Most of life’s intricacies can be explained by evolution — as organisms encounter new challenges, subsequent generations evolve and become better equipped to survive.
But one evolutionary peculiarity has baffled scientists since its discovery in 1848, with no evident reason as to why life developed the way it did. The mystery is “chirality,” a scientific property that can be thought of in terms of human “handedness.” Just as a person has two hands that look symmetrically identical but can’t replace one another, a molecule is chiral if it has two structurally similar forms that cannot be superimposed on top of one another.
As it turns out, life favors one side.
“We know for a fact that long ago — we’re talking back to the origin of life — somehow there was a choice made, just by the laws of physics and chemistry, to use homochiral systems and molecules,” said David Deamer, a biomolecular engineering professor at the University of California Santa Cruz.
Why was that choice made? A new theory, published in Astrophysical Journal Letters on May 20, may help put that question to rest.
While scientists tend to look for Earthly pressures to explain evolution, physicists Noémie Globus and Roger Blandford believe the answer may be nothing short of cosmic.
On the one hand
DNA is an example of a chiral molecule. The famous double helix curves to the right in living organisms but has a defunct twin that curves to the left. This classifies DNA, along with other important biological molecules such as amino acids and RNA, as “homochiral,” meaning it exists today in only one form of handedness.
Racemic mixtures — sets of molecules that display both right and left-handed chiral forms — can be created in a lab, but chiral molecules don’t play well with their counterparts. If a wrong-handed sugar were to be inserted into a piece of DNA, for instance, the entire strand would cease to function.
Humans are chiral beings and can be harmed if this “handedness” is incongruous in consumable substances such as medications. In the mid-20th century, before scientists understood the importance of distinguishing between chiral molecules and their mirrors, a mix-up in a drug called thalidomide, intended to reduce morning sickness, caused some pregnancies to end in miscarriages and others to produce children with severe birth defects.
“When we produce drugs in laboratories, we produce 50-50 of each mirror image, and one of them can have the intended effects, but the other can be lethal,” Globus said.
Looking to Occam
As is often the case in biomedical science, fixing the problem proved easier than understanding why it exists in the first place. Over the years, drug developers have come up with a number of ways to separate and produce molecules with only the intended chiral arrangement. But the question nonetheless vexed researchers – particularly those who were looking at the mystery through the lens of classical evolution.
“You’ve probably heard the term Occam’s razor, which is a sort of philosophical thing that says if you have a complicated explanation and a simple explanation, choose the simple explanation,” Deamer said. “Most scientists take Occam’s razor into consideration, and the simplest idea would be that there is no cause for it.”
Or, at least, no Earthly explanation. And it turns out that’s what Louis Pasteur figured, too. The renowned biologist first uncovered the phenomenon of chirality in 1848 while inspecting tartaric acid build up in wine vats. Unable to provide an explanation that would satisfy his understanding of biology, Pasteur later proposed that some mysterious, dissymmetric cosmic force was behind the homochirality of many biomolecules.
A continuous cosmic shower
Globus and Blandford’s paper and its proposed experiments build on Pasteur’s early postulation. The researchers believe the cause of homochirality may lie in the interaction between cosmic rays and ancient helical molecules — the initial building blocks of life.
When cosmic rays hit Earth’s atmosphere, they break down into secondary particles called muons, which in turn break down to magnetically polarized electrons. This, the hypothesis suggests, is where the dissymmetric forces promote one chiral form over the other. The researchers believe because of their uniform polarization, “spin-polarized” particles cause more damage to a certain chiral form than its mirror, thus promoting evolution-favoring mutation in one chiral form and the eventual disappearance of the other.
“In the long run, if you have a different mutation rate between life and its mirror image… if you induce this bias which is permanent — because cosmic rays are falling continuously on us — then you can make a preference in the long term,” Globus said.
Deamer, who has gained fame for his novel ideas about the origins of life on Earth, was smitten with the hypothesis. No other scientists have thought about the problem in this way, he said.
“So that’s why we’re having this conversation, because everybody loves new ideas,” Deamer said. “And Noémie, to her credit, is trying to test it.”
Recreating early Earth
The testing itself will be an interdisciplinary effort, requiring collaboration among scientists from different fields including physics and biology.
Deamer, who served as an adviser for Globus on the paper, helped come up with an initial way to begin testing the theory. The scientists plan to conduct an Ames test, which is designed to assess the mutagenic effect of substances on the bacteria salmonella. In the experiment, the scientists will subject the helical bacteria to opposite forms of polarized radiation to discover if there is a difference in the amount of damage each inflicts.
“Here’s the most important point: If the spin-polarized electrons are in one direction, they may not produce this effect, because it is the wrong direction to produce damage in the DNA,” Deamer said. “But if the spin polarization is in the right direction to interact with the DNA, they will be more mutagenic than the spin polarized in the other direction. And that would be a first and very nice, very strong test, I think, of her idea.”
In other words, they are looking for proof that these polarized particles can precipitate damage — and thus mutation — in helical molecules, as they believed happened billions of years ago.
“It is really exciting,” Globus said. “If the bacteria would respond to polarized radiation of opposite polarities, I would be thrilled.”
Answers beget questions
Proof is an elusive quarry. Even if the experiments work, Deamer cautions that—while drawing a step closer to validation—the theory will remain largely unproved.
“If the experiment works,” Deamer said, “that will be novel and interesting. Whether it is significant is a different matter… There is going to be a lot more work going back in time, because we just don’t know how much cosmic radiation was coming into the Earth back then; we don’t know about the atmosphere. So there’s a whole bunch of unknowns.”
Deamer noted that the hypothesis begins with the premise that helical molecules existed to be radiated, “and yet a helical molecule already has to be homochiral, or it can’t be helical,” he continued. “So there’s a big gap in the story right there — where did the first helical molecules arise?”
Richard Rosenberg, a senior chemist at Argonne National Laboratory, believes spin-polarized particles could indeed be behind the homochirality we see on Earth today — but in his view, it's likely low-energy particles.
Cosmic rays are a form of high-energy radiation that thermalize when they interact with a solid, said Rosenberg, who has studied and written about theories similar to that of Globus and Blandford. “In other words,” he said, “they cascade about, lose their energy, lose their spin polarization, and they create a lot of low-energy electrons which aren’t polarized, which don’t have one polarization more than the other.”
Rosenberg noted, however, that he wasn't dismissing the new theory outright. “I’m not saying that it is out of the question or anything," he said, "but it makes it less probable.”
Here — and there?
Questions regarding chiral preference are not limited to Earth. In fact, Globus first became interested in the subject during college, when one of her professors was tasked with studying the chirality of amino acids found on a meteorite as part of the Rosetta space probe launch.
The scientists discovered there were more left-handed amino acids than right-handed ones on the meteorite. And researchers are now studying other cosmic bodies, trying to discover if homochirality may truly extend further than the confines of Earth.
These findings have far-reaching implications, and some researchers believe that meteorites delivered the biased amino acids to Earth and thus were the cause of biological homochirality.
Brett McGuire, an assistant professor of chemistry at MIT, is not so sure.
McGuire was part of the team that uncovered the first chiral molecule in interstellar space in 2016 — a discovery which may allow scientists to better study how the imbalance of chiral amino acids on meteorites came to be.
However, he is not convinced that the meteorite theory is correct — or even that homochirality is due to cosmic forces.
“There are a number of possibilities that all have — I wouldn't say an equal likelihood — but they are all reasonable possibilities,” he said. “I would not place a bet on any of them versus any of the others. I think that there is no strong evidence pointing us in one direction or the other.”
McGuire said that’s precisely why it’s important to do studies like the one Globus and Deamer are undertaking. “Maybe that really is the answer,” he said. “It certainly could be. And we need to figure out what the most efficient pathway is.”
Trying to find the actual cause of homochirality requires first testing theories in a lab — like Globus and Blandford plan to do — and then trying to piece together what conditions were like billions of years ago to see if the theory matches up with the circumstance.
But there is another challenge: Even if something works theoretically, it doesn’t mean that’s what actually nudged life toward homochirality.
“In fact, probably all of them would work to some degree,” McGuire said of the multitude of theories that have been discussed. “So which one was the dominant force that drove it in one direction or the other?”
According to Globus, NASA is currently planning a mission to Europa, one of Jupiter’s moons, to study molecules on its surface. Although the planetary body is icy and has little atmosphere, if cosmic rays were to interact with the moon’s ice and create muons with the same polarization as the ones on Earth, they could potentially produce the same homochiral effect on molecules there.
If that were so, it would strengthen their theory.
“This is why we actually like this mechanism,” Globus said. “If it can make a differential effect on mutation of life in every form, then it should work everywhere — everywhere you can have this polarized component.”
And that, she noted, would include life elsewhere in the universe.
Daedan Olander is a student in the Department of Journalism and Communication at Utah State University. The UnDisciplined Deep Dive is funded by a grant from the Maxium and Doreen LaPlante Fund for Health and Science Journalism.