Abiogenesis, In 1924, Oparin hypothesised that simple

                

Abiogenesis, or the Origin of Life, is the process in which Earth
migrated from an abiotic planetary mass into a thriving biotic environment ~3.7
billion years ago (Nutman et al.,
2016). Almost all biologists agree that simple cells cannot be synthesised from
abiotic molecules in a singular step. Intermediate forms must have subsisted,
known as “precellular life”, to achieve the diversity of life found on Earth
today. The surface of primitive Earth was molten (Hashizume, 2012), and as the surface
temperature progressively decreased, magma solidified, forming the primitive
oceans and paleoatmosphere. In 1952, Urey suggested the paleoatmosphere was composed
of anoxic gases (CH4, NH3) and water vapour (H2O),
conversely, atmospheric chemists now hypothesise that the paleoatmosphere was
redox neutral: dominated by carbon dioxide (CO2) and nitrogen (N2)
(Freeman & Herron, 2004). Numerous hypotheses for abiogenesis have been
established since the 20th Century; most focus on a reducing paleoatmosphere,
though more conventional theories now persist: specifically, autotrophic
origins of hydrothermal vents.

 

In
1968, Crick suggested that RNA was the first genetic molecule, which may act as
an enzyme to catalyse its own self-replication. RNAs have ribozyme function: the
process required for replicating nucleic acids (Cech, 1986). RNA can also act as a genome, i.e. in viruses such as HIV (Bernhardt, 2012). Hence, RNA
can simultaneously possess a genotype and phenotype. Is RNA with ribozyme
function the transitional form between non-living matter and living cells? The RNA World Hypothesis, a term coined by Walter Gilbert, 1986, proposes
that RNA was the first self-replicating molecule on Earth, and catalytic RNA preceded
the current world of DNA-RNA-protein (Martin et al., 2008). RNA replication has evolved to utilise ribosomal
proteinaceous function. If the RNA World preceded the origin of protein
synthesis, the debate of ‘metabolism first’ versus ‘gene first’ still stands. Non-coding
RNA such as ribosomal RNA and transfer RNA have the ability to replicate, prior
to the evolution of ribosomal protein synthesis (Bernhardt, 2012). The RNA World concept
encounters criticism: RNA is fragile, complex, unstable and difficult to
synthesize in abiotic conditions. The catalytic range of ribozymes is narrow,
and the origin of a synthesizing apparatus for the first RNA is not yet decided
(DeVincenzi,
1996). It is universally accepted that the RNA World functioned as a transitional
stage between the first building blocks of life and complex, multicellular
organisms (Orgel, 2004).

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The
most conventional hypothesis is the Oparin-Haldane Theory, colloquially acknowledged
as the ‘Primordial Soup’ Theory. In 1924, Oparin hypothesised that simple molecules
(CH4, NH3) reacted to form organic molecules, thus, more
complex bio-polymers (i.e. nucleotides, peptides), which evolved into
multicellular systems, and ultimately, life. Haldane, in 1929, independently
proposed a similar hypothesis: that in a reducing atmosphere, supplied with
energy (lightning or ultraviolet light), a range of organic molecules could be
synthesised. Both Oparin and Haldane hypothesised that any organic molecules formed
would dissolve into the primordial ocean: ‘Primordial Soup’. This pioneering
theory inspired research by Miller and Urey.

 

In
1959, Miller and Urey were able to synthesise bio-molecules from abiotic precursors,
by circulating anoxic gases (CH4, NH3 and H2) over
water and artificial lightning. Organic molecules, notably amino acids, were
produced (Table 1), which dissolved into the ‘ocean’, forming the ‘soup’ (Fry,
2006). The observations concluded that 10-15% of the 710mg of carbon from CH4
was now in the form of organic compounds. 13 of the 22 essential amino acids
used in protein production were formed: the most abundant being glycine. Alas,
hydrolysis of bio-polymers occurs in water. Whilst the experiment proves organic
molecules can be created from abiotic components, many scientists now predict
that early Earth had a redox neutral atmosphere, with CO2 and N2,
rather than CH4 and NH3 as dominant gases (Cairns-Smith et al., 1992). Large scale bio-synthesis
seems less likely as conditions become decreasingly reducing.

 

In
1951, Bernal suggested a plausible mechanism to overcome hydrolysis in the
‘Primordial Soup’: the role of clay minerals. Clay’s ability to absorb organic
molecules, protect against UV radiation, concentrate and catalyse polymerisation,
alongside its abundance on primitive Earth, makes it a viable theory (Hashizume,
2012). The formation and replication of polynucleotides is possible
on clay. Cairns-Smith, in 1982, suggested clay minerals were the basis of the
first organisms, and the genes responsible. In 1996, Ferris et al. successfully adhered nucleotides
to Mg-montmorillonite, and Mg-montmorillonite catalysed phosphodiester formation,
creating polynucleotide chains. Chain formation occurred faster than hydrolysis
whilst bound to clay. This model is a potential method of polynucleotide
formation on Earth (Ferris et al.,
1996). Bio-molecules may be able to select different clay minerals, i.e.

serpentine has a limited capacity for amino acid absorption (Hashizume,
2012): making natural selection, thus evolution possible.

 

Hydrothermal
vents, since their initial 1977 discovery, are vital in theorising the origin
of life. The concept, known as autotrophic origins, postulates life arose from
CO2, and the first organisms were chemo-autotrophs. There are two
types of hydrothermal vent: the ~360oC black smoker vents,
volcanically driven by the magma-chamber below the ocean; and the cooler,
~50-90oC Alkaline vents, driven by serpentinisation, found in the
“Lost City” (Fig. 1) (Lane, 2010). Black smokers continuously pump ‘smoke’: in
the form of acidic (~pH 2-3) metal sulphides, reaching temperatures up to 405oC
(Lane, 2010; Martin et al, 2008). These
sulphide chimneys are imitations of primordial Earth. 2oC seawater
is superheated and charged with minerals and gases. This fuels communities,
serving as the base energy within ecosystems. In 1992, Wächtershäuser explored chemo-autotrophic abiogenesis, based upon black smokers. The
conditions found within black smokers, consisting mainly of pyrite, are a prerequisite
for chemo-auto-origin (Wächtershäuser,
1992).

           

Alkaline
vents are located in the Lost City hydrothermal field (LCHF). The alkaline
fluid (~pH 9-11) and porous structure are generated by fresh rock exposure by
the movement of tectonic plates on the sea floor (Martin et al, 2008). The reaction between fresh rock and seawater alters the
structure, by a process called Serpentinisation. Natural proton gradients at
alkaline vents allow electron transfer from H2 to CO2.

This reaction forms either methane or acetate, producing a high-energy
thioester bond on acetyl CoA (Lane et
al., 2010). The acetyl CoA pathway produces pyruvate, as well as acetyl
phosphate. These products drive the reverse Krebs cycle; a primary theory of organic
molecule production. The acidic primitive oceans, with high partial pressure of
CO2, made the vent membranes chemiosmotic, due to the opposing
alkalinity. The three-dimensional, semi-permeable microenvironments produced by
serpentinisation-driven synthesis allow electron flow through the membrane;
thus, providing the substrate for chemiosmosis to occur. Russel et al., 1993, proposed that proton
gradients, which allow for chemiosmosis, are fundamental to life. This hypothesis
is compatible with the RNA World: the micro-compartmentalisation of the vents
may produce a conductive environment for the synthesis and polymerisation of
the first RNAs (Martin et al., 2008).

 

The
final hypothesis discussed is Panspermia: the concept that life migrated
through space to Earth. The first variation is interplanetary panspermia: life
originated within our Solar System; impact-expelled rocks from a planet’s
surface transported biological life from one planet to another. McKay et al., 1996, discovered evidence for the
first variation: the Martian Meteorite ALH84001 had markings of bacterial
fossils, or bacterial processes. Jupiter’s moon, Europa, also insinuates life in
our Solar System: water deposits, active volcanoes and energy capable of
sustaining life in Jupiter’s magnetosphere provide a suitable interface for
life (Freeman & Herron, 2004). The second variation is interstellar
panspermia: biological material was transported from another solar system to ours.

A prerequisite is that bacterial spores would need sufficient velocity to
escape the gravitational pull of their own planet; this force could be supplied
by radiation pressure (Secker et al.,
1994). The third, final, and most experimental theory: directed panspermia,
whereby the microbes which generated all life on Earth were sent intentionally,
by an extra-terrestrial civilisation, to spread life throughout the universe
(Crick & Orgel, 1973). It is plausible that this occurred, but there is insufficient
research. Moreover, panspermia would only clarify abiogenesis on Earth, rather
than the origin of all life.

 

In
~4.5 billion years of Earth, complex life only arose once. All extant life on
Earth stems from common origin: the Last Common Universal Ancestor (LUCA). It
is possible that abiogenesis occurred numerous times, due to the ease of
generating organic molecules, except the resulting organisms desiccated, or
were not preserved in early fossil records. LUCA most likely used DNA rather
than RNA, and was packaged inside a membrane which allowed chemiosmosis, and the
production of complex proteins (Lane et
al., 2010). It is hypothesised that LUCA was hypothetically a community of
interbreeding species (Freeman & Herron, 2004). This proposition would
suggest that there were multiple lineages formed after the initial origin of
life, but only LUCA survived.

 

The
most influential hypothesis discussed is the theory of autotrophic origin in
Alkaline vents. This theory provides insight into chemiosmosis, a prerequisite
for life, within an oceanic environment, outside of the debated atmospheric
conditions of early Earth. Ultimately, organic molecules were synthesised,
joined and replicated at the start of the epoch of life on Earth.

 

 

 

 

 

 

 

 

 

 

 

 

 

                                                                                                 

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