The molecular basis of all life is
mysteriously asymmetric, only using
molecules on one side of what should be
the equivalent mirrored pairs. The
universe has a similar mirror asymmetry.
And it may be that our very DNA
inherited its twist from the underlying
handedness of reality.
black hole. Links in the description. Our universe is beautifully symmetric.
For the most part, the laws of physics
work the same if we swap left with
right, positive with negative charge,
and even the future with the past. But
there are subtle violations to these
symmetries. For example, a perfect
mirror reflection of our universe would
work very slightly differently. This
asymmetry is thought to be the reason
why our universe is made of matter
rather than antimatter. and we've talked
about it before,
but there's another violation of mirror
symmetry that is 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
helix always curls one way and never the
other. And the amino acids and sugars
used by life are never used by life in
their mirror reflected forms. 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 Nomi Globus and Roger Blandford that
proposes that this asymmetry may have
been delivered to Earth by cosmic rays.
But before we get to the cool physics
and space stuff, let's do some
biochemistry.
Okay, so something is chyal if its
mirror reflection can't then be rotated
to perfectly match the original. Your
hands are chyal because your left hand
can't be rotated to match your right.
Molecules can also be chyal if their
mirror image versions are similarly
non-equivalent.
By analogy, we label chyal molecules as
either left or right-handed. Although
the assignment of the label for each
molecule is somewhat arbitrary, these
mirror image forms called enanti are
chemically similar but can behave very
differently in biological systems.
Sometimes only one enanti is
biologically active. Sometimes one is
medically useful while the other is
dangerous. For example, the right-handed
anantimer of theomide has anti-nausea
properties while the left-handed causes
terrible birth defects.
This chyro bias is even more profound
for molecules that make up living
organisms. For example, life only uses
left-handed amino acids and right-handed
sugars. These molecules join together to
construct DNA. 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 the opposite handedness, but we
never ever see that nature. This uniform
selection of a singlehandedness across
biological molecules is known as
homocyality. The homocyality of life was
discovered at the same time as the
kirality of molecules back in 1848 by a
young Louie Pau. He found that crystals
of synthetic tataric acid came in two
distinct mirror image forms while those
from natural sources like grapes only
displayed one form. These crystals even
rotated the polarization of light in the
opposite direction to each other. We now
know that these mirror image forms are
due to the mirror image enantiurs of
tataric acid and that life only produces
the right-handed version of it. A
century later, the Miller Uray
experiment further demonstrated this
divide by showing that under simulated
prebiotic conditions, amino acids and
other organic compounds form
spontaneously when simple compounds are
hit with an energy source, but always as
a 50/50 mixture of kiralities, what we
call a recemic mixture. But in life,
only the right-handed amino acids are
formed and used. The millet ure
experiment was meant to approximate the
conditions of the earth pre-life or
prebiotic earth that is the primordial
soup on that earth surely started out as
roughly recemic. But life that sprang
from that soup was homocyal or at least
the life that survives to this day. So
when and how did that homocyality
emerge?
We don't know exactly how the first life
formed on Earth, but we're probably safe
in breaking it up into three phases. In
the first phase, we have only simple
organic molecules like amino acids, and
these are still monomers. They haven't
yet formed complex polymer chains. This
is the world simulated in the Miller ure
experiment. Following the lead of 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 like autoc catalyzation and
self-replication. RNA is the prime
candidate for the molecule driving this
phase. Technically this is also
prebiotic but again following the
researchers lead will call it
transbiotic. the transition between
prebiotic and the upcoming biotic phase,
which is everything after the
development of the first very simple
single-sellled organism, the first true
life form. At some point during these
development phases, the stuff of life
became homocyal. Perhaps, for example,
the primordial soup ended up with a
significant excess of one chirality of
amino acid. Or maybe herocarality
emerged in the transbiotic stage with
one winding of RNA winning out over the
other or did pre-life evolve in parallel
in both kiralities all the way to the
first cell?
There's no known process to wholesale
wipe out one kirality. All proposed
processes at best give one side a small
numerical advantage.
So in order to achieve full
herocarality, this advantage needs to be
amplified via positive feedback loops
that can lead to exponential domination
of one kirality until the other is wiped
out. Example mechanisms include
autocatalysis, the chyro molecule
promotes chemical synthesis of its own
kind, or anti-catalysis,
the opposite kirality is suppressed, or
just self-replication. more and more
material is locked into one kirality
starving out the other. Auto and
anti-catalysis are potentially possible
as early as the prebiotic era. Although
the known mechanisms for simple monomers
don't seem efficient enough to achieve
full homocyality. But once we get to the
transbiotic phase with the complex
molecular machinery enabled by for
example RNA catalysis mechanisms and
reproduction could and probably should
lead to homocality as long as one
corality has an adaptive or numerical
advantage and this should happen very
quickly given the exponential nature of
the process. This would probably lead to
complete homocorality long before the
first self formed. But we've yet to
explain how that advantage first
appeared. There are some nonf
fundamental options. It could be that
Earth was seeded with amino acids that
already had a chyro bias brought by
comets and asteroids. Those in turn
could have picked up that bias due to
polarized light selectively destroying
one inantimer. There's disputed evidence
of this chyrobias in meteorite material,
although it's hard to tell if this stuff
was contaminated by the very biased
earth amino acids. This chyal bias could
also have arisen right here on Earth.
For example, there have been experiments
with magnetite that find a kind of
cooperative feedback between coral
molecules and magnetic surfaces that
could induce homocorality.
Or perhaps selective absorption of one
enantimer by the mineral surfaces where
these molecules were stewing in the
prebiotic era did the same thing.
Another pretty reasonable possibility is
that the advantage to one kirality was
random. Perhaps one side gained a slight
numerical advantage just by random
chance and this snowballed. Or perhaps
one side reached an important adaptive
step first. For example, the first RNA
molecule just happened to be
right-handed and again snowballed from
there. In this case, the winner of the
kirality race would be completely
random.
But we promised to try to link the
mirror asymmetry of life to something
more fundamental. And so let's do that.
The chyal asymmetry of the universe is
most clearly manifest in the weak
interaction. And this is how physicists
have tried to connect it to the
homocyality of life. One possibility
stems from the fact that the two
anantimers of a chyal molecule have
different stabilities. Atoms are bound
into molecules mostly by the
electromagnetic force and the weak force
being true to its name contributes of
order one part in a quintilion of the
binding energy. On its own this 10 ^ of
minus 18 difference in stability
wouldn't be enough of a seed to push
earth's pre-biochemistry to homocyality.
But Abdus Salam, one of the co- Nobble
laurates for electroeak unification,
proposed a mechanism for enhancing this
difference. If amino acids formed in
extremely cold environments like the
outer solar system, they could enter a
superfluid state in which the less
stable kirality gets converted to the
more stable. These could then be
delivered to Earth by the comets and
asteroids I mentioned earlier with
enough of the weak interaction induced
chyro bias to then snowball to
homocyality. But there may be a way to
extract the handedness of the weak
interaction without so many steps. This
chyal asymmetry manifests most strongly
in particle decay products mediated by
the weak interaction. For example, when
a pion decays via the weak interaction
into an electron or a muon, the magnetic
moment of that resulting particle is
always in the opposite direction of the
particle motion. And this handedness of
decay products gives us a new way to
impose the handedness of the universe on
all of life. And the delivery mechanism
will be cosmic rays, high energy
particles from energetic space phenomena
like the sun or supernova explosions.
When a cosmic ray arrives at Earth, it
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 more particles,
just like an accelerator collision. At
Earth's surface, you are receiving a
constant radiation dose from cosmic ray
showers. In fact, the cosmic ray
radiation dose is an important variable
that sets the mutation rate in life on
Earth. While the bulk of the particles
end up as electrons and photons, the
atmosphere does a good job filtering
them out before they get to us. It is
the rare and highly penetrating muons
that are responsible for about 85% of
our cosmic ray radiation dose despite
being only 1 to 2% of the particles
generated in the initial shower. As high
energy radiation, these muons can break
up molecules. But the ability of muons
to dump energy into a molecule to break
it depends in part on the relative
kirality of the muon and the molecule.
So if muons have a kirality preference
due to the fundamental asymmetry of the
universe, this can be translated into an
evolutionary pressure against a
particular molecular kirality on the
early earth. The paper by Globes and
Blandford tries to estimate the strength
of the effect and whether it's strong
enough to initiate the cascade of
homocirality.
They used computational models to
approximate the damage caused to
different types of chyal molecules based
on these polarized muon passages.
Although they're not full quantum
mechanical calculations, they have
models for simpler chyal monomers like
amino acids and helical polymers like
RNA that lets them understand how the
difference in ionization can lead to
molecular damage. The difference for
monomers like amino acids was very small
indicating that significant movement
towards homocorality is unlikely to have
occurred in the prebiotic era. But they
found that the difference for helical
polymers like RNA was much stronger.
Left-handed RNA is preferentially
damaged and perhaps by enough to
initiate the path to a right-handed RNA
world in the transbiotic era. At least
give it a strong enough rate of cosmic
ray muons. While muons make up a large
fraction of the cosmic rays that reach
the ground, it's questionable whether
there are enough to initiate a kirality
snowball at the current levels. However,
the cosmic ray flux changes over time.
For example, nearby supernova can
dramatically increase that flux. We see
geological evidence for supernova source
cosmic ray spikes from more recent
times. We know that star formation and
so supernova activity must have been
higher when the earth first formed,
giving more chances for 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
time when they're needed to kickstart
life.
So, this is a lot of speculation, but we
are scientists and so we want to find
tests. The molecular inhabitants of the
pre and transbiotic earth are long gone
and with them any record of
non-homocality.
But this hypothesis that homocyality
originates from the chyro symmetry of
the weak interaction does make a
dramatic prediction that we could maybe
test. It suggests that all life across
the universe should have the same
handedness.
We should never find mirror reflected
life. If we do, that's bad news for our
kirality being cosmically ordained. We
should also probably not try to shake
hands with that mirror life. It would be
socially awkward and a potential
biohazard due to those mirror pathogens
that we have no defenses against.
But the alien test is probably a very
long way off, if we could ever perform
it at all, that is. But there's
something that we can try much sooner.
We can look for chyrobiases 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 that they
are free of earthly contamination. We've
also found amino acids on comets, but
haven't yet been able to test for chyro
bias. If we find that these molecules
are always biased to left-handed
anantias, it's evidence of a fundamental
bias. If we see different biases in
different samples, then it might suggest
a less fundamental source such as
polarized light. When we finally get
samples from the subsurface oceans of
the moon's Europa and Enceladus, well,
maybe we'll just find life there. But if
we only find amino acids, that's still
exciting. A measure of the chyal bias of
those will tell us a lot about any
mechanism for the amplification of that
bias. A significant bias would actually
be a point against the cosmic ray
hypothesis because we expect the surface
ice of these moons to stop those muons.
On a similar note, if we ultimately
determined that Earth life started at
the bottom of the ocean as has been
proposed, that's another ding against
the muon hypothesis because the ocean
stops muons. Muon sourced kirality must
have started in tidal pools where it's
exposed to cosmic rays. If you were
paying close attention when we flashed
the paper earlier, you might have
noticed that it was from 2020. So, we
caught up with Dr. Globus for this video
and asked a few questions about what's
new. So right now they are doing new
experiments at the ISIS neutron and muon
source blasting both left and
right-handed RNA with beams of spin
polarized muons to measure reaction
rates and see how the biochemistry
responds. We might know a lot more about
the origins of life on Earth in just a
few months. These experiments are also
going to fill in key gaps in our models
for understanding radiation effects on
human health. After all, particles
breaking your DNA is one of the main
ways that radiation kills us. So, yeah,
astrophysics is saving lives 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 to the deeper broken symmetry
of spaceime.