The molecular basis of all life is mysteriously聽 asymmetric, only using molecules on one side of what should be the equivalent mirrored pairs.聽 The universe has a similar mirror asymmetry, and it may be that our very DNA inherited its聽 twist from the underlying handedness of reality.

Our universe is beautifully聽 symmetric.

For the most part, the laws of physics work the same if we swap聽 left with right, positive with negative charge, and even the future with the past.

But there are聽 subtle violations to these symmetries—for example, a perfect mirror reflection of our universe聽 would work very slightly differently.

This asymmetry is thought to be the reason聽 why our universe is made of matter rather than antimatter.

We’ve talked about it before.

But there’s another violation of mirror symmetry that’s much closer to home.

Many of the most  important molecules used by life are exclusively from one side of the mirror.

For example, the DNA聽 helix always curls one way and never the other, and the amino acids and sugars used by life are聽 never used by life in their mirror-reflected form.

The reason for this .. still isn’t known,  and there are several competing hypotheses.

Today we’re going to look at the idea that the  handedness of life is connected to the fundamental mirror-asymmetry of the universe itself.

We’ll  focus on a paper by Noemie Globus and Roger Blandford that proposes that this asymmetry may聽 have been delivered to Earth by cosmic rays.

But before we get to the cool physics and聽 space stuff, let’s do some biochemistry.

Something “chiral” if its mirror reflection  can’t then be rotated to perfectly match the original.

Your hands are chiral because your聽 left hand can’t be rotated to match your right.

Molecules can also be chiral if their mirror-image聽 versions are similarly non-equivalent.

By analogy, we label chiral molecules as either left-handed聽 or right-handed, although the assignment of the label for each molecule is somewhat arbitrary.聽 These mirror-image forms, called enantiomers, are chemically similar but can behave very聽 differently in biological systems.

Sometimes only one enantiomer is biologically active.聽 Sometimes one is medically useful while the other is dangerous.

For example, the聽 right-handed enantiomer of thalidomide has anti-nausea properties, while the聽 left causes terrible birth defects.

The chiral bias is even more profound for the聽 molecules that make up living organisms.

For example, life only uses left-handed聽 amino acids and right-handed sugars.

These molecules join together to construct DNA,聽 and so the resulting helix only winds one way.

We say that DNA is right-handed, as is all RNA.

In principle, you could build a perfectly stable organism with all molecules having聽 the opposite handedness, but we never, ever see that in nature.

This uniform聽 selection of a single handedness across biological molecules is known as homochirality.

The homochirality of life was discovered at the same time as the chirality of molecules,聽 back in 1848 by a young Louis Pasteur.

He found that crystals of synthetic tartaric聽 acid came in two distinct mirror-image forms—mirror images of each other—while those from  natural sources like grapes only displayed one form.

These crystals even rotated the polarization聽 of light in the opposite direction to each other.

We now know that these mirror-image forms聽 are due to the mirror-image enantiomers of tartaric acid molecules, and that life only聽 produces the right-handed version of it.

A century later, the Miller-Urey experiment聽 further demonstrated this divide by showing that, under simulated prebiotic conditions, amino acids聽 and other organic compounds form spontaneously when simple compounds are hit with the energy聽 source—but always as 50-50 mixtures—what we call a racemic mixture.

But in life, only the聽 right-handed amino acids are formed and used.

The Miller-Urey experiment was meant to聽 approximate the conditions of the Earth, pre-life or prebiotic Earth.

The primordial聽 soup on that Earth surely started out racemic.

But the life that sprang from this soup was聽 homochiral … or at least the life that survives to this day is.

So when and how did that happen?

We don’t know exactly how life first formed on Earth, but we’re probably safe in  breaking it up into three phases.

In the first phase we have only simple聽 organic molecules like amino acids that are still monomers—they haven’t yet formed complex  polymer chains.

This is the world simulated in the Miller-Urey experiment.

Following the lead of聽 the authors of the paper we’re about to get to, we’ll call this the prebiotic phase.

Then we have the phase with increasingly complex molecular chains with emerging abilities聽 like auto-catalyzation and self-replication.

RNA is the prime candidate for the molecule driving聽 this phase.

Technically this is also prebiotic, but again following our researcher’s lead  we’ll call it transbiotic—transition between prebiotic and the biotic phase—which is everything  after the development of the first very simple single-celled organism—the first true lifeform.

At some point during these development phases, the stuff of life became homochiral.

Perhaps,聽 for example, the primordial soup ended up with a significant excess of one chirality of amino acid.聽 Or maybe homochirality emerged in the transbiotic stage, with one winding of RNA winning out over聽 the other.

Or did pre-life evolve in parallel in both chiralities all the way to the first cell?

There’s no known process to wholesale wipe out one chirality.

All proposed processes at best give聽 one side a small numerical advantage.

In order to achieve full homochirality, this advantage聽 needs to be amplified via a positive feedback loop that can lead to exponential domination聽 of one chirality until the other is wiped out.

Example mechanisms include autocatalysis—the  chiral molecule promotes chemical synthesis of its own kind.

Or anticatalysis—the  opposite chirality is suppressed.

Or self-replication—more and more material is  locked into one chirality, starving the other.

Auto- and anti-catalysis are potentially possible聽 as early as the prebiotic era, although the known mechanisms for simple monomers don’t seem  efficient enough to reach full homochirality.

But once we get to the transbiotic phase, with聽 the complex molecular machinery enabled by RNA, catalysis mechanisms and reproduction could and聽 probably should lead to homochirality, as long as one chirality has an adaptive or numerical聽 advantage quickly, and given the exponential nature of that process this would probably聽 complete long before the first cell formed.

But we’re still to explain how that advantage  first appeared.

There are some non-fundamental options.

It could be that Earth was seeded with聽 amino acids that already had a chiral bias, brought by comets and asteroids.聽 Those in turn could have picked up that bias due to polarized light聽 selectively destroying one enantiomer.

There’s disputed evidence of this  chiral bias in meteorite material, although it’s hard to tell if this stuff was  contaminated by the very biased Earth amino acids.

The chiral bias could also have arisen聽 right here on Earth.

For example, there’s been some experiments with magnetite  find a kind of ‘cooperative feedback’ between chiral molecules and magnetic surfaces, a sort聽 of positive feedback loop that could induce homochirality.

Or perhaps selective absorption聽 of one enantiomer by the mineral surfaces where these molecules were stewing in the prebiotic era.聽 Another pretty reasonable possibility is that the advantage to one chirality was random—perhaps one  side gained a slight numerical advantage just by random chance and this snowballed, or perhaps one聽 side reached an important adaptive step first—for example the first RNA molecule just happened聽 to be right-handed—and again snowballed from there.

In this case, the winner of the聽 chirality race would be completely random.

But we promised to try to link the聽 mirror-asymmetry of life to something more fundamental, and so let’s do  that.

The chiral asymmetry of the universe is most clearly manifest in the weak聽 interaction, and this is how physicists have tried to connect it to the homochirality of life.

One possibility stems from the fact that the two enantiomers of a chiral molecule have different聽 stabilities.

Atoms are bound into molecules mostly by the electromagnetic force, and the weak聽 force, being true to its name, contributes of order one part in a quintillion of the binding聽 energy.

On its own, this 10^-18 difference in stability wouldn’t be enough of a seed to push  Earth’s pre-biochemistry to homochirality.

Abdus Salam, one of the co-Nobel laureates for聽 electroweak unification, proposed a mechanism for enhancing this difference.

If amino acids formed聽 in extremely cold environments like the outer solar system, they could enter a superfluid state聽 in which the less stable chirality gets converted to the more stable.

These could then be delivered聽 to Earth by the comets and asteroids I mentioned earlier, with enough of a weak-interaction-induced聽 chiral bias to then snowball to homochirality.

But there may be a way to extract the handedness聽 of the weak interaction without so many steps.

This chiral asymmetry manifests most strongly聽 in particle decay products mediated by the weak interaction.

For example, when a pion decays via聽 the weak interaction into an electron or muon, the magnetic moment of that particle is always聽 in the opposite direction to the particle motion.

And this handedness of decay products, gives聽 us a new way to impose the handedness of the universe on all of life.

And the delivery聽 mechanism will be cosmic rays—high energy particles from energetic space phenomena聽 like the Sun or supernova explosions.

When a cosmic ray arrives at earth, it聽 collides with other particles in the air, creating an ‘air shower’ – the kinetic energy  of the cosmic ray is converted into new pairs of particles that cascade into more collisions and聽 particles, just like in an accelerator collision.

At earth’s surface, you’re receiving a constant  radiation dose from cosmic ray showers.

In fact, the cosmic ray radiation dose is an important聽 variable that sets the mutation rate in life on earth.

While the bulk of the particles end up聽 as electrons and photons, the atmosphere does a good job filtering them out before they get聽 to us.

It is the rare and highly penetrating muons that are responsible for about 85% of our聽 cosmic ray radiation dose despite only being 1% to 2% of the particles in the shower.

As high energy ionizing radiation, these muons can break up molecules.

But聽 the ability of muons to dump energy into a molecule to break it depends in part on the聽 relative chirality of muon and molecule.

So, if muons have a chirality preference due to the聽 fundamental asymmetry of the universe, this can be translated into an evolutionary pressure against a聽 particular molecular chirality on the early Earth.

The paper by Globus & Blandford聽 tries to estimate the strength of this effect and whether it’s strong enough  to initiate the cascade of homochirality.

They use computational models to approximate聽 the damage caused to different types of chiral molecules based on these polarized muon passages.聽 Although they’re not full quantum mechanical calculations, they have models for simpler聽 chiral monomers like amino acids and helical polymers like RNA that lets them understand聽 how the differences in ionization can lead to molecular damage.

The difference for monomers聽 like amino acids was very small, indicating that significant movement towards homochirality is聽 unlikely to have occurred in the prebiotic era.

But they found that the difference for helical聽 polymers like RNA was much stronger.

Left-handed RNA is preferentially damaged, and perhaps by聽 enough to initiate the path to a right-handed RNA world in the transbiotic era.

At least,聽 given a strong enough rate of cosmic ray muons.

While muons make up a large fraction of聽 the cosmic rays that reach the ground, it’s questionable whether there are  enough to initiate a chirality snowball at current levels.

However, the cosmic聽 ray flux changes over time.

For example, nearby supernovae can dramatically increase聽 that flux.

We see geological evidence for supernova-sourced cosmic ray spikes from more聽 recent times.

We know that star formation and so supernova activity must have been higher聽 when the Earth first formed, giving more chances for the cosmic ray spikes.

It’s also true  that the Sun was more active when it was younger, which means more solar cosmic rays at just the聽 time when they’re needed to kick-start life.

So far this is a lot of speculation.

But聽 we’re scientists, and so we want to find tests.

The molecular inhabitants of the聽 pre- and transbiotic Earth are long gone, and with them any record of non-homochirality.

But聽 this hypothesis that homochirality originates in the chiral asymmetry of the weak interaction聽 does make a dramatic prediction that we can test.

It suggests that all life should have the聽 same handedness, everywhere in the universe.

We should never find mirror-reflected life.

If聽 we do that’s bad news for our chirality being cosmically ordained.

We also should probably not聽 shake hands with it—it’ll be socially awkward and a potential biohazard due to those mirror聽 pathogens that we have no defences against.

But the alien test is probably a very long way聽 off, if we can ever perform it at all.

But there’s something we can try much sooner.

We can look聽 for chiral biases in amino acids from space.

I mentioned that there’s disputed evidence of this  in meteorite samples.

Well, we need pristine samples to make sure they’re free of Earthly  contamination.

We’ve found amino acids on comets, but haven’t yet been able to test for chiral bias.

If we find that these molecules are always biased to left-handed enantiomers it’s evidence of a  fundamental bias.

If we see different biases in different samples then it might suggest a less聽 fundamental source such as polarized light.

When we finally get samples from the sub-surface聽 oceans of the moons Europa and Enceladus—well, maybe we find life there.

But if we just find聽 amino acids, that's still exciting.

A measure of the chiral bias will tell us a lot about any聽 mechanism for the amplification of that bias.

A significant bias would actually be a point聽 against the cosmic ray hypothesis because we expect the surface ice sheets to stop those muons.

On a similar note, if we ultimately determine that Earth life started at the bottom of the ocean,聽 that’s a ding against the muon hypothesis because the ocean stops muons.

Muon-sourced聽 chirally-bias life probably started in tidal pools where it’s exposed to cosmic rays.

If you were paying close attention when we flashed up the paper earlier, you聽 might have noticed it was from 2020, so we caught up with Dr. Globus for this video to聽 ask about what’s new.

Right now they’re doing new experiments at the ISIS Neutron and Muon Source,聽 blasting both left and right handed RNA with beams of spin-polarized muons to measure reaction聽 rates and see how the biochemistry responds.

We might know a lot more about the origins聽 of life on earth in just a few months.

These experiments are also going to fill in key gaps in聽 our models for understanding radiation effects on human health.

After all, particles breaking your聽 DNA is one of the main ways radiation kills us.

So, yeah, astrophysics is saving lives聽 again!

Which is a nice consolation prize, even if we don’t find out why all of biochemistry  is one-handed.

But personally, I’m hoping that the invariant winding of my DNA really is connected聽 to the deeper broken symmetry of spacetime.