On the Role of 40K in the Origin of Terrestrial Life

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Citation: Vladilo, G. On the Role of

40K in the Origin of Terrestrial Life.

Preprints 2022,12, 1620.

https://doi.org/10.3390/life12101620

Academic Editors: Claudia Pacelli,

Francesca Ferranti and Marta del

Bianco

Publisher’s Note: MDPI stays neutral

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Submitted to Preprints for possible

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4.0/).

Article

On the Role of 40K in the Origin of Terrestrial Life

Giovanni Vladilo

INAF-Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy; giovanni.vladilo@inaf.it

Tel.: +39-0403199216

Abstract:

The abundance and biological role of potassium suggest that its unstable nuclide was present

in all stages of terrestrial biogenesis. With its enhanced isotopic ratio in the Archean eon,

K may have

contributed to the special, perhaps unique, biogenetic conditions that were present in the primitive Earth.

Compared to the U and Th radionuclides,

K has a less disruptive radiochemical impact, which may

drive a moderate, but persistent evolution of the structural and functional properties of proto-biological

molecules. In the main

-decay route of

K, the radiation dose generated by an Archean solution with

potassium ions can be larger than the present background radiation on Earth by one to two orders of

magnitude. Estimates of the rates of organic molecules indirectly affected by

decays are provided for

two schematic models of the propagation of secondary events in the solvent of prebiotic solutions. The

left-handed

β−

particles emitted by

K are the best candidates to trigger an enantiomeric excess of L-type

amino acids via weak nuclear forces in the primitive Earth. The concentration-dependent radiation dose

K ﬁts well in dry–wet scenarios of life’s origins and should be considered in realistic simulations of

prebiotic chemical pathways.

Keywords: origins of life; radiation chemistry; potassium; molecular chirality

1. Introduction

In the most-common scenario of life’s origins, initially proposed by Oparin one century

ago [

], terrestrial life emerges as a result of natural processes driving an increase in molecular

complexity and functionality. However, despite remarkable progress in proposing plausible

prebiotic chemical pathways [

], the reconstruction of the sequence of physico-chemical

processes that led to the emergence of life in different environments of the primitive Earth

[

–

] remains one of the most challenging problems of science. Among the physical processes

considered in studies of the origins of life, here, we focus on the radiochemical effects generated

by natural sources of radiation.

Throughout the history of the Earth, radionuclides of astrophysical origin captured by

our planet have generated radiogenic heating and triggered radiation chemistry

reactions [9].

Radiogenic heating of the Earth’s mantle facilitates plate tectonics [

], which is believed to

support the long-term habitability of our planet [

]. Radiation chemistry is well known

for its biological effects [

], but only in the last two decades has it been investigated as a

potential trigger of prebiotic chemical reactions [

–

]. Indeed, natural radioactivity affects

the bonds in organic molecules, which could be broken and reformed to build larger molecules,

ancestors of extant biopolymers. Furthermore, the radiolysis of water can be a starting point for

a chain of chemical reactions of prebiotic interest. Most studies of natural radioactivity in the

prebiotic scenario have considered the radionuclides of uranium and thorium [

], which

provide most of the internal heating of the Earth’s mantle. In this work, we instead investigate

the potential role of a lighter radionuclide,

K, which has been rarely and only marginally

discussed in previous studies of life’s

origins [16,21,22].

The general purpose of this paper is to

investigate the potential role of

K in the stages of prebiotic molecular evolution when early

polymers were starting to emerge. Two speciﬁc points deal with the biogenic conditions of the

Archean and the origin of biomolecular chirality.

arXiv:2210.13995v1 [physics.bio-ph] 24 Oct 2022

2 of 17

The physico-chemical conditions of the Earth during the Archean were different from

those of the following eons in many respects (e.g., higher rotation rate, lower fraction of

continents, anoxic atmosphere, high rate of UV photons, fainter, but more active Sun, etc.).

Since terrestrial life’s originated in that epoch, it is important to understand which Archean

conditions might have been essential for the emergence of life. A speciﬁc motivation of this

study is to understand if the radiochemical effects of

K, whose isotopic ratio was signiﬁcantly

enhanced in the Archean, may have contributed to the special conditions that allowed life to

emerge.

The origin of biomolecular homochirality is an open question in studies of life’s

origins [

]. According to some authors [

], the homochirality must have originated

before the prebiotic synthesis of monomers, since polymerization tends to be inhibited in

racemic mixtures of nucleotides. The general idea for the origin of molecular chirality is that,

after an initial, small enantiomeric enhancement, some form of chirality ampliﬁcation [26–29]

allowed homochiral polymers to be assembled from enantiomerically pure building blocks.

Different types of natural processes have been invoked to explain the generation of an initial

enantiomeric excess [

]. Processes forcing a well-deﬁned chirality (either left or right) are

appealing in this context because a steady forcing in a constant direction would increase the

chance of accumulating an enantiomeric excess. This is important in the prebiotic scenario,

where racemization converts optically active molecules back into a racemic mixture in geologi-

cally short time scales [

]. Parity violation of electroweak forces [

] is a natural process,

which, in principle, may provide a universal explanation of an initial enantiomeric excess

[

–

]. In the context of parity violation interpretations, here, we examine the possibility that

the spin-polarized

β−

particles emitted by

K may have generated a small enantiomeric excess

of early polymers. The possibility that spin-polarized electrons produced by UV irradiation of

Archean magnetite deposits may have triggered a prebiotic chiral excess has been considered

in a recent paper [37].

The paper is structured as follows. The reasons why, at variance with heavier radionu-

clides,

K was likely to be present in all stages of prebiotic chemistry are discussed in Section 2.

Quantitative estimates of the radiochemical impact generated by

β−

decays

K in a prebiotic

solution of the primitive Earth are provided in Section 3. The possibility that

K can play

a role in the origin of biomolecular chirality is discussed in Section 4. The conclusions and

suggestions for future experiments are summarized in Section 5. Basic properties of the

decay can be found in Appendix A. We assumed that life emerged at

≈

4 Ga, i.e., in the time

span between 4.3–4.2 Ga, the age of the formation of oceans [

], and 3.75–3.5 Ga, the age of

the oldest, best-established traces of life [39].

2. Sources of Natural Radiation in the Early Archean: The Case for 40K

Several sources of natural radiation were present on Earth in the early Archean. In

principle, any of them may have inﬂuenced the prebiotic chemical evolution leading to the

emergence of the ﬁrst functional molecules with catalytic–genetic properties. To ascertain the

relative importance of the different sources of radiation, we considered the following aspects:

(1) the abundance and distribution of the sources on the primitive Earth, (2) the potential role

of the stable isotopes in prebiotic chemistry, (3) the compatibility of the decay products with

protobiological molecular structures, (4) the continuity with present-day terrestrial life, and (5)

the enhancement of the radiation activity in the early Archean. The properties of some natural

radionuclides present on Earth are summarized in Table 1.

3 of 17

Table 1. Properties of natural radionuclides in the Earth’s crust and sea [40,41].

Elem. Abund. Abund. Role in Unstable Isotopic Half Decay piDecay

in the Crust in the Sea Biology Nuclide Ratio Life Mode Product

(mg/kg) (mg/L) (%) (Gyr)

K 20900 399 yes 40K0.0117 1.248 β−

0.893

40Ca

EC,γ

0.107

40Ar

Th 9.6 1×10−6no 232Th 100 14 α228Ra

U 2.7

3.2

−3no 235U0.72 0.704 α231Th

238U99.27 4.46 α234Th

2.1. Abundance and Distribution of Natural Radiation Sources on the Primitive Earth

Potassium is among the eight most-abundant elements in the Earth’s crust and sea, where

it is several orders of magnitude more abundant than thorium or uranium [

]. While these

heavier elements are concentrated in speciﬁc environments [

], potassium is widespread

and likely to be present in environments considered to be plausible sites for the emergence of

terrestrial life, such as evaporative alkaline lakes [

] and subaerial hot spring ﬁelds [

The concentration of potassium was moderately higher in Archean oceans (

= 14–24 mM

[

]) than in present-day oceans (

= 10.2 mM [

]). Since it is abundant both in the crust and

in the sea, potassium and its unstable isotope

K should be considered in all possible scenarios

of life’s origins, whether subaerial in early emerged lands or in shallow waters [5,6,23].

2.2. Potential Role of the Stable Potassium Isotope in Prebiotic Chemistry

Laboratory studies have shown that potassium can be important in some stages of pre-

biotic chemistry. Speciﬁcally, potassium has the potential to assist the formation of the ﬁrst

membranes [

] and also the assemblage of peptides [

]. Remarkably, the molar concentration

found in many Archaea,

CK≈

1 M [

], lies in a range that appears to be optimal for the

assemblage of peptides [47].

2.3. Compatibility of Decay Products with Protobiological Molecular Structures

Among the radionuclides listed in Table 1,

K is the only one that has decay particles

and decay products that can coexist with the early functional molecules and protocells. This

conclusion, which is in line with the presence of

K and the absence of heavy radionuclides in

extant life, is based on the following arguments. The radionuclides of Th and U emit

particles

with energies of

4 MeV and generate unstable nuclides (Table 1), which in turn produce

chains of decays. In each of these chains, several

particles with

energies >4 MeV

are emitted.

At variance with this behavior,

K emits

and

rays with

energies ≤1.5 MeV

(Appendix A)

and generates stable nuclides, such as

Ca, which is used in terrestrial biochemistry, and

Ar,

which might play some secondary biological role [

]. Another remarkable difference is the

linear energy transfer (LET), i.e., the mean energy transferred to the medium per unit path

length traveled by the ionizing particle. Typical values of LET in water [

] are three orders of

magnitude larger for

particles (

1.5

keV/

m) than for

particles (

0.2 keV/

m); the

LET of

rays is lower than that of

particles (Appendix A). These facts suggest that the strong

activity of Th and U radionuclides may disrupt proto-biological molecular structures, whereas

40K can coexist with such structures, affecting their evolution in a non-disruptive way.

2.4. Continuity with Present-Day Terrestrial Life

The role of potassium in present-day life makes this element special compared to other

elements that possess natural radioisotopes. While thorium and uranium do not have any

biological role, potassium is an essential ingredient of ionic channels. The present-day potas-

4 of 17

sium channel is an archetype of other structures of ionic channels [

], suggesting that

potassium was essential also in the earliest forms of life. In line with this possibility is the large

concentration of intracellular potassium found in Archaea organisms, which lie close to the root

of the phylogenetic tree [

]. Indeed, the molar concentration (

M = mole/liter

) of potassium

(

) in the cytosol of Archaea is generally above 0.5 M [

]. For Archaea growing in saline

habitats, such as Halobacterium halobium, this high

can be attributed to osmoadaptation.

However, osmoadaptation does not explain Archaea that grow in low-ionic-strength habitats,

such as Methanobacterium thermoautotrophicum, which has a cytosol

CK=

0.65

−

1.1 M [

]. In

cases of this type, the high

could be a relic of ancient conditions, rather than the result of

adaptation. If unicellular organisms close to the root of the phylogenetic tree preserve memory

of their past history [

], the potassium-rich cytosol of Archaea is consistent with a scenario

in which life emerged in an environment with a high concentration of potassium.

2.5. Enhancement of the Radiation Activity in the Early Archean

Radiation sources that were enhanced in the primitive Earth might have contributed to

the special conditions that allowed life to emerge in the early Archean. If radiochemistry did

play a role in life’s origins, we can assess the relative biogenetic importance of different sources

of natural radiation by comparing their strength at 4 Ga and at the present time.

Since

K has a half-lifeof 1.248 Gyr, the isotopic ratio

K/K was one order of magnitude

higher in the early Archean than today. Therefore, in addition to the properties discussed

above,

K is also a potential contributor to the biogenetic conditions of the Archean. This

is not the case for the other natural sources of radiation, as we will now discuss. For the

reasons explained in Section 2.3, we did not consider

-particle emitters, such as the heavy

radionuclides shown in Table 1.

2.5.1. Short-Lived Radionuclides of Astrophysical Origin

Among the radionuclides of astrophysical origin,

Al and

Fe decay without emitting

particles.

Al disintegrates by electron capture and

β+

emission.

Fe disintegrates by

β−

emission to

Co, which is unstable and decays in a short time to

Ni through different routes

with

β−

and

emissions. The half-lives of these nuclides (

= 0.71 and

2.6 Myr [55–57],

for

Al and

Fe, respectively) are much shorter than the time scale required for the Earth to

become habitable. As a result, the

Al and

Fe incorporated in the Solar Nebula did affect

the geochemical processes at the time of the formation of the Solar System (4.55 Ga [

]), but

completely faded out at the epoch of life’s origins, a few hundred Myr later. Even if both

radionuclides could have been delivered to Earth at later epochs from explosions of nearby

supernovae, there is no reason why such explosions should have been more frequent in the

Archean than at later stages.

2.5.2. Radiation Sources Generated by Galactic Cosmic Rays

The interactions of galactic cosmic rays (GCRs) with the molecules in the highest at-

mospheric levels generate radionuclides and energetic particles that may have affected the

atmospheric and surface chemical processes in the primitive Earth. Among the products of

GCRs,

C is particularly interesting because: (i) similarly to

K, it decays to a stable nuclear

product (

N) emitting a

β−

particle and, (ii) given the biological role of carbon,

C could have

been incorporated in prebiotic molecules, generating internal

β−

radiation from within the

molecules themselves [

]. However, there are reasons to believe that the Archean production

rate of

C was smaller than today. The ﬂux of GCRs arriving at our location in the Solar System

is partially shielded by the solar wind and the solar magnetic ﬁeld [

]. The existence of this

shielding effect is supported by the observation that the

C production rate is modulated

by the solar cycle, being lower when the solar activity is higher [

]. Owing to the enhanced

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Citation:Vladilo,G.OntheRoleof40KintheOriginofTerrestrialLife.Preprints2022,12,1620.https://doi.org/10.3390/life12101620AcademicEditors:ClaudiaPacelli,FrancescaFerrantiandMartadelBiancoPublisher'sNote:MDPIstaysneutralwithregardtojurisdictionalclaimsinpublishedmapsandinstitutionalafl-iations.Copyrig...

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