Natural Radioactivity and Chemical Evolution on the Early Earth: Prebiotic Chemistry and Oxygenation

Abstract

It is generally recognized that the evolution of the early Earth was affected by an external energy source: radiation from the early Sun. The hypothesis about the important role of natural radioactivity, as a source of internal energy in the evolution of the early Earth, is considered and substantiated in this work. The decay of the long-lived isotopes 232Th, 238U, 235U, and 40K in the Global Ocean initiated the oxygenation of the hydro- and atmosphere, and the abiogenesis. The content of isotopes in the ocean and the kinetics of their decay, the values of the absorbed dose and dose rate, and the efficiency of sea water radiolysis, as a function of time, were calculated. The ocean served as both a “reservoir” that collected components of the early atmosphere and products of their transformations, and a “converter” in which further chemical reactions of these compounds took place. Radical mechanisms were proposed for the formation of simple amino acids, sugars, and nitrogen bases, i.e., the key structures of all living things, and also for the formation of oxygen. The calculation results confirm the possible important role of natural radioactivity in the evolution of terrestrial matter, and the emergence of life.

Full text

Molecules 2022, 27, 8584. https://doi.org/10.3390/molecules27238584 www.mdpi.com/journal/molecules
Review
Natural Radioactivity and Chemical Evolution on the Early
Earth: Prebiotic Chemistry and Oxygenation
Boris Ershov
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
Leninsky pr. 31-4, 119071 Moscow, Russia; ershov@ipc.rssi.ru
Abstract: It is generally recognized that the evolution of the early Earth was affected by an external
energy source: radiation from the early Sun. The hypothesis about the important role of natural
radioactivity, as a source of internal energy in the evolution of the early Earth, is considered and
substantiated in this work. The decay of the long-lived isotopes 232Th, 238U, 235U, and 40K in the
Global Ocean initiated the oxygenation of the hydro- and atmosphere, and the abiogenesis. The
content of isotopes in the ocean and the kinetics of their decay, the values of the absorbed dose and
dose rate, and the efficiency of sea water radiolysis, as a function of time, were calculated. The
ocean served as both a “reservoir” that collected components of the early atmosphere and products
of their transformations, and a “converter” in which further chemical reactions of these compounds
took place. Radical mechanisms were proposed for the formation of simple amino acids, sugars,
and nitrogen bases, i.e., the key structures of all living things, and also for the formation of oxygen.
The calculation results confirm the possible important role of natural radioactivity in the evolution
of terrestrial matter, and the emergence of life.
Keywords: early earth; ocean; radioactivity; radiolysis; chemical evolution; prebiotic molecules;
oxygenation
1. Introduction
Chemical elements, including unstable radioactive isotopes, are products of stellar
evolution. Therefore, natural radioactivity has been an intrinsic component of the natural
environment in all stages of Earth’s evolution. The isotopes from the lifetimes which are
comparable with the Earth’s age, and which possess high specific activities are of special
interest. This refers to heavy elements, 232Th, 235U, and 238U, and a light element,
radioactive potassium 40K. These elements have long half-lives and high specific
radioactivity. The decay of these isotopes served as a potent internal source of energy
and, in addition to the gravitational contraction and impacts of falling meteorites, this
promoted melting and differentiation of matter and tectonic activity of the early Earth.
The subsequent chemical evolution of the Earth, and emergence and development of life
on the Earth, also took place under a considerable influence of natural radioactivity. The
presence of radioactive elements has always been an integral condition, and a necessary
component of the external and internal environment of the Earth.
The evolution of the early Earth, starting from its origin 4.6 Ga ago, proceeded
under the radiation of the early Sun. Extensive physico-chemical transformations in the
early atmosphere were initiated by hard UV radiation, powerful electric discharges,
meteorite fall, high temperature, and volcanic activity. This, apparently, was among the
factors that affected the transformation of inorganic components of the early Earth to
more complex organic matter. Currently, the existence and development of the biosphere
is determined by the presence of molecular oxygen in the atmosphere. However, in an
early stage, the planet’s atmosphere contained mainly CH4, H2O, H2, СО2, N2, NH3,
Citation: Ershov, B. Natural
Radioactivity and Chemical
Evolution on the Early Earth:
Prebiotic Chemistry and
Oxygenation. Molecules 2022, 27,
8584. https://doi.org/10.3390/
molecules27238584
Academic Editor: José A.
González-Pérez
Received: 24 October 2022
Accepted: 1 December 2022
Published: 5 December 2022
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Molecules 2022, 27, 8584 2 of 27
nitrogen, and sulfur oxides, which released during the mantle degassing [1–4]. Anaerobic
atmosphere predominated for at least the first two billion years. The appearance of
oxygen during the oxygenic photosynthesis in cyanobacteria is attributed to a later
period. Before this, simple anaerobic life forms emerged and developed on the Earth for a
long time. These forms appeared very early (3.6–3.8 billion years ago), which is
evidenced by the biogenic nature of the earliest layered stromatolites found in
north-western Australia [5–10]. Their appearance was preceded and accompanied by the
transformation of inorganic to organic matter and formation of prebiotic molecules. The
change in the composition of the Earth’s atmosphere, towards the predominance of
oxygen, took place approximately 2.4 billion years ago [1–4]. This change was called the
Great Oxidation Event (GOE). However, it is important to note that, according to
geochemical data, traces of oxygen have always been present in the Earth’s atmosphere
[11–15]. The sharp change in the atmosphere composition led to mass extinction of
previously existing anaerobic life forms and spread of energetically more favorable
oxygen-breathing species.
The transformation of inorganic to organic matter, including the formation of
prebiotic compounds, and subsequent transformation of non-living to living matter, are
important parts in the appearance of life on the Earth. The simplest biological life
appeared and started to develop using this organic protomatter as the nutritious base
[16–21]. Charles Darwin believed that the primary life spark could appear in a “warm
little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, &c.,
present, that a protein compound was chemically formed, ready to undergo still more
complex changes.” [22]. In essence, this idea forms the grounds of the known
Oparin-Haldane hypothesis, which implies that the first molecules that constituted the
earliest cells were slowly self-organized from the primordial soup [23,24]. This soup is
considered to contain prebiotic molecules, i.e., molecules that form living matter and
have been derived from the molecules of organic precursors. This idea was confirmed in
the 1950s by known experiments of Miller and Urey [25]. They reproduced the
atmospheric conditions that presumably existed on primitive Earth by subjecting a
mixture of water vapor and volcanic gases (CH4, NH3, and H2) to UV light and electric
discharge at high temperature (≤100 °С). The formation of a mixture of amino acids and
some other prebiotic molecules was established [26,27].
Throughout most of the Earth’s history, the existing life forms differed considerably
from the species observed today. According to [16–18], the first billion years of biological
and ecological evolution showed that bacteria and archaea live in oceans containing large
amounts of iron, and traces of oxygen. During the next billion years, where the amount of
iron in the oceans decreased and the amount of oxygen increased, the metabolic diversity
of prokaryotic microorganisms extended aerobic metabolism; while, eukaryotes, hybrid
cells with a new and a different organization, further increased the diversity and
ecological complexity of the microbial community. In the first two billion years (Hadean
and Archean), biogeochemical fundamentals of carbon, sulfur, nitrogen, and phosphorus
cycles, that is, microbial processes, which still underlie all Earth’s ecosystems, were
created in the early oceans. New genetic and cellular biological features were engrained
in the new eukaryotic cells. This finally resulted in the appearance and development of
complex multicellular organisms. Thus, from ecological and evolutionary standpoints,
the modern highly organized forms of life are products of this far-away world of simple
microorganisms of the early Earth. It is very important to pay attention to the fact that the
simple organisms that appeared almost 4 Ga ago and are found in Archean rocks were
preceded by a very important stage of Earth development, in particular, the
transformation of inorganic matter to organic matter. This organic protomatter initiated
the development of the simplest biological forms of life, and oxygenation of the
atmosphere accelerated the formation of highly organized forms of life.
In our opinion, the radiation-induced degradation of oxygen-containing substances,
first of all, water of the Global Ocean, was an important source of oxygen in the early

Molecules 2022, 27, 8584 3 of 27
stage of existence of the Earth where the atmosphere was mainly anaerobic. The radiation
also initiated the transformation of inorganic matter into organic matter, and the
synthesis of prebiotic molecules.
This review further develops and substantiates the hypothesis proposed previously
[28–32], stating an important role of natural radioactivity in the chemical evolution of the
early Earth. The goal is to rationalize and evaluate the radiation mechanism of the
formation of organic matter and prebiotic molecules, as well as oxygenation of the hydro-
and atmosphere. According to the hypothesis, an important part of the “radiation”
mechanism of chemical evolution is radiation-induced transformation of the early Earth
matter in the Global Ocean. The decay of radioactive isotopes in the ocean initiated
extensive chemical transformations of dissolved compounds. The Ocean served as a
reservoir for components of the early atmosphere and the products of their reactions, and
simultaneously as a converter for radiation-induced reactions. As a result, organic matter
and oxygen were formed.
2. Natural Radioactivity: Long-Lived Isotopes 232Th, 235U, 238U, and 40K
Similarly to stable isotopes, radioactive isotopes appeared as a result of nuclear
synthesis of stars of various mass [33]. The decay of isotopes is accompanied by emission
of γ-ray or α- and β-particles. Currently, more than 300 radionuclides that were formed
simultaneously with the solar system are present on the Earth. They are also permanently
formed upon the natural decay of long-lived radionuclides or in nuclear reactions
induced by cosmic rays [34–36].
Table 1 presents characteristics of the isotopes that made the crucial contribution to
the chemical evolution of the early Earth. The calculation of the total energy released
during the decay of heavy isotopes 232Th, 235U, and 238U took into account the α- and
β-particles energies of all intermediate isotopes of the radioactive family [32]. Among
light elements, only 40K makes not just a merely significant, but a crucial contribution to
the radiation environment of the Earth. It has a long half-life and a high specific
radioactivity. The 40K isotope (natural abundance of 0.0117 %) decays along two
pathways: about 89% of 40K atoms undergo β-decay to 40Ca, and the rest of 40K decays are
via capture of an electron from the own electron shell by the nucleus (K electron capture),
thus forming 40Ar. In the prebiotic era, the radiation level on the Earth was very high. It
was supported by the energy released upon the isotope decay, which was described by
an exponential law.
Nt = N0e–λt
(1)
where N0 and Nt are the initial and final (at time t) numbers of atoms of the isotope, and λ
is the decay constant equal to 0.693
T1/2 .
Table 1. Long-lived natural radioactive elements in the earth’s crust and ocean and their
characteristics.
Isotopes
T1/2, bn
Years
Radiation
Energy, MeV
Specific
Activity ×10−4, Bq
g−1
Crust
Ocean
Mass ×10−19, g
Released
Energy ×10−30, J
Mass ×10−15, g
Released
Energy ×10−26, J
4.6 Ga
Now
4.6 Ga
Now
235U
0.713
α
β
48.306 *
6.671 *
8.014
2.973
0.034
0.661
810
68.5
0.56
238U
4.47
α
β
57.902 *
17.544 *
1.246
9.57
4.69
1.488
2.84
0.033
0.97
232Th
14.05
α
β
40.134 *
7.057 *
0.407
22.67
18.07
0.801
~3 × 10−4
~2 × 10−4
~1 × 10−5
40K
1.28
β
γ
0.455
1.46
25.89
131.04
10.86
1.618
810
68.5
10.1
* Total energy released in the form of α- and β-particles, respectively.

Molecules 2022, 27, 8584 4 of 27
The contents of the radioactive elements in the inaccessible Earth’s interior remain
unknown. According to [37,38], after differentiation of the Earth’s matter, approximately
77% of radioactive isotopes were concentrated in the Earth’s crust where they occupy the
15–20 km thick near-surface layer. The masses of isotopes in the crust, given in Table 1,
were calculated considering the Clarke numbers of elements—the crust mass, which is
2.8 × 1022 kg, according to Taylor [38,39], and the known isotope abundances (in %). The
corresponding amounts of isotopes in the Global Ocean were calculated [29,32] from
their content in sea water [36,40]. The isotope masses 4.6 Ga ago were calculated from
Equation (1) and correspond to their fractions retained currently in the Earth’s crust.
Figure 1 illustrates the decay kinetics of 40K; 238U; 235U; and 232Th from the origin of the
Earth to the present time.
Figure 1. Decay kinetics of the 232Th, 238U, 40K and 235U isotopes since the origin of Earth.
The concentration is presented in relative units corresponding to the fraction of the
initial amount. The absolute amount (mass) can be easily calculated by multiplying the
fraction (Figure 1) by the isotope mass in grams, given in Table 1. The content of 40K
decreased approximately 10-fold relative to the initial amount. The radioactive isotopes
that quantitatively prevail now are those of thorium and uranium (approximately 80%
and 50% relative to the initial amounts).
Table 1 also presents the calculated radiogenic energies released due to decay of
radioactive isotopes during 4.6 billion years. The calculation takes into account the decay
of isotopes by Equation (1), and includes the energies E0 released in one decay event and
specific activity λ.
Er = N0×E0(1 − e–λt)
(2)

Molecules 2022, 27, 8584 5 of 27
Analysis of the data of Table 1 indicates that the radioactivity of the Earth is
concentrated in the crust almost completely. The mass of water in the ocean is 0.02% of
the Earth’s mass, and the content of 40K, which is the main radiation source, in the ocean
is 0.06% of its total amount on the Earth (~77% is in the crust). The greatest contribution
to the energy produced by radioactive isotopes on the Earth (1.62 × 1030 J) is made by 40K.
Approximately 36% of the total amount of energy released by now in the crust is caused
by the decay of this isotope, while in the ocean this is almost 90%. The relative contents of
heavy isotopes in the ocean are largely determined by their solubilities. The thorium
solubility is low; therefore, the 232Th contribution to the total radioactivity is negligible.
The energy released in the crust upon the decay of radioactive isotopes over 4.6 billion
years is in total 4.6 × 1030 J. Considering the differentiation of the Earth’s matter in the
indicated period, this energy should more correctly be referred to the whole Earth’s
mass. A greater energy release, amounting to 1.46 × 1031 J [41,42], was caused by only
gravitational forces.
Figure 2 shows the time dependence of radiogenic energy release in the ocean. To
date, the total value for all isotopes is 1.16 × 1027 J. All energy release calculations are
based on the assumption that the initial radionuclide is in secular equilibrium with all its
decay products, i.e., the activity of all radionuclides that make up the decay series is the
same. However, in a complex system of “ocean-host rocks”, a deviation from secular
equilibrium is inevitable over billions of years due to geochemical differentiation of
isotopes of one series in an environment where some of the intermediate radionuclides
are less soluble (mobile) than others. Nevertheless, it can be assumed that a significant
part of the radionuclides will not leave the “ocean-host rocks” sphere and will have a
radiative effect on water. Therefore, we calculated their possible contribution to the total
energy release. It turned out that the contribution of the 238U and 235U series to the overall
energy release pattern is very small, compared to the 40K contribution (see Table 1 and
Figure 2). Potassium makes a decisive contribution, and the contribution of
representatives of the uranium series is approximately 10% or less (if we restrict
ourselves to the decay of uranium and some other elements of the uranium series).
Figure 2. Time dependence of the energy release upon the decay of 40K, 238U, and 235U in the ocean.

Molecules 2022, 27, 8584 6 of 27
The decay of isotopes 232Th, 238U, 235U, and 40K also served as a powerful energy
source in the early stage of evolution (the first 500 million years). Therefore, it would be
reasonable to expect that natural radioactivity has considerably affected the chemical
evolution of the Earth: transformation of matter, oxygenation of the atmosphere, and
finally the appearance of life.
3. Global Ocean and Sea Water Radiolysis
The Global Ocean acted as both a reservoir for inorganic compounds that entered
the ocean from the Earth’s atmosphere and crust, and a converter for the chemical
conversion of compounds induced by the radiation emitted by radioactive isotopes.
3.1. Global Ocean
Water has played an important role in the formation of the planet and in the
emergence and evolution of life. This is due to quite a number of factors. Among them,
noteworthy is the accumulation in the ocean of inorganic compounds and products of
their photo- and thermochemical reactions that took place in the atmosphere, and on the
surface of the planet. The ocean was a propitious environment for their subsequent
physicochemical conversion into organic matter. According to the hypothesis we
developed, the crucial role in this process belonged to natural radioactivity [30,32]. This
transformation was initiated by the decay of radioactive isotopes.
Judging by geological and geochemical data, the Earth’s surface oceans have
apparently existed from the very early period of Earth’s history, perhaps since the Earth’s
origin [43–45]. Apparently, the earliest indications of the existence of the hydrosphere on
Earth (3.7–4.0 Ga) are volcanoes in the south-western part of Greenland [46], and gneisses
in north-western Canada [47] with signs of water-lain sediments. The zircon crystals
found in Western Australia [48,49], which might have formed in the presence of water
according to isotope and geochemical indications, probably provide evidence for even
earlier existence of liquid water on Earth (4.4 Ga). The currently prevailing view is that
the Earth has gained most of its water by accretion of carbonaceous chondrite material,
particularly CI-like chondrites, from beyond the snow line in the solar nebula [50–53].
This is supported by the fact that the H/D isotope ratio, in the structurally bound water in
carbonaceous chondrites, (140 ± 10) × 10−6 is a reasonable match to the terrestrial value.
Water in comets is usually characterized by higher D/H ratios, which can be up to 300 ×
10−6 [54,55], while hydrogen in the solar nebula has D/H ≈ 21 × 10−6 [56].
The amount of water in the oceans is 1.4 × 1021 kg. The total mass of water in these
reservoirs, i.e., besides the ocean water, is estimated at approximately (5–7) × 1020 kg [57],
i.e., about a half of the ocean’s mass.
3.2. Sea water radiolysis
The decay of 40K; 235U; 238U; and 232Th initiated the degradation of sea water. The
mechanism and energy balance of the interaction of ionizing radiation with water have
now been thoroughly studied [58–60]. The radiolysis of water can be expressed by the
following equation:
Н2O /\/\/\→ eaq− (0.28); •H (0.06); •OH (0.28); H2O2 (0.075); H2 (0.045)
(3)
The action of γ-ray and high-energy β-particles (fast electrons with energy of ≥1
MeV) gives rise to radical fragments of the water molecule which are highly reactive:
hydrated electron eaq−, hydrogen atom •H, and hydroxide radical •OH; as well as
molecular products: hydrogen H2, and hydrogen peroxide H2O2. The values given in
parentheses in Equation (3) are the radiation chemical yields of products for the indicated
types of radiation (рН 4–9). They are expressed in µmol J−1, that is, they are equal to the
concentration of particles (in µmol) formed upon the absorption of 1 Joule of radiation
energy in 1 kg of water. The yields of eaq−, •H, and •OH are substantially higher for γ-ray

Molecules 2022, 27, 8584 7 of 27
and β-particles than for α-particles, which are mainly generated upon the decay of
uranium and thorium. This is due to the fact that for α-particles the concentrations of
radicals in the track are very high. Due to recombination, their yields decrease, while the
yields of molecular products, on the contrary, increase. Moreover, the action of
α-particles, unlike γ-ray and β-particles, gives rise also to the hydroperoxide radical
HO2•. The yields of products of water radiolysis induced by α-particles are: G(eaq+H) =
0.025; GOH = 0.045; GH2 = 0.165; GH2O2 = 0.155 [58] and GHO2 = 0.01 [61] (µmol J−1).
Hydrogen peroxide is unstable and tends to decompose
Н2О2 → H2O + 1/2 O2
(4)
The decomposition of Н2О2 is considerably affected by heat and light, rock catalysis,
and the presence of transition metal ions [62].
Sea water contains various compounds, first of all, halide ions Cl−, Br−, and I−, which
react with the products of water radiolysis. Halide ions catalyze the formation of oxygen
in sea water. This is caused by oxidation of chloride ions to hypochlorite ions, which
decompose by the reaction [29,30]:
2ClO− → 2Cl− + O2 4 × 109
(5)
Hereinafter, the reaction rate constants are given in L mol−1 s−1.
The radiation gives rise, simultaneously, to reactive oxygen species (•OH, HO2•,
H2O2) and species possessing pronounced reducing ability (eaq- and H•). The standard
electrode potentials of eaq−, •H, and •ОН, HO2• are −2.9 V, −2.3 V, and +2.8 V, +1.44 V,
respectively [63]. The reactions involving these species proceed by a radical mechanism
at high rates. The rate constants of eaq-, •H, and •OH with numerous compounds,
including those present in the early Earth’s atmosphere (CH4, NH3, CO, CO2, H2S,
nitrogen and sulfur oxides, HCN, cyanogen, and many others), were studied by pulse
radiolysis and are summarized in a review [59]. These reactions were analyzed in
previous studies [29,30,32].
Let us note the following important properties of the primary ion-radial products of
water radiolysis. The hydrated electron is capable of being transformed to a •H atom via
the following reaction:
eaq− + H+ → •H 2.3 × 1010
(6)
As a result, the yield of these species increases, in comparison with the initial yield
of •H atoms. In the early period of the Earth’s evolution (Hadeon), the volcanic activity
was accompanied by the release of acidic sulfur and nitrogen oxides. Therefore, the
features of reactions involving •H were probably significant in the local regions of the
planet at that time period. The reactions of •H and •OH with saturated organic
compounds (RH) mainly proceed as dehydrogenation
RH + •H (•OH) → •R + H2 (H2O)
(7)
while reactions with unsaturated compounds proceed as addition
RH + •H (•OH) → •RH2 (•RHOH)
(8)
An important feature of the radiation-induced reactions of compounds dissolved in
sea water is that these reactions proceed by a radical mechanism involving radical ion
products of water radiolysis, i.e., the action is indirect.
The absorbed dose rates caused by the decay of isotopes are low, with the large total
absorbed doses being due to long time of irradiation. The crucial contribution is made by
the decay of 40K. The absorbed dose rate 4.6 Ga ago for potassium-40 was approximately
1.3 × 10−10 Gy s−1, while those for 235U and 238U were 1.2 × 10−12 Gy s−1 and 0.7 × 10−12 Gy s−1,
respectively [32]. The dose rate decreases by law (1) according to the decay constants λ
for these isotopes. As a result, the absorbed dose rate has currently decreased,
approximately to 1.0 × 10−11 Gy s−1 for 40K and to 1.0 × 10−14 Gy s−1 and 0.35 × 10−12 Gy s−1 for

Molecules 2022, 27, 8584 8 of 27
235U and 238U, respectively. The absorbed doses increase over time according to Equation
(2), being currently 7.1 × 105 Gy for 40K, and approximately 1.0 × 105 Gy for the sum of 235U
and 238U [32].
4. Radiation-Induced Transformation of the Earth’s Matter: Formation of Organic
Molecules
The advent of life on Earth was preceded by transformation of inorganic substances
into organic matter, and formation of prebiotic molecules. The chemical evolution of the
primary matter of the planet required a source of energy to sustain chemical reactions. It
is generally accepted that the chemical transformation of matter, during the first 500
million years after the formation of Earth, was initiated by hard solar radiation, intense
electric discharges, fall of meteorites, and volcanic activity. This provided the conversion
of simple inorganic compounds containing carbon, nitrogen, sulfur, phosphorus, and
other to more complex organic molecules. The arising primary organic molecules were
subsequently utilized as primary “structural elements” for the formation of more
complex molecules, and/or as the nutrient medium for bacteria (prebiotics). However,
the presence of radioactivity on the Earth means the existence of also an internal,
powerful, and permanent energy source; the important role of which, in the chemical
evolution of the planet, is still to be evaluated. The radical and ionic products of water
radiolysis in the ocean could have served as “activators” of inorganic matter conversion
into organic. This is evidenced by their high reactivity towards inorganic compounds
containing carbon, nitrogen, and sulfur present in the atmosphere and the hydrosphere
of the early Earth [30]. The new radicals containing C, N, P, and S atoms, which were
formed in the reactions, underwent subsequent radical chain reactions and chain
termination reactions upon recombination, and thus formed the organic matter of the
planet. Thus, the Global Ocean functioned as a converter of inorganic compounds to
organic matter using the energy released upon the decay of radioactive isotopes. This
process, which lasted for hundreds of millions of years, was apparently an important
constituent of the Earth’s chemical evolution.
The amount of the resulting radicals was sufficiently high to considerably influence
the chemical composition of sea water. Indeed, calculations show that by the time of the
Great Oxidation Event, i.e., 2.5 Ga ago, the number of radicals formed by the radiation
mechanism in the ocean were approximately 2 × 1044 species (Figure 3). This was about
0.5 mol of species per liter of water. It is clear that this amount of active species should
have markedly affected the chemical evolution of the planet.

Molecules 2022, 27, 8584 9 of 27
Figure 3. Time dependence of the number of radical species in sea water formed upon decay of
isotopes 40K, 235U, and 238U.
5. Radical Mechanism of Formation of Amino Acids and Sugars
A generally accepted view is that hydrogen cyanide (HCN) and carbon oxide (CO)
served as the sources of carbon and nitrogen to manufacture building blocks from amino
acids, nucleotides, sugars, and lipids. There is every reason to define these molecules as
“God’s molecules.” These compounds and their derivatives have an unsaturated bond
and, as a consequence, they tend to undergo condensation and polymerization reactions,
giving rise to prebiotic molecules, and organic matter as a whole.
HCN was formed in an atmosphere containing N2 and rich in methane (CH4) or
acetylene (C2H2), but it could also arise in considerable amounts (>1 ppm) in a
CO-dominated atmosphere [64]. A necessary condition was the absence of oxygen or,
more precisely, low oxygen content in the atmosphere (С/О > 1). The reactions of these
gases to give HCN require drastic conditions. HCN appeared by the photochemical
mechanism involving the action of hard solar radiation [65,66], high-energy cosmic rays
[67], lightning [68,69], and meteorite falls [70].
Over the last 20 years, the classical Miller-Urey method, for the synthesis of
prebiotic molecules [25–27], has been markedly extended by analogous syntheses under
various drastic conditions of high-energy impacts in the presence of HCN and
formamide (HCONH2), resulting from HCN hydration [70–81]. HCN is a perfect
“chemical” block for the construction of prebiotic complex molecules. Owing to the
presence of a triple bond, HCN tends to bind other molecules with unsaturated bonds to
form organic matter under the action of excess energy [82]. This situation, mimicking the
harsh conditions of the early Earth’s atmosphere, was implemented in the synthesis of
prebiotics according to the Miller-Urey mechanism. However, the radiation-induced

Molecules 2022, 27, 8584 10 of 27
synthesis proceeds at ambient temperature via the chemical reactions of dissolved
compounds present among volcanic gases, and the products of their reactions [28–32]. A
specific feature of this process is that the reactions involve free radicals formed upon the
radiolysis of water. In other words, the transformation of inorganic molecules is not
directly induced by exposure to high-energy radiation (light, plasma, electric discharge,
mechanical shock, etc.), but occurs indirectly by a radical mechanism. Currently, many
reactions that represent the intermediate steps of this transformation have been studied
in sufficient detail by pulsed radiolysis; the reaction rate constants and the nature of the
final products have been measured. The results of these studies are summarized in
review [59], and in publications [30,32]. Radical reactions are characterized by very
high-rate constants. As an example, consider some important reactions involving
water-derived radicals that are formed in the ocean upon the decay of radioactive
isotopes [59]:
CН4 + •OH → •CН3 + H2О 1.1 × 108
(9)
CO + •OH → •CO2H 2 × 109
(10)
NH3 + •OH → •NH2 + H2O 9.7 × 107
(11)
H2S + •OH → HS• + H2O 1.5 × 1010
(12)
HCN + •OH → HOCH=N• 6 × 107
(13)
HCHO + •OH → •CHO + H2O ~1 × 109
(14)
H2PO4– + •OH → OH– + H2PO4• 2 × 104
(15)
CO + •H → •CHO 3.3 × 107
(16)
HCN + •H → HN=C•H 3.7 × 107
(17)
HCHO + •H → •CHO + H2 5.0 × 106
(18)
CO + eaq– (Н+) → •CO– (•CHO) 1.6 × 109
(19)
CO2 + eaq– → CO2•– (•CO2H) 7.7 × 109
(20)
(CN)2 + eaq– → (CN)2•– 2.1 × 1010
(21)
HCHO + eaq– → •CH2OH + OH− ~1 × 107
(22)
It is evident that these, and many other radiation-induced reactions, took place in
the ocean in the early stage of Earth’s existence. These reactions are of interest since they
generate fragment radicals which are combined with each other (with termination of the
radical state) to give a set of diverse and important organic “molecules of life”, such as
amino acids, sugars, and nucleotides. The radical recombination gives a set of organic
molecules of the first level of complexity. The subsequent participation of these species in
secondary reactions may give larger molecules and/or even macromolecules. Consider,
in particular, the mechanism of formation of the simplest amino acid—glycine. This
important molecule can be obtained upon the recombination of the primary radicals
•CHO and HN=C•H, which arise in the reactions of •OH and •H with CO, HCHO, and
HCN (reactions 14, 16-19):
HN=CH• + •CHO + H2O→ NH2-CH2-COOH
(23)

Molecules 2022, 27, 8584 11 of 27
The ease of formation accounts for the predominant presence of glycine in various
protocols of implementation of prebiotic chemistry. This amino acid, apparently, was the
starting compound in the synthesis of other α-amino acids which form the polypeptide
chain in protein biosynthesis. The general α-amino acid structure NH2-RCH-COOH
includes a glycine moiety; the backbone radical NH2-•CH-COOH which is common to
different amino acids, and the radical R•, which is different for various α-amino acids.
The radical NH2-•CH-COOH appears upon radiation-induced reactions of glycine with
H• [83] and •OH [84]:
•H + NH2RCHCOOH → NH2•CHCOOH + H2 7.1 × 104
(24)
•OH + NH2RCHCOOH → NH2•CHCOOH + H2O 1.7 × 107
(25)
Further, it can be assumed quite reasonably that simple α-amino acids are formed
via the addition of radicals R• to the “glycine” radical NH2-•CH-COOH in the α-position:
R• + NH2-•CH-COOH → NH2-RCH-COOH
(26)
The radicals R• present in simple amino acids, alanine and serine, were formed in
the primary reactions of “volcanic” gases with the radical ion products of water
radiolysis. These are radicals •CH3 (reaction 9) and •CH2OH (reaction 22), respectively.
Other amino acids are generated in several steps via reactions of organic molecules. For
obtaining the radicals R• present in threonine, cysteine, asparagine, glutamine, and
aspartic and glutamic acids, it is necessary first to assemble the molecules CH3CH2OH,
CH3SH, CH3CONH2, CH3CH2CONH2, CH3COOH, and CH3CH2COOH, respectively.
More complex amino acids are formed under specific conditions, and apparently in the
presence of enzymes. It is necessary to emphasize that the radiation-induced synthesis of
amino acids in water does not require harsh conditions that were required in the
Miller-Urey and other experiments [70–80]. The major difference is that organic synthesis
takes place not in a gas-vapor environment at high temperature, but in a condensed
medium (water) at any temperature.
The initial molecule for the radiation-induced formation of sugars is, probably,
carbon monoxide CO. Like HCN, the carbon monoxide molecule has a triple bond. At
room temperature, CO is non-reactive, but the reactivity markedly increases on heating
and in solutions. However, the situation is sharply different when СО reacts with
radicals. In this case it is highly reactive and, such as HCN and other unsaturated
compounds, tends to add radicals to give adducts. When CO reacts with oxygen, carbon
dioxide СО2 is formed. However, the lack of free oxygen in the early period allowed CO
to react with other species in the hydro- and atmosphere of the planet. Reactions (10, 16,
19) illustrate the involvement of СО in chemical reactions with radical species eaq–, H•,
and •OH in water. It is known [85] that HCHO is formed in aqueous solutions containing
CO and/or CO2 on exposure to radiation. In other words, this ancient abiogenic organic
molecule also appeared, most likely, in the early Earth in sea water, for example, via the
reduction of СО with •H according to reactions (16) and (27), and with eaq– according to
(19) and (28)
•CHO + •H → HCHO
(27)
•CHO + eaq– (Н+) → HCHO
(28)
The condensation of formaldehyde is a well-studied reaction [86]. In aqueous
solutions, formaldehyde polymerizes at relatively high concentrations (≥10−3 mol L–1) to
give sugars of various complexities, including ribose and/or deoxyribose. In our opinion,
polymerization in sea water follows a radical mechanism where formaldehyde adds to
the •CHO radical to give successively radical fragments of glycolaldehyde, triose, tetrose,
and, finally, ribose •C5H9O5:

Molecules 2022, 27, 8584 12 of 27
•CHO + 4HCHO → •C5H9O5
(29)
This mechanism implies the possibility of condensation of HCHO, present in a low
concentration in solution. The radical nature of ribose and deoxyribose •C5H9O5 ensures
high chemical reactivity for the subsequent participation in the synthesis of not only ATP,
but also DNA and RNA in the molecules of which it is also present.
This assumption is confirmed by the fact that HCHO does, indeed, arise in aqueous
solutions containing CO, on exposure to radiation. According to [85], HCHO, glyoxal
H2C2O2, HCOOH (in 0.05, 0.03, and 0.04 µmol J−1 yields), and CO2 are formed in acidic CO
solutions (4.8×10−4 mol L–1). The first step is the addition of •OH and •H to CO to give
•CO2H (k = 2×109 L mol–1 s–1) [59] and •CHO (k = 3.3×107 L mol–1 s–1) [83], which further
recombine. That is, formaldehyde and the simplest product of its condensation (glyoxal)
are actually formed upon the radiolysis of an aqueous solution of СО. This fact provides
a reasonable conclusion that the oldest abiogenic organic HCHO molecule also, most
likely, appeared on the early Earth in sea water in the indicated CO reactions with radical
products of water radiolysis. The subsequent polymerization of HCHO affords sugars of
various complexity. The condensation of HCN proceeds, apparently, by a similar radical
mechanism involving the intermediate HСN•− (or HN=C•H) radical, and gives rise to the
adenine •−C5H4N5 radical:
HСN•− + 4HCN → •−C5H4N5
(30)
An important role in biological processes belongs to adenosine triphosphate – ATP
(Figure 4). It is a versatile source of free energy participating in all biochemical reactions
that absorb energy, such as formation of enzymes. ATP is composed of three parts: a
nucleic base (adenine), a sugar (ribose), and phosphate groups.
Figure 4. Structure of ATP.
The ATP molecule, being involved in a biochemical reaction, gives off energy as a
result of hydrolysis to adenosine diphosphate (ADP) or adenosine monophosphate
(AMP) and phosphate groups:
ATP + H2O → ADP (or AMP) + H3PO4 (or 2 H3PO4)
(31)
Probably, the possibility of abiogenic synthesis of this molecule, at an early stage of
evolution of the Earth’s matter, predetermined the emergence of life. It can be seen that
the combination of the ribose •C5H9O5 and adenine •−C5H5N5 radicals produces the ATP
backbone. The phosphate group completes the ATP molecule.
The radical mechanism of formaldehyde and hydrogen cyanide condensation
explains the selectivity of this process, and the possibility for the process to occur at low
concentrations of HCHO and HCN in the ocean. Indeed, the •CHO and HСN•− radicals
catalyze the addition of unsaturated HCHO and HCN (reactions 29 and 30) to yield
ribose and adenine, respectively. Adenine and the phosphate group are also parts of
DNA and RNA. Apart from adenine, DNA has three more bases—thymine C5H6N2O2,
guanine C5H5N5O, and cytosine C4H5N3O, and ribose is replaced by deoxyribose. RNA

Molecules 2022, 27, 8584 13 of 27
has the same structure and composition as DNA, except that the sugar is ribose, and
thymine is replaced by another base, uracil. Presumably, the mechanism of formation of
DNA and RNA fragments is generally the same as that considered above in relation to
ATP formation. In other words, the principle of formation of larger groups, by a
combination of radical groups that arose in radiation-induced reactions, is preserved. It is
clear that the radiation mechanism is just a part of a complex and multistage process of
the evolution of Earth’s matter, which ended in the appearance of life. It is noteworthy
that relying on the proposed radical mechanism of the formation of adenine and sugar by
polymerization of HCN and HCHO, the formation of the cytosine molecule C4H5N3O can
be interpreted as a “mixed” condensation of three HCN molecules, and one HCHO
molecule. It is important to emphasize that the stationary character of the radiolytic
synthesis, over hundreds of millions of years, ensured the continuous production of
amino acids, DNA, RNA, and ATP fragments and, as a result, the evolutionary nature of
the ordering of organic molecules.
6. Formation of Organic Matter and Purification of the Ocean
The presence of HCN, CO, and other compounds with unsaturated groups (HCNO,
HCONH2, H2NCN, (CN)2, HCSN, and many others) in water, with their proneness to
polymerization and condensation under irradiation, was apparently responsible for the
transformation of the Earth’s inorganic matter into organic matter. These compounds
acted as active sites that initiated the formation of condensed matter. In the aqueous
medium, the molecules were selected according to their water solubility and reactivity
towards the radical ion products of water radiolysis; this selection allowed for the
subsequent chemical transformations by a radical mechanism. Thus, the presence of
HCN, (CN)2, HCHO, and CO molecules in the atmosphere and hydrosphere—even in
very low concentrations—and the acting selectivity mechanisms in place, ensured their
involvement in the chemical reactions in sea water, and accumulation of products with
time.
The possibility of radiation-induced formation, in the ocean, of organic molecules
included in important biochemical processes, cannot be interpreted as the origin of life on
the Earth. It can only be reasonably argued that the conditions on the primitive Earth
were favorable for radiation-induced chemical reactions that resulted in the formation of
racemic mixtures (containing both L and D enantiomers) of complex organic compounds
from simpler inorganic precursors. Nevertheless, these organic molecules could probably
serve as the building material for the fabrication of more complex “biomolecules”, and
act as prebiotics of simple bacteria.
The radiation-induced chemical reactions of inorganic compounds dissolved in sea
water promoted the implementation of two interrelated processes important for
evolution: formation of organic matter, and purification of the ocean from toxic
impurities. In turn, purification of the ocean was favorable for the formation of an
environment that enabled the origin of life. The formation of organic compounds via
transformation of inorganic matter of the Earth contributed to the same goal. The fact that
the first signs of simple organisms were found to exist 4 Ga ago [5–10] indicates that the
transformation of matter has actively proceeded, even during the planet’s formation and
then during the formation of the ocean, i.e., in the first 500 million years (Hadean).
Unfortunately, there are virtually no reliable data on the geochemical state of the early
Earth. There are two points of view on the composition of the primitive atmosphere: (1) it
mainly consisted of CH4, CO, and NH3, i.e., the atmosphere was reducing; (2) carbon
mainly existed as the dioxide CO2, i.e., the atmosphere was oxidative [1–4]. It was noted
above that the simplest amino acid, glycine, which served as the basis for the formation
of other vital amino acids, was most likely generated upon the reaction of •H with CO
and HCN. Additionally, ribose and adenine, which are the major parts of ATP, DNA, and
RNA, were produced by the condensation of HCHO and HCN, respectively. In our
opinion, these facts most likely provide evidence in favor of the reducing atmosphere on

Molecules 2022, 27, 8584 14 of 27
the early Earth. Analysis of possible reactions shows that HCN, CO, and HCHO had a
significant predominance over many other compounds that would be expected to appear
in the early ocean. This follows on from the relatively high-rate constants for the reactions
of these molecules (107–109 L mol−1 s−1). For example, CO2 and NH3 exist in water as CO32−
and NH4+, which have low reactivity towards eaq−, •H, and •OH (≤104 L mol−1 s−1).
Therefore, CO, and HCHO should be present in approximately 103–105 higher
concentrations than HCN to be competitive with it. That is, the presence of HCN, CO,
and HCHO in water, even in rather low concentrations, gives them a pronounced
advantage for reactions with eaq−, •H, and •OH, and hence for the subsequent
involvement in the formation of amino acids, sugars, and nucleic bases. One more
specific feature of radiation-induced reactions of these molecules is the ability of •CHO
and HСN•−, derived from these molecules, to act as condensation centers and to initiate
radical polymerization, giving rise to macromolecular products. These substances are
separated into an insoluble phase and are removed from the area under irradiation. Thus,
the continuous supply of HCN, CO, and HCHO into sea water should have ensured the
transformation of the inorganic matter of the Earth, into organic matter. In addition to
HCN, cyanogen (CN)2 might also play an important role in the radiolytic transformations
leading to the formation of organic matter. Like HCN, cyanogen has the reactive −C≡N
group, and therefore it can be readily transformed in the radiation-induced chemical
reactions into organic compounds, including polymers. In addition, cyanogen is
hydrolyzed in water to give HCN and cyanic acid HОCN (or isocyanic acid HNCО). All
compounds containing a −C≡N group with an unsaturated bond tend to undergo
radiation-induced chemical reactions to give organic amino compounds, and polymers.
These compounds accumulate various radicals that arise upon radiolysis, and act as
centers of formation of condensed matter. Table 2 gives the reactions and rate constants
for reactions of eaq–, •H, and •OH with some carbon- and nitrogen-containing compounds.
These compounds were present among volcanic gases, or were formed in the early
atmosphere via chemical reactions. Most likely, they were the initial species for the
formation of organic matter.
Table 2. Rate constants of eaq–, •H, and •OH with carbon- and nitrogen-containing compounds.
Radical
Reaction
k, L mol−1 s−1
Reference
•H
HCN + •H → HN=•CH
3.7 × 107
[87]
(CN)2 + •H → products
<1 × 107
[88]
H2NCN + •H → H2NCH=N•
6.9 × 106
[89]
N3– + •H → HN3•–
2.4 × 109
[90]
HN3 + •H → products
7.2 × 107
[91]
SCN– + •H → products
2.3 × 108
[92]
CN– + eaq– → products
3 × 105
[93]
eaq–
(CN)2 + eaq– → (CN)2•–
2.1 × 1010
[88]
HCONH2 + eaq– → •CONH2
2 × 107
[94]
N3– + eaq– → products
<1.5 × 106
[95]
HN3 + eaq– → HN3•–
1.2 × 1010
[96]
H2NCN + eaq– → H2NCN•–
1.5 × 109
[89]
SCN– + eaq– → products
<1 × 106
[97]
HCN + •OH → HOCH=N•
6 × 107
[98]
•OH
CN– + •OH → •C(OH)=N–
8 × 109
[93]
NCO– + •OH → •NC(OH)O–
4.8 × 107
[99]
(CN)2 + •OH → •CNCNOH
<1 × 107
[88]
N3– + •OH → •N3 + OH–
1.4 × 1010
[100]
H2NCN + •OH → H2NC(OH)=N•
8.7 × 106
[89]
SCN– + •OH → HOSCN•–
1.2 × 1010
[100]

Molecules 2022, 27, 8584 15 of 27
NH3 + •OH → •NH2 + H2O
9.7 × 107
[101]
Another important circumstance that promoted the radiolytic transformation of
HCN and (CN)2, as well as HCHO, into biological compounds is, as indicated above,
their high solubility in water. Therefore, they had preference in the migration from the
atmosphere to sea water. Indeed, HCN and HCHO are infinitely soluble in water, while
the volume coefficient of solubility (mL of a solute in 100 g of water) for (CN)2 is
approximately 450. The solubility of gases O2, H2, CO, CH4, and N2 is in the range of 1.5–
3.5. This was a criterion for the selection of molecules, promising for prebiotic chemistry,
from the atmosphere to the hydrosphere. In water, molecules were selected in terms of
their reactivity towards the radical and ionic products of water radiolysis, which
provided their subsequent chemical transformations by the radical mechanism. Finally,
the unsaturation of the −C≡N group and the С≡О molecule enabled their subsequent
polymerization to give organic matter, which formed a separate phase. It was shown by
pulsed radiolysis [98] that Cyanide-H• (HN=•CH adduct) and Cyanide-•OH (HOCH=N•
adduct) recombine, with the reaction rate constants being very high: ~1.4 × 109 L mol−1 s−1.
High reactivity of radicals with CN groups, and their ability to act as condensation
centers for compounds present in the ocean, was also demonstrated in relation to cyanic
acid HCNO [99]. The Cyanate-•OH adduct formed in the reaction (see Table 2):
NCO– + •OH → •NC(OH)O–
(32)
was shown to tend to add additional OCN– ions, i.e., it is able to act as a center for radical
chain polymerization
•NC(OH)O–+ NCO–→ −OC(OH)NNC•O– 4.3 × 106
(33)
The cyanate radical ion −OC(OH)NNC•O– thus formed is also highly chemically
reactive towards various organic compounds. The rate constants of its reaction with the
ascorbate ion, hydroquinone, methoxyphenol, phenylenediamine,
tetramethyl-p-phenylenediamine, and urate ion are approximately (7 × 107–4 × 108) L
mol−1 s−1, while the rate constants for the reactions with aniline and phenol are <5 × 106 L
mol−1 s−1. There is a tendency for radical transfer to these compounds. Thus, the action of
radiation on aqueous solutions containing CN compounds, first, initiates their
polymerization and, second, makes them react with other organic compounds according
to a chain mechanism. A similar behavior of the СО molecule is evidenced by the data on
the formation of formaldehyde, the product of its primary condensation (glyoxal), and
more complex sugars upon the radiolysis of an aqueous solution of СО [85].
Thus, the appearance of HCN, (CN)2 and HCHO, CO molecules in the atmosphere
and the hydrosphere, even in a very low concentration, provided the subsequent
accumulation of the products they form with time. The most important factor in this
process was the stationary character of the radiation, which ensured the evolutionary
changes of molecules, and accumulation of organic matter over hundreds of millions of
years.
Hydrogen cyanide, cyanide salts, and cyanogens are highly potent poisons. Other
compounds containing a cyano group –C≡N (or =C=N) are also toxic. In particular, this
refers to organic compounds—nitriles and isonitriles, cyanic and isocyanic acid and their
derivatives, and many others. Toxicity is also inherent in CO and HCHO. It is amazing
that Nature chose these toxic compounds to design the molecules of life. However, high
reactivity of these compounds apparently implied the possibility of their easy
degradation on exposure to radiation. The decay of natural radioactive isotopes initiated
their degradation, and hence this promoted the radiation purification of ocean water
from toxins. Note also the proneness of cyano derivatives to hydrolysis, giving organic
acids and ammonia, which is thus accompanied by the loss of toxic properties of cyano
compounds. The accumulation of organic matter took place gradually over a long period
of time, and simultaneously purification of the ocean took place. The conditions in the

Molecules 2022, 27, 8584 16 of 27
aquatic environment were thus prepared for the subsequent origin of life. The efficiency
of radiation purification of water was recognized to its full extent only nowadays. Now
the use of radiation for water purification, from toxic impurities, is considered to be one
of the most efficient and promising methods [58,102–104]. The radiation treatment of
water refers to advanced oxidation technologies (AOP technologies). High penetrating
power of radiation ensures the destruction of both dissolved, and suspended impurities.
The purifying effect of radiation is due to its ability to inactivate toxic and chromophore
functional groups, transform impurities into an easily extractable form, damage the DNA
of microorganisms and their spore forms, and increase the biodegradability of organic
impurities. The concentrations of radical ion products (of the order of decimols per liter),
which are attained upon sea water radiolysis, are markedly higher than the
concentrations of impurities (about ~10−7–10−3 mol L−1) that could be expected to be
present in water in the early stage, as a result of volcanic activity (methane, ammonia,
carbon oxides, etc.). The radiation-induced transformations of sea water affect the
chemical composition of water. The presence of transition metal ions and organic
compounds—as well as the appearance and accumulation of oxygen (see the next
chapter)—may enhance the radiation effect of chemical transformations of dissolved
compounds by a large factor, by initiating chain reactions involving radicals [58].
7. Quantitative Evaluation of the Formation of Organic Matter
Reliable information on the amounts of radioactive isotopes on the Earth since its
formation, and the radiation energy released upon their decay, enables a rational
evaluation of the formation of organic matter from inorganic matter by the radiation
mechanism. This synthesis is related, first of all, to the involvement of unsaturated
molecules (HCN, (CN)2, СО, HCHO, and other) and the products of their reactions in the
successive condensation and polymerization reactions initiated by the radical ion
products of water radiolysis. The inorganic to organic transformation also involved other
compounds present in the ocean (CH4, NH3, CO2, H2S, inorganic acid anions, and other).
The process ended in the isolation of the organic matter as a separate phase. The chain
mechanism includes successive initiation (34), chain propagation (35), and chain
termination (36) steps:
M + eaq− (or •H and •OH) → M•
(34)
M• + (n−2)M → M•n−1
(35)
M•n−1 + M• → Mn
(36)
For quantitative estimation, it is necessary to use the chain propagation number n,
which is unknown. Therefore, we assume that the radiation-initiated condensation of
molecules M occurs only as “fusion” of radicals M•, resulting from the reaction of
molecules M (i.e., HCN, (CN)2, HCHO, СН4, and other) with eaq−, •H, and •OH, according
to Equations (9)–(22) indicated in Table 2. Thus, we have
(х−2)M• → M•х−1
(37)
M•х−1 + M• → Mх
(38)
The difference between the mechanisms described by reactions (34–36), on the one
hand, and reactions (37, 38), on the other hand, is as follows. According to the former
mechanism, each water-derived radical (eaq−, •H and •OH) consumes n molecules M for
the chain formation of organic matter, while in the latter mechanism, it is only one
molecule M. Since the n value is unknown, we use the latter mechanism of the formation
of organic matter in the calculation. This calculation gives markedly lower yields of
organic matter. We take the molecular weight of M to be 30, which is approximately the
average of the molecular weights of indicated HCN, (CN)2, HCHO, and other molecules.

Molecules 2022, 27, 8584 17 of 27
We assume that the final molecule Mх, which has a weight of approximately 300–1000,
i.e., contains 10–30 molecules M, is isolated from the aqueous solution as a separate phase
and no longer participates in the radiation-induced chemical reactions. This results in
accumulation of organic matter. Further, we assume that all radical products of water
radiolysis are captured by the dissolved compounds. Then, the amount of organic matter
formed in the ocean upon the decay of the radioactive 40K isotope alone is described by
the equation
[Pr]t = [СK×AK−1×E×G(Pr)×MH2O−1×10−2]×(1 −e−λt)
(39)
where СK is the amount of 40K in the ocean 4.5 Ga ago (g); AК is the atomic weight of
potassium (g); E is the average energy released during the decay of a 40K atom (5.9×105
eV); G(Pr) is the total radiation-chemical yield of ion-radical products of water radiolysis
eaq- (0.28 µmol J−1), •H (0.6 µmol J−1), and •OH (0.28 µmol J−1), equal to 0.62 μmol J–1 (see
Equation (3)); MH2O is the amount of water in the ocean involved in the radiation chemical
synthesis (1.4 × 1021 kg). Figure 4 illustrates the build-up of the weight of organic matter
caused by water radiolysis, and induced by the decay of 40K alone, during the early
period of Earth’s existence (Hadean) characterized by volcanic and tectonic activity. After
this period, in the beginning of the Archean (4.0–3.8 Ga), the simplest anaerobic forms of
life already appeared and developed.
Figure 4. Accumulation of organic matter in the ocean upon the radiation-induced transformation
of dissolved inorganic substances.

Molecules 2022, 27, 8584 18 of 27
It can be seen that the accumulation of mass during the short Hadean period,
followed nearly linear time dependence, and in the period from 4.5 Ga to 4.0 Ga,
approximately 3 × 1021 g was formed. The contribution of 235U; 238U; and 232Th was
markedly lower; these isotopes increased the indicated yield of organic matter by
approximately 30%. The mass of organic matter currently present on the Earth, including
all representatives of flora and fauna, is roughly estimated to be 1018 g. However, this
matter continuously forms in various processes, degrades, and forms again. On the early
Earth, organic matter also existed in the same equilibrium state between formation and
degradation. Therefore, its steady-state amount was always significantly lower than that
obtained in our calculations, for the whole period of 500 million years. If we take the
degradation and restoration period to be, for example, 100 years, the steady-state amount
of organic matter that was permanently present on the Earth between 4.5 and 4.0 Ga ago
is found to be approximately 6 × 1014 g. This amount is three to four orders of magnitude
lower than the amount of organic matter on today’s Earth. The difference is quite
understandable and explainable, in view of the fact that at present, organic matter is
formed upon photosynthesis involving oxygen, and is mainly composed of plants. On
the early Earth, there was little organic matter, and its appearance and mass build-up
implied the coming of the planet to life. The organic molecules were gradually converted
to the simplest living species along the obscure paths of evolution.
8. Oxygenation of Hydro- and Atmosphere
As the volcanic activity was attenuated and the Earth cooled down, the composition
of the Earth’s atmosphere gradually changed. A crucial change in the atmosphere, in
which volcanic gases were replaced by oxygen, took place approximately 2.4 billion years
ago due to the development of oxygen photosynthesis by blue-green algae
(cyanobacteria). This brought about mass extinction of previously existing anaerobic life
forms, and the spread of energetically more favorable oxygen-breathing species, i.e.,
GOE, took place [1–4,19–23]. The transition from anaerobic fermentation to oxygen
respiration was gradual and required the presence of free oxygen in a relatively low
concentration in the atmosphere, even in the prebiogenic period of Earth’s development.
Presumably, only in this case, cyanobacteria could appear and switch to the
photosynthetic assimilation of carbon dioxide, and production of oxygen. Indeed,
according to geochemical studies [20,24–26], a minor amount of oxygen was present in
the atmosphere even at the very early stages of Earth’s existence, since around 3 billion
years ago or even earlier. Perhaps, the laminated iron ores detected in the south-western
part of Greenland attest to the early oxygenation of the atmosphere (approximately 3.8
billion years ago) [46]. The formation of such ores requires the presence of free oxygen to
oxidize divalent iron to the trivalent state. In other words, even at that time, there were
some sources of oxygen supplied to the exosphere. Water and carbon dioxide
photodissociation, in the upper layers of the Earth’s primary atmosphere, is considered
as the most probable (external) source [105–107]. Early oxygen photosynthesis, as a
source of oxygen, cannot be ruled out either.
In our opinion, in the early stages of the existence of Earth, free oxygen was mainly
generated by natural radioactive isotopes [29–32]. As indicated above (section 3.2), their
radioactive decay initiated water splitting to give oxygen. Molecular oxygen is not a
primary product of water radiolysis, it is formed via decomposition of hydrogen
peroxide H2O2 (reaction 4). According to the mass balance of decomposition of a water
molecule
Н2О → H2 + 1/2 O2
(40)
the radiation yield of О2 should be equal to the half of the yield of Н2, i.e., for γ-ray or
β-particles, it is approximately 0.022 μmol J–1, while for α-particles, it is about 0.07 μmol J–
1 (see section 3.2). The oxygen formation upon water radiolysis and determination of the
radiation chemical yield were discussed in detail previously [29,30]. The time

Molecules 2022, 27, 8584 19 of 27
dependence of oxygen accumulation in the atmosphere under the radiation of 40K and
235U, 238U in the ocean is shown in Figure 5.
Figure 5. Time dependence of oxygen formation upon radiolysis of sea water.
The major contribution to oxygenation is associated with the decay of 40K. The
contribution of 235U and 238U is approximately 25% of the total amount. It can be seen that
the decay of isotopes could give rise to approximately 6.2 × 1020 g of О2 entering the
hydro- and atmosphere of the early Earth, in the period between 4.5 Ga ago and GOE (2.4
Ga ago). This amount is comparable with its current content in the atmosphere (1.2 × 1020
g). Thus, the natural radioactive isotopes 40K, 235U, 238U, and 232Th could serve as the
internal source of energy that provided continuous oxygenation of the hydro- and
atmosphere of the early Earth, as a result of ocean water radiolysis.
The oxygenation and the evolution of life are linked by several threshold points. It is
generally accepted that the formation of oxygen was caused by the photodissociation of
H2O and CO2 in the upper layers of the Earth’s primary atmosphere, under hard UV
radiation from the Sun [105–107]. With this oxygenation mechanism, the accumulation of
O2 cannot exceed the threshold value, equal to 0.001, of the current oxygen content of 1.2
× 1021 g (Urey point). The concentration of 1.2 × 1018 g should be maintained by itself
because of absorption of UV radiation by the formed oxygen. That is, as this
concentration has been attained, oxygen starts to shield further decomposition of water
by the photochemical mechanism. With this oxygen content, only anaerobic life could
exist on the Earth. It is obvious that the external source does not affect the oxygen
formation from the internal source (radioactive isotopes). With the radiation-induced
degradation of sea water, the threshold concentration of oxygen (Urey point) is attained
rapidly on the geological time scale, within approximately 4–5 million years. The next

Molecules 2022, 27, 8584 20 of 27
threshold point (Pasteur point) is attributed to the possibility of appearance and existence
of oxygen-breathing living cells. This corresponds to oxygen content in the atmosphere
equal to 0.01 of the current level, that is, 1.2 × 1019 kg. This oxygen level is favorable for
the appearance of organisms that can reversibly switch their energy metabolism from
respiration to fermentation, as the oxygen content varies in the vicinity of the Pasteur
point. The restriction of the development of oxygen-breathing life is due to the
deleterious action of UV radiation, caused by the weak ozone shield. With the radiation
mechanism of oxygenation, this amount of oxygen should have been formed in
approximately 40–50 million years, i.e., this point also could have been crossed in a very
early period of the Earth’s existence. Finally, the third threshold amount of oxygen in the
atmosphere (the Berkner-Marshall point [108,109]) corresponds to 0.1 of the current level,
i.e., 1.2 × 1020 g. This content of oxygen enables the formation of the protective ozone
shield to preserve oxygen-breathing life. It can be seen in Figure 5 that this oxygen
content in the atmosphere was attained 4.25 Ga ago. In other words, the threshold
amount of oxygen that allows for the development of oxygen-breathing life
(Berkner-Marshall point) could have been attained long before GOE. These
considerations are purely speculative, because they refer to the unlikely situation in
which oxygen is formed, but not consumed. Certainly, this is not the case. Actually, the
rising oxygen, and most likely also H2O2, its precursor, were consumed in diverse
reactions such as oxidation of iron and other metals, formation and decomposition of
inorganic and organic compounds, and a multitude of other reactions. An important
consequence of the presence of natural radioactive isotopes in sea water was the
existence of a continuous, and powerful internal source of energy. This source could
provide the supply of oxygen to the Earth’s hydro- and atmosphere; thus it could bypass
the indicated barriers (threshold points) to the oxygen and ozone shielding of energy
supply from an external source (Sun). Over time, sea water was purified and spots with
elevated oxygen content appeared, and hence oxygen consumption in biochemical
reactions became possible. Therefore, the situations corresponding to the Urey, Pasteur,
and Berkner-Marshall threshold points should be considerably shifted towards the
present time. However, it is obvious that the natural radioactivity on Earth is an
important source of oxygen in the early stages of the Earth’s existence, and an important
factor promoting the formation of the oxygen atmosphere of the Earth.
In the case of an external source of oxygenation (Sun), oxygen gets into the ocean
from the atmosphere, or is formed in a thin near-surface active zone. This means that the
oxygen concentration gradient is directed from the atmosphere towards the ocean depth,
i.e., the outer layers may be saturated, while no oxygen may be present in deep-water.
The radiation-induced oxygenation follows an entirely different pattern. Apparently,
oxygen arises uniformly throughout the ocean bulk, which corresponds to the uniform
distribution of the dissolved radioactive isotopes. As the saturation is reached, oxygen
moves upwards, and migrates to the atmosphere. This mechanism implies the possibility
of retardation in the oxygen saturation of the atmosphere, thus highlighting the essential
lag between atmospheric and oceanic oxygenation, and setting the stage for a generation
of research in Precambrian oxygenation. This delay, before the atmospheric oxygenation,
could be enhanced by slow oxygen diffusion from the ocean depth at a high pressure of
water mass. Oxygen appeared throughout the ocean bulk and was simultaneously
consumed in a variety of redox reactions. This should have resulted in a stationary,
although relatively low, level of oxygen in seawater for tens and hundreds of millions of
years. This was favorable for the appearance, and then development, of
oxygen-breathing microorganisms in the ocean.
9. Isn’t the Role of Natural Radioactivity Exaggerated?
The substantiation of the hypothesis about the important role of natural
radioactivity in the chemical evolution and formation of life on the primitive Earth,
presented in this paper, is based on quite obvious and reliable facts. These facts include

Molecules 2022, 27, 8584 21 of 27
(i) the presence of radioactive isotopes since the Earth’s origin, and the amounts of the
isotopes; (ii) types of decay, and the released energy of ionizing radiation; (iii) the
mechanism of water radiolysis, and yields of the radical or ionic and molecular products;
and (iv) particular radical reactions of compounds dissolved in water on the early Earth,
and the measured rate constants for these reactions. In the calculations, we assumed that
the amount of water on the planet was constant, and equal to that in the currently
existing ocean. This corresponds to the modern views on the appearance of water by
accretion of chondritic meteorites during the formation of Earth. Moreover, as indicated
in section 3.2, the amount of water on the Earth is 1.5–2.0 times greater than the amount
of water in the ocean. In other words, the amounts of organic matter and oxygen formed
upon the radiolysis of water, and aqueous solutions of compounds present in the
atmosphere and geosphere of the early Earth, may well be higher. The quantitative
estimate of the chemical transformation of matter of the early Earth, made in the present
study, should be certainly regarded as underestimated. The calculations considered the
ocean to be the only source of the evolutionary process. Meanwhile, the mass of ocean
water is 5% of the mass of Earth’s crust, and the content of 40K (the main source of
radiation) in the ocean is less than 0.06% of the total amount of 40K on the Earth (the
greater part of ~77% is in the crust). Therefore, the radiation effect of the transformation
of the matter in the crust can be more pronounced than that estimated only for the ocean.
This is indicated by active microbial life deep below the Earth’s surface. There are reasons
to believe that molecular hydrogen and oxidizing agents, formed as a result of water
radiolysis, are a constant source of energy for these microbial communities [110–112].
However, the chemical evolution of molecules in the earth’s crust, apparently, was
significantly limited in comparison with the ocean due to the difference in the phase state
of the media. The radiation-induced decomposition of rocks requires special
investigations, and therefore it is beyond the scope of the current discussion. It can be
concluded, with confidence, that the calculations were performed at the minimum level
without making any doubtful assumptions, and were based on known and reliable
parameters. The proposed hypothesis of the important role of natural radioactivity and
radiation-induced chemical reactions in the chemical evolution of the early Earth is
certainly sufficiently substantiated, and has prospects for further development.
10. Conclusions
It is clear that an internal potent source of energy for chemical transformation of
inorganic to organic matter, that is, natural radioactive isotopes (40K; 235U; 238U, and 232Th),
has always existed on the Earth. The molecules that formed the first prokaryotic and
eukaryotic living organisms arose in the atmosphere under harsh conditions, and under
the action of solar radiation, plasma, electrical discharges, impacts of falling meteorites,
and the heat of the Earth. Then, they were washed into the ocean. However, the ocean
was not only a reservoir for these compounds, but also an efficient converter for their
subsequent reactions. Particularly in the ocean, the key stage of the prebiotic chemistry
took place, most likely. Natural radioactive isotopes served as the source of energy for
the synthesis and accumulation of organic matter in the ocean. The potential of
radiation-induced synthesis is vividly demonstrated by the mentioned experiment using
a CO solution [85]. It is hardly possible to choose a simpler chemical composition.
Nevertheless, radiolysis resulted in a variety of products: CO2, HCHO, glyoxal, CHO
CHO, and HCOOH. Other products were present in amounts below the level of
analytical determination. However, the formation of formaldehyde and glyoxal (as the
first formaldehyde condensation product) reasonably suggests the possibility of
subsequent formation of various sugars and alcohols, with a continuous supply of CO
and irradiation. Abiotic organic synthesis based on radiolytic transformations of CO2 and
HCO3− and CO32- anions in water is much less efficient. The absence of unsaturated bonds
in them affects this. However, in this case, the formation of formate and oxalate is fixed
also [113–119]. These low molecular weight carboxylate compounds, along with H2, can

Molecules 2022, 27, 8584 22 of 27
play an important role in maintaining subsurface lithoautotrophic microbial ecosystems.
The efficiency of radiation-induced synthesis is also supported by computer simulation
of the radiolysis of simple organic compounds—oxalic and acetic acids. It was shown
that the decomposition of oxalic acid gives tartaric, tartronic, glyoxylic, and many other
organic acids [113,114], while the decomposition of acetic acids gives glycolic and
glyoxalic acids, formaldehyde, and other products [115]. It can be stated, with sufficient
certainty, that the radiation-induced chemical transformations of aqueous solutions of
CO, CH4, HCHO, HCN, and other simple organic compounds makes it possible, in
principle, to synthesize an almost infinite set of organic products. Necessary conditions
are continuous supply of the starting compounds, and exposure to ionizing radiation, i.e.,
the conditions that existed on the early Earth. The accumulation of organic compounds in
the ocean gave rise to an “organic soup”, in which additional reactions, and the
formation of more complex organic molecules took place for hundreds of millions of
years under the action of internal radiation, and external light and heat. The gradual shift
of the place of synthesis from the toxic early atmosphere to the hydrosphere protected the
arising molecules from the deleterious action of light, electrical discharges, and plasma; it
also promoted further evolution of molecules during the decline of the active phase of the
Earth’s development in the first hundreds of millions of years. It is noteworthy that the
radiation stage occurred under stationary conditions of internal supply of radiation
energy, and under greenhouse conditions of the ocean. The nutrition medium of
prebiotic molecules needed for the simplest organisms was permanently reproduced in
the aquatic environment. The simulation of radiation-induced reactions and
experimental observations show that amino acids, sugars, fatty acids, and nitrogenous
bases were formed in the ocean on the early Earth, under the action of indicated factors.
There compounds are the building blocks that constitute the basis for all living forms
nowadays, 4.6 billion years later. The radiation-induced splitting of water to give
oxygen-ensured oxygenation of the hydro- and atmosphere. There are also no grounds to
rule out the possibility that some organic compounds arrived on the early Earth from
outer space. This is evidenced by the discovery of many molecules, including amino
acids, bases, and fatty acids in meteorite fragments (1969, Australia, Murchison), and the
presence of complex organic carbon compounds in carbonaceous chondrites [120].
Apparently, outer space could have been an important source of organic matter coming
to the Earth in the early stage of formation of the solar system. The size and the surface of
the ocean have changed over billions of years, which promoted redistribution of organic
matter on the Earth. There appeared shallow and well-warmed lagoons, open and dry
places and, hence, new opportunities for the emergence and development of life. The
performed calculations show that the radiation mechanism could account for the
formation of significant amounts of organic matter and oxygen, as a result of water
radiolysis in the ocean. The radical products of radiolysis of sea water, apparently,
played an important role in the purification of water from toxic impurities, formation of
various simple “biomolecules”, and transformation of inorganic matter of the Earth into
organic matter. The radiation-induced transformations proceeded on the Earth
non-uniformly, while the calculations were performed for average conditions over the
whole Global Ocean. Indeed, different areas on the Earth’s surface and in the ocean were
apparently characterized by different radioactivity levels, temperatures, salinities,
contamination levels of sea water, and many other parameters. In other words, there
could appear a “warm little pond...” in which “…a protein compound was chemically
formed ready to undergo still more complex changes”, as was suggested by Darwin [22].
This suggests a non-uniform appearance and distribution of organic matter, including
prebiotics and oxygen on the Earth, and accordingly the non-uniform appearance and
development of life forms. One can reasonably assume the presence of compartments
with an increased, but not detrimental, radioactivity level in which an internal (radiation)
source of oxygen was continuously operating. In the early Archean period, most bacterial
groups in an anaerobic environment did not generate oxygen following photosynthesis.

Molecules 2022, 27, 8584 23 of 27
In order to trigger this process, free oxygen was required in the prebiogenic stage of the
Earth’s development. Only in the presence of free oxygen in the atmosphere were
cyanobacteria able to switch to the photosynthetic assimilation of carbon dioxide, and
production of oxygen. In the above-mentioned compartments, conditions were suitable
for the modification of anaerobic forms of life and appearance of oxygen-breathing life
forms, with participation of natural radioactive isotopes. Apparently, the very early
radiation-induced oxygenation of the atmosphere and the ocean was only able to initiate
the emergence of oxygen-breathing biological forms, but this oxygenation was not
always high. This does not rule out the early origin of oxygen photosynthesis. The
possibility of oxygenic photosynthesis appeared with the advent of cyanobacteria. They
created the stable oxygen-containing atmosphere of the Earth.
The proposed hypothesis implies further development and substantiation. In the
current state, it should be considered as the starting point for the subsequent
consideration of the important role of natural radioactivity in the general picture of
chemical evolution of the Earth, and the appearance of life.
Funding: This research was supported by the Ministry of Science and Higher Education of the
Russian Federation, project number 122011300061-3.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The author declares no conflicts of interest.
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