There one another, all proposed theories imply

There is an overwhelming amount of support for theories on the
origin of life. Although each theory differs to one another, all proposed
theories imply that life has evolved from single-celled microorganisms to the
complex multicellular life forms that have existed over millions of years.

The first
hypothesis I am evaluating is the Panspermia hypothesis. Panspermia, from
Ancient Greek is defined as (pan) meaning ‘all’, and (sperma)
meaning ‘seeds’. It is the hypothesis that states life exists all throughout
the Universe, and life is propagated throughout space from location to location
via meteoroids, comets (Wickramasinghe, 2011), and planetoids (Rampelotto,
2010). To some people, this is how life on Earth originated.

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Panspermia
is a theory that has three popular variations: Lithopanspermia, Ballistic
panspermia, and Directed panspermia. Lithopanspermia, or interstellar
panspermia is the belief that planetary rocks are the mechanisms of transfer
for life from one solar system to another. Ballistic panspermia, or
interplanetary panspermia is the belief that planetary surface rocks serve as
transport of life between planets within the same solar system. Directed
panspermia is the spreading of life intentionally by extra-terrestrial
civilisations, or from Earth to other planets by humans (Crick & Orgel,
1973)

Evidence in supporting the Panspermia hypothesis comes from a meteorite
ALH84001 from the surface of Mars ~15MYA and arriving on Earth ~13,000YA
(Antretter et al., 2003). The meteorite was discovered in Antarctica in 1984,
and in 1996 supposed remains of a bacterial-like lifeform from Mars (McKay et
al., 1996). Bearing similar proportions to theoretic nanobacteria of diameters
20nm to 100nm in diameter, similar to those found in calcified arteries (Miller
et al., 2004). However, the announcement of the ‘discovery’, sparked
considerable controversy – many rejected the possibility of this being solid
evidence of extra-terrestrial life. McKay argued that, previous Martian
meteorites analysed have contaminations that do not bear resemblance to those
found on ALH84001, which appear embedded within the matter (McKay et al., 2003).

Despite some supporting evidence, there is no clear proof that
these fossilised bacterial shapes are indeed extra-terrestrial life forms from
space. Some proposed that evidence for Panspermia seems weak, and there is the
view of Crick (Crick, 1993) who suggests that the implausibility of the RNA
world may imply that life that we know of did not originate on our planet, but
instead on a planet where there might have been conditions that were
selectively favourable for the evolution of RNA. Implications of this are that
it is much less adverse for RNA to have existed on a more favourable planet
than existing on Earth at a time which would not have favoured the origin of
the RNA world (Crick, 1993).

The second hypothesis that I will be evaluating is the Oparin-Haldane
hypothesis, a theory put forward separately by biochemists Alexander Oparin and
J. B. S.Haldane who both subsequently released independent papers. Their
theories stated that life started to appear in the oceans during a time of
atmospheric reduction. According to the theory, organic compounds were
synthesis nonbiologically via ultraviolet light, made possible due to the lack
of an ozone shield, allowing for light to penetrate the upper layers of the
ocean surface (Haldane, 1929., Oparin, 1924).

Oparin came up with the hypothesis in 1924 that Earth’s atmosphere
was extremely reducing in its early stages of development, meaning that the
atmosphere had an excess of negative charge and could cause reducing reactions
by adding electrons to compounds (Oparin, 1924). Oparin suggested that these
organic compounds could have undergone a series of reactions leading to more
and more complex molecules (Oparin, 1924). Under these circumstances, it was
hypothesized that organic molecules could have formed from simple inorganic
molecules (Oparin, 1924).

The Oparin-Haldane theory helps theorise chemical evolution, that
evolution occurred through chemical reactions from a primitive, uninhabitable
Earth. A primitive Earth is said to have contained gases different to what we
have today, the primitive atmosphere containing: methane, carbon monoxide,
carbon dioxide, ammonia, nitrogen, hydrogen and water. “Evolutionists are thus
forced to assume that the primitive earth atmosphere contained no oxygen but
contained hydrogen and that carbon existed in the form of carbon monoxide or
methane” (Brinkman, 1969), which provides some evidence towards the possibility
of life originating within the oceans. If a primitive Earth consisted of
similar atmospheric conditions to our present atmosphere, then it is highly
unlikely that complex organic molecules would have been synthesised. However,
some biologists have examined difficulties with the assumptions. Brinkman has
stated, that “a high rate of photolysis of atmospheric water vapour by
ultraviolet light would have generated a significant quantity of oxygen very
early in the earth’s history” (Brinkman, 1969). Davidson has also stated that
there is no evidence that the atmosphere ever differed greatly from that of the
present (Davidson, 1965). There is also no geochemical evidence supporting the
presence of methane within the atmosphere at any given time (Abelson,1966),
providing some weighted evidence against chemical evolution and the
Oparin-Haldane theory.

The final hypothesis I am going to evaluate is the Deep Ocean
Hydrothermal Vents hypothesis, that life originated from hydrothermal vents
along the seafloor. In 1977, scientists discovered biological communities
living within the proximity of a hydrothermal vent discovered in the East
Pacific Rise mid-oceanic ridge (Martin et al., 2008). These biological
communities were surviving on a chemical soup rich in hydrogen, carbon dioxide
and sulphur far away from sunlight. The idea proposed that these hydrothermal
vents provided the necessary conditions for microbial life to flourish,
suggesting the potential for life to have originated within these chemically
reactive environments.

In 2000, a new type of vent system was discovered, dubbed the Lost
City hydrothermal field (LCHF) (Kelley, 2005). The LCHF provides insights into
past mantle geochemistry and presents a better understanding of the chemical
compositions that existed during the evolutionary transitioning, from
geochemical processes to biochemical processes (Baker & German, 2004).
However, there is a reason to doubt the origin of life from hydrothermal vents.
One of the main arguments against the origin of life from the deep sea, is the
fact that so many macromolecules are found in biology. Molecules such as DNA,
RNA, proteins and lipids are all polymers and form via condensation reactions –
a wet environment is required for molecules to mix, but water then needs to be
removed in order to form a polymer (Da Silva, 2015)

The hypothesis has sound evidence towards life being able to be
sustained within the proximity of hydrothermal events, and helps to disprove
the chemical evolution theory, those complex organic molecules were synthesised
from vents, not from ultraviolet light penetrating the ocean surface. All three
hypotheses have their validities and invalidities, but neither has been
completely disproven – there is potential that particular species on Earth
arose from ancestors with different points of origin. After reviewing these
three hypotheses I personally believe that the origin of life is indeed a
complicated process. However, given the potential of evidence for and against
the theories, there is a possibility that the process for the origin of life
may have occurred more than once, from arising on Earth from multiple origins.