Cosmic rays can explain life bias for right-handed DNA
The universal bias that seeks an explanation has to do with the movement of the genetic helix of any organism, if we could be so small as to travel through it, we would find that its rotation is always clockwise.
There is no reason why all known life prefers this model, the molecules exist in paired forms that reflect each other, all chemical reactions produce mixtures of both, in principle, a DNA or RNA chain made of bricks of Left-handed nucleotides should work just as well as one made from right-handed bricks.
However, life today uses only one of the two Lego sets available from chemistry. Many researchers believe the selection is random: Those dexterous genetic strands have just appeared first, or in a slightly larger number. But for more than a century, some have pondered whether biology’s innate hand has deeper roots.
“This is one of the links between life on Earth and the cosmos,” wrote Louis Pasteur, one of the first scientists to recognize asymmetry in life molecules, in 1860.
Now two physicists may have validated Pasteur’s instincts by connecting the invariable turn in natural DNA with the behavior of fundamental particles. The theory, which appeared in The Astrophysical Journal Letters in May, does not explain every step of how life took its hand from the current lesson, but it claims that the shape of terrestrial DNA and RNA is not an accident. Our spirals could be traced back to an unexpected influence of cosmic rays.
This work “points to a new chiral agent that we weren’t considering,” said Dimitar Sasselov, an astronomer at Harvard University and director of the school’s Origins of Life Initiative, who was not involved in the research. “It seems to be very good.”
Cosmic rays are bullets from deep space, atomic shrapnel constantly raining down on our heads. These violent objects are the quarry for Noémie Globus, a high-energy astrophysicist at New York University and the Flatiron Institute Center for Computational Astrophysics. (Quanta Magazine is an editorially independent publication sponsored by the Simons Foundation, which also funds the Flatiron Institute.) But Globus didn’t think much about how cosmic rays could affect life until 2018 when she was a visiting scholar at the Kavli Institute for Astrophysics in Particles and Cosmology, where he met Roger Blandford, also an astrophysicist and former director of the Stanford University Institute.
They started with the fact that cosmic ray showers, like DNA strands, have a hand. Physical events often break to the right as often as they break to the left, but some of the particles in cosmic ray showers take advantage of one of nature’s rare exceptions. When the high-energy protons in cosmic rays crash into the atmosphere, they produce particles called pions, and the rapid decomposition of pions is governed by the weak force, the only fundamental force with known mirror asymmetry.
Pions that crash into the atmosphere produce showers of particles including electrons and their heavier siblings, muons, all of which are equipped by the weak force with the same chiral magnetic orientation relative to their path. The particles bounce as they traverse the atmosphere, Globus said, but generally they tend to maintain their preferred chirality when they hit the ground.
The earliest organisms on Earth, which were perhaps little more than bare barber poles of genetic material, probably came in two varieties, the researchers assumed. Some had twisted DNA or RNA strands like ours, which she and Blandford dubbed “live” molecules (chiral naming conventions differ by field), and others had inverted strands with mirrors: “evil” life. Using a series of toy models, the researchers calculated that skewed cosmic ray particles were increasingly more likely to drop an electron from a “live” propeller than an “evil” one, an event that theoretically causes mutations.
The effect would be small: Millions, if not billions, of cosmic ray strikes could be required to produce an additional free electron in a strand “live”, depending on the energy of the event. But if those electrons changed letters in the genetic codes of organisms, those adjustments may have added up. For a million years, Globus suggests, cosmic rays could have accelerated the evolution of our earliest ancestors, allowing them to compete with their “evil” rivals. “If you don’t have mutations, you don’t evolve,” he said.
The researchers’ next task is to see if the actual particle hand can actually cause the rapid mutation seen in their model. After they published their research, Globus reached out to David Deamer, a biologist and engineer at the University of California, Santa Cruz, for help. Impressed by his ideas, he suggested the simplest biological test he could think of: a ready-to-use test known as the Ames test that exposes a bacterial colony to a chemical to find out if the substance causes mutations. But instead of evaluating a chemical, the researchers plan to roast the microbes with chiral electron beams or muons.
Proof that the particle hand can actually mutate microbes would strengthen their case that cosmic rays pushed our ancestors off the evolutionary starting block, but still would not fully explain the uniform chirality of life on Earth. The theory does not address, for example, how “living” and “malignant” organisms managed to materialize from a primordial shake containing right-handed and left-handed building blocks.
“That is a very difficult step”, said Jason Dworkin, a senior astrobiologist at NASA’s Goddard Space Flight Center and a researcher with the Simons Collaboration on the Origins of Life, “but if this [theory] can provide a mechanism different, another Darwinian pressure, that would be interesting”.
Even before genetic evolution enters the picture, another unknown process seems to harm “evil” life. Simple amino acid molecules that make proteins also exist in “live” configurations favored by life and “evil” configurations that are not (although the preferred chirality for “living” amino acids is almost exclusively left-handed). Careful analysis of meteorites by Dworkin and others has found that certain “live” amino acids outperform “malignant” ones by 20% or more, a surplus that they may have transmitted to Earth. Too many molecules could be the lucky survivor of billions of years of exposure to circularly polarized light, a collection of coiled beams in the same direction that experiments have shown can destroy a type of amino acid slightly more thoroughly. than the other.
But, like cosmic rays, light beams have a marginal effect. Countless interactions would be required to leave a noticeable imbalance, so some other force might also be at work. The light would have to spray unsustainably huge amounts of molecules to account for the excesses on its own, Dworkin says.
References:
· https://iopscience.iop.org/article/10.3847/2041-8213/ab8dc6
· https://www.cfa.harvard.edu/~sasselov/
· https://pubs.acs.org/action/cookieAbsent
· https://iopscience.iop.org/article/10.1088/2041-8205/727/2/L27
