Imagine a world where everyone is right-handed. The world may not look different, but eventually, the question might arise: Why is no one left-handed? In the world of the molecules that make up the bodies of living things like DNA and RNA, this is a real question—and astrophysics researchers think they might have an answer.
Molecules that have two different structures that are mirror images but can’t be superimposed possess chirality—or handedness. Our hands are good visual representations of chirality: When you stack your hands, back of hand to palm, it’s apparent that, while being mirror images, they can’t be superimposed as your thumbs jut out to the sides. While molecules have the option of being left- or right-handed, biomolecules such as amino acids, RNA, and DNA only occur in one form in nature. DNA, for example, is only ever a right-handed helix, sugar molecules are also right-handed, while amino acids are the lefties of the biomolecular world.
These preferences each biomolecule has towards only one chiral form is called homochirality, a concept first described by French biologist and chemist Louis Pasteur (best known for his invention of the process of pasteurization) in 1848. He wrote in a paper describing biological homochirality that he suspected “cosmic forces” might be a reason for this quirk of nature. A new paper, published in The Astrophysical Journal Letters, proposes what Pasteur’s cosmic forces might be and how they affected early evolution to produce the homochirality we see in today’s world.
The authors of the paper, Noemie Globus, and Roger D. Blandford, speculate that a constant shower of energetic particles from outer-space impacted the early evolution of biomolecules, producing a preferred chirality. These particle showers are produced by cosmic rays—high energy particles from space—colliding with atoms in the Earth’s atmosphere to create different particles called muons.
“We are irradiated all the time by cosmic rays,” said Globus, a postdoctoral researcher at New York University and the Simons Foundation’s Flatiron Institute. “Their effects are small but constant in every place on the planet where life could evolve, and the magnetic polarization of the muons and electrons is always the same. And even on other planets, cosmic rays would have the same effects.”
The magnetic polarization of muons—meaning they all share the magnetic orientation as they pass through the atmosphere and interact with the Earth’s surface, like raindrops all falling at the same angle—creates a type of radiation, which researchers think could have been what caused certain small differences in mirror-image lifeforms. This radiation doesn’t cause any danger to humans or the relatively stable DNA helix. But in the early stages of Earth’s evolution, a young, active sun emitting more cosmic rays and a different atmospheric makeup may have allowed these cosmic visitors to nudge primitive, fragile biomolecules towards their forms we see today.
|Showers of high energy particles originating from the sun and our galaxy collide with nitrogen and oxygen in the upper atmosphere. At ground level, the shower is dominated by magnetically polarized muons. The magnetically polarized radiation preferentially ionized one type of ‘handedness’ leading to a slightly different mutation rate between the two mirror proto-lifeforms. Over time, right-handed molecules out-evolved their left-handed counterparts. Credit: Simons Foundation|
“This is a little bit like a roulette wheel in Vegas, where you might engineer a slight preference for the red pockets, rather than the black pockets,” said Roger Blandford, a professor at Stanford University. “Play a few games, you would never notice. But if you play with this roulette wheel for many years, those who bet habitually on red will make money and those who bet on black will lose and go away.”
If Globus and Blandford’s hypothesis is correct, they expect to find that all life in the universe will have the same preference for homochirality as cosmic rays would have the same effects. Since we can’t yet travel to a far-away planet for samples of alien DNA, the researchers hope to test samples of organic material collected from asteroids and Mars.
“As cosmic rays provide a natural connection between the weak interaction and living systems, we predict that, if ever indigenous biopolymers are found (i.e., traces of living
systems), they will have the same handedness as life on Earth,” they wrote in their paper, published in May.
Other ways of testing their hypothesis include testing how bacteria evolve when exposed to radiation with slightly different magnetic polarization than that of muons. Such a study could help indicate whether magnetically polarized radiation has any measurable impact on how living structures evolve.
“This idea connects fundamental physics and the origin of life,” said Blandford. “Regardless of whether or not it’s correct, bridging these very different fields are exciting and a successful experiment should be interesting.”