Life’s molecules insist on being right- or left-handed:
Before there were animals, bacteria or even DNA on Earth, self-replicating molecules were slowly evolving their way from simple matter to life beneath a constant shower of energetic particles from space.
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. (Image credit: Simons Foundation)
In a new paper, a Stanford professor and a former postdoctoral scholar speculate that this interaction between ancient proto-organisms and cosmic rays may be responsible for a crucial structural preference, called chirality, in biological molecules. If their idea is correct, it suggests that all life throughout the universe could share the same chiral preference.
Chirality, also known as handedness, is the existence of mirror-image versions of molecules. Like the left and right hand, two chiral forms of a single molecule reflect each other in shape but don’t line up if stacked. In every major biomolecule – amino acids, DNA, RNA – life only uses one form of molecular handedness. If the mirror version of a molecule is substituted for the regular version within a biological system, the system will often malfunction or stop functioning entirely. In the case of DNA, a single wrong handed sugar would disrupt the stable helical structure of the molecule…
In their paper, published on May 20 in Astrophysical Journal Letters, the researchers detail their argument in favor of cosmic rays as the origin of homochirality. They also discuss potential experiments to test their hypothesis.
Taylor Kubota, “Cosmic rays may have left indelible imprint on early life, Stanford physicist says” at Stanford News Service
The claim is that “Over time, right-handed molecules out-evolved their left-handed counterparts.”
Paper. (open access)
Our physics color commentator Rob Sheldon responds,
This press release from Stanford U, suggests “yet another chirality hypothesis” (YACH), attempting to explain why all life has left-handed amino acids and right-handed sugars. Because if non-biological chemistry makes amino acids, you get a 50/50 mixture of left and right-handed versions. Somewhere, someplace in this universe, some non-biological process had to prefer L-amino acids. Now we know that if you shine polarized light on these molecules, there will be a preference for L-amino to react differently than R-amino, but then we need to find a way to make polarized light! And that turned out to be another big puzzle.
Back in 1956, CS Wu discovered that radioactive nickel decays in one polarization more than another, and every particle physicist worth his salt has tried to convert that nuclear polarization into polarized light. It hasn’t gone well. This is a good example of that genre.
At 70, Stanford astrophysicist, Roger Blandford has decided to leave a legacy, suggesting that cosmic ray showers might do the trick.
Cosmic rays are extremely high energy photons or protons that, when they hit the Earth’s atmosphere, generate showers of subatomic particles as their energy is turned into mass. The most penetrating of these particles, which make it down to the ground, are the muons. A muon is unstable, and turns into an energetic electron that then collides with more electrons, ending up in a dense cone or “track” as seen in a cloud chamber. Approximately 1/3 of the radiation you get every day comes from these cosmic rays, and 85% of that from muons.
Roger supposes that if the cosmic rays are polarized, they will generate polarized muons, that then decay into polarized electrons, which somehow are low enough in energy to electromagnetically interact with helical molecules such as DNA. In the middle of the paper he admits that it is a 3rd order perturbation (meaning really, really small), which somehow has to be amplified. That is tricky, because this is all supposed to happen before life, so we have to find some chemical process that amplifies the signal. And entropy always pushes chemistry the other direction–toward L/R balance, or racemization. But no matter, Roger wants to fund an experiment to see if (a) cosmic rays really are polarized, and (b) if it makes any difference on life.
Is this likely?
No. To begin with, muons are much more likely to destroy molecules than to select them. For another, the chiral interaction between radiation and molecules is at a maximum when they are in resonance–the wavelength of the radiation and the “wavelength” of the helical twist are the same. DNA has a helical twist some 3.5 nm long. This corresponds to a 350 eV photon or soft X-ray energy. Photons of this energy are likely to destroy everything, regardless of resonance. But the muon has a much shorter wavelength, closer to 0.0001 of this length. It can’t be made resonant with chiral molecules no matter how hard you tried.
Roger, as a typical astrophysicist, leaves everything as a formula and never actually calculates any numbers. Mind you, if the numbers had been favorable, he would have included them in his paper. But when the calculation looked improbable, he didn’t want to waste his effort so he published it as a formula with a plea to experimentalists to search for some effect.
Why review this paper then?
I have been following the crisis in particle physics where experiment after experiment has failed to turn up predicted SUSY particles, or Dark Matter particles, or axions, or Majorana fermions, or right-handed sterile neutrinos, and on it goes.
Sabine Hossenfelder has said that particle physicists are “lost in math,” > trained to play with “beautiful” equations rather than actual physics. Here we have a similar example in astrophysics. Roger makes one unsupported assumption after another, leading to an effect so small as to “need amplification.”
But the field is enamored of the math, not the physics, and that is what Roger delivers in spades. Just as in particle physics, it gets added to the stack of YACH papers, now about waist-high. The Origin-of-Chirality subproblem of Origin-of-Life looks to be as hard a problem as Origin-of-Consciousness, or Origin-of-Language. 150 years after Darwin’s Origin, and we have only multiplied all the origin problems. Maybe Sabine is right, maybe our whole approach is off.
Rob Sheldon is the author of Genesis: The Long Ascent and The Long Ascent, Volume II.