By Noémie Globus and Roger Blandford
“What I cannot create, I do not understand.” This was written on Richard Feynman’s blackboard, and we could say this for living organisms as we do not know how to define life or create life from non-living matter. Biopolymers (large molecules operating in living systems, such as DNA or proteins) possess a unique architecture: the arrangement of their atoms in space has the property of specific chirality (or handedness; the word “chiral” comes from the Greek for “hands”). How this happened is unknown and solving it is central to understanding the origin of life because the property of homochirality—where all biomolecules of a certain type have the same chirality—allows the biopolymers to adopt stable helical structures. As a result, their helices spiral in only one direction, and this direction is the same for all living organisms (see Figure 1, below).
We do not know how biological homochirality happened. A second puzzle is that DNA and protein helices select a specific spin direction of the electrons that propagate through the molecule (electron transport is a regular function of biological molecules). This effect is called "chiral-induced spin selectivity," and it is not fully understood yet. This property seems to be critical for molecular recognition and replication processes.
Most explanations for the origin of biological homochirality have focused on prebiotic molecules—in other words, the biopolymers' building blocks [1,2]. For instance, it has been proposed that an astrophysical source of circularly polarized ultraviolet light destroyed slightly more building blocks of one chirality. This may be because the particular area from which the Solar System emerged in the original protosolar nebula preferentially received a particular sign of circularly polarized light [3]. However, based on laboratory results, it is unlikely that the slight induced initial asymmetry in chirality leads directly to the homochiral biopolymers seen today. One missing link in this question, though, is how the initial excess in chirality of the building blocks became amplified throughout time.
However, in a model we have proposed [4], we hypothesize that chirality is imposed in biopolymers, possibly at the biological level when they made the transition to self-replication—in which case chiral radiation would act as a chiral evolutionary pressure. To investigate this possibility, we are now designing experiments, working with colleagues at UC Santa Cruz (including Eefei Chen, David Deamer, David Kliger, and Enrico Ramirez-Ruiz), to understand chiral-selective interactions between circularly polarized light and biopolymers. These experiments will allow us to better understand the influence of chiral radiation on biological evolution, and truly require interdisciplinary research efforts to achieve.
One might wonder: what kind of chiral radiation incident on the Earth can naturally occur? While circularly polarized light is a possible chiral agent as already mentioned [5], another possible chiral agent is muons, secondary particles created by cosmic rays hitting Earth’s atmosphere—about ten thousand of them passing through our bodies every minute. The physics of their origin in our atmosphere gives them a specific handedness which can then influence the chirality of biological molecules [6].
To test our hypothesis, we calculated [7] the muon radiation on different objects in our solar system, especially those that have been prime targets for the search of life: Mars, Titan, Venus, and small bodies with a negligible atmosphere such as Enceladus. While cosmic rays provide a free available source of muons everywhere, the majority of the muons arise at different depths in the atmosphere depending on the environment (see Figure 4). Interestingly, Earth is the only body in the solar system where muons dominate the cosmic radiation at ground level!
Although the radiation from cosmic rays may be smaller than that from natural radioactivity in the present time, nearby supernovae that occur every (roughly) 5 million years in our stellar neighborhood are able to enhance the cosmic-ray flux by several orders of magnitude, making muons the dominant source of radiation for thousands of years after the supernova explodes. Such an event may have occurred on the early Earth, and it is quite possible that it was during this period that the biopolymers chose a specific chirality and life originated.
Footnotes
[1] See Brandenburg, 2021, for a review.
[2] Takahashi & Kobayashi, 2019.
[3] Bailey, 2001.
[5] D’Hendecourt, et al., 2019.
[6]. For particle physicists: The parity-violating weak force produces spin-polarized muons, and since the magnetic moment is proportional to the product of the charge of the particle times its spin, their magnetic moment is on average always anti-aligned with their velocity vector. This is for both positive and negative muons. Hence this magnetic polarization would be the same sign for all muons, as illustrated in the right panel of Figure 3.
[7] Globus, Fedynitch, & Blandford, 2021.
