids, starts to seem like evidence for a common source.

On the other hand: If we find multiple molecules with matching chirality, on multiple asteroids, and none of them match with earth... well, we should probably start preparing for the invasion.

> There might be another asteroid flying around that is 100% chiral, or maybe 50% of them and we were just unlucky.

The more asteroids we look and not find any asymmetry, the more evidence we have that life never existed on any of them.

The thing with panspermia is if you believe in emergent life you almost have to believe in the possibility of panspermia as well. It is why NASA uses clean rooms after all.

I don’t know a ton about how chirality works. Couldn’t it just be that half(or some number) the asteroids contain left handed and half contain right? We only have a sample of one. Or is there something fundamental about left handed molecules that gives us reason to believe that if we see right handed ones once we would rarely see left handed ones in similar disconnected systems?

Chirality means that there is a mirror image of a molecule that cannot be twisted into the original shape, despite being structurally identical. Due to the particular ways molecules tickle each other in living organisms to do interesting things, that means that the mirror image (racemate) of a molecule does something different.

In chemical synthesis, most (but not all) processes tend to preserve chirality of molecules: replacing a bunch of atoms in a molecule with another set will tend to not cause the molecule to flip to a mirror image. If you start from an achiral molecule (one where its mirror image can be rotated to the original), almost all processes tend to end up with a 50-50 mix of the two racemates of the product.

In biochemistry, you can derive all of the amino acids and sugars from a single chiral molecule: glyceral. It turns out that nearly all amino acids end up in the form derived from L-glyceral and nearly all sugars come from D-glyceral. The question of why this is the case is the question of homochirality.

There's as yet no full answer to the question of homochirality. We do know that a slight excess in one racemate tends to amplify into a situation where only that racemate occurs. But we don't know if the breakdown into L-amino acids and D-sugars (as opposed to D-amino acids and L-sugars) happened by pure chance or if there is some specific reason that L-amino acids/D-sugars is preferred.

"I Applied Wavelet Transforms to AI and Found Hidden Structure" https://news.ycombinator.com/item?id=42956262 re: CODES, chirality, and chiral molecules whose chirality results in locomotion

Do any of these affect the fields that would have selected for molecules on Earth? The Sun's rotation, Earth's rotation, the direction of revolution in our by now almost coplanar solar system, Galactic rotation

That’s definitively helpful thank you. I guess I’m still surprised that this finding is being taken to mean left handed molecules couldn’t have come from asteroids. If anything it seems to me to increase the likelihood of the panspermia hypothesis.

The reason this makes panspermia less likely is that the panspermia hypothesis would prefer to put the point at which all amino acids become L-amino acids before arrival on Earth, whereas the evidence is that the amino acids on this asteroid is before that point.

That makes sense. I’m just thinking statistically though. All we’ve proven is that some asteroids have R-amino acids. If it’s a 50/50 shot you could easily find a dozen R asteroids because it starts to look really bad for it being a 50/50 split.

I guess the reason it makes things more likely is because homochirality happens so aggressively that finding right handed molecules at all drastically reduces the odds of left.

This is a good explanation of chirality but I think the parent meant to say enantiomer instead of racemate:

– enantiomer refers to each mirror image of the molecule. https://en.wikipedia.org/wiki/Enantiomer

– racemate refers to a 50-50 mix of the two enantiomers. https://en.wikipedia.org/wiki/Racemic_mixture

I think the main point is that if amino-acids in the sample were left-handed they might have come from Earthly contamination of the sample.

Since they are a mixture of right and left-handed they definitely come from space.

Chirality of life is not a consequence of panspermia, just a consequence of incompatibility so one had to win eventually.

I don’t quite see how it disproves or undermines panspermia - surely if earth began with a racemic mixture of the building blocks, life could have started with either chirality (both being an unlikely scenario what with it vastly increasing complexity and therefore decreasing probability of evolution), and would then select for that chirality exclusively - so it’s no great surprise we have ended up with a single-handed biosphere.

An explanation is required for why that selection occurred here, and not in whatever biosphere from which this one was panspermically inseminated.

I wonder how much deep sea research we'd have to do to find all the possible impact craters. There's gotta be lot of evidence at the source.

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