There is an overwhelming amount of support for theories on theorigin of life. Although each theory differs to one another, all proposedtheories imply that life has evolved from single-celled microorganisms to thecomplex multicellular life forms that have existed over millions of years.The firsthypothesis I am evaluating is the Panspermia hypothesis. Panspermia, fromAncient Greek is defined as (pan) meaning ‘all’, and (sperma)meaning ‘seeds’. It is the hypothesis that states life exists all throughoutthe Universe, and life is propagated throughout space from location to locationvia meteoroids, comets (Wickramasinghe, 2011), and planetoids (Rampelotto,2010). To some people, this is how life on Earth originated.
Panspermiais a theory that has three popular variations: Lithopanspermia, Ballisticpanspermia, and Directed panspermia. Lithopanspermia, or interstellarpanspermia is the belief that planetary rocks are the mechanisms of transferfor life from one solar system to another. Ballistic panspermia, orinterplanetary panspermia is the belief that planetary surface rocks serve astransport of life between planets within the same solar system. Directedpanspermia is the spreading of life intentionally by extra-terrestrialcivilisations, or from Earth to other planets by humans (Crick & Orgel,1973)Evidence in supporting the Panspermia hypothesis comes from a meteoriteALH84001 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 etal., 1996). Bearing similar proportions to theoretic nanobacteria of diameters20nm to 100nm in diameter, similar to those found in calcified arteries (Milleret al.
, 2004). However, the announcement of the ‘discovery’, sparkedconsiderable controversy – many rejected the possibility of this being solidevidence of extra-terrestrial life. McKay argued that, previous Martianmeteorites analysed have contaminations that do not bear resemblance to thosefound on ALH84001, which appear embedded within the matter (McKay et al., 2003).Despite some supporting evidence, there is no clear proof thatthese fossilised bacterial shapes are indeed extra-terrestrial life forms fromspace. Some proposed that evidence for Panspermia seems weak, and there is theview of Crick (Crick, 1993) who suggests that the implausibility of the RNAworld may imply that life that we know of did not originate on our planet, butinstead on a planet where there might have been conditions that wereselectively favourable for the evolution of RNA. Implications of this are thatit is much less adverse for RNA to have existed on a more favourable planetthan existing on Earth at a time which would not have favoured the origin ofthe RNA world (Crick, 1993).
The second hypothesis that I will be evaluating is the Oparin-Haldanehypothesis, a theory put forward separately by biochemists Alexander Oparin andJ. B. S.Haldane who both subsequently released independent papers.
Theirtheories stated that life started to appear in the oceans during a time ofatmospheric reduction. According to the theory, organic compounds weresynthesis nonbiologically via ultraviolet light, made possible due to the lackof an ozone shield, allowing for light to penetrate the upper layers of theocean surface (Haldane, 1929., Oparin, 1924).
Oparin came up with the hypothesis in 1924 that Earth’s atmospherewas extremely reducing in its early stages of development, meaning that theatmosphere had an excess of negative charge and could cause reducing reactionsby adding electrons to compounds (Oparin, 1924). Oparin suggested that theseorganic compounds could have undergone a series of reactions leading to moreand more complex molecules (Oparin, 1924). Under these circumstances, it washypothesized that organic molecules could have formed from simple inorganicmolecules (Oparin, 1924).The Oparin-Haldane theory helps theorise chemical evolution, thatevolution occurred through chemical reactions from a primitive, uninhabitableEarth.
A primitive Earth is said to have contained gases different to what wehave today, the primitive atmosphere containing: methane, carbon monoxide,carbon dioxide, ammonia, nitrogen, hydrogen and water. “Evolutionists are thusforced to assume that the primitive earth atmosphere contained no oxygen butcontained hydrogen and that carbon existed in the form of carbon monoxide ormethane” (Brinkman, 1969), which provides some evidence towards the possibilityof life originating within the oceans. If a primitive Earth consisted ofsimilar atmospheric conditions to our present atmosphere, then it is highlyunlikely that complex organic molecules would have been synthesised. However,some biologists have examined difficulties with the assumptions. Brinkman hasstated, that “a high rate of photolysis of atmospheric water vapour byultraviolet light would have generated a significant quantity of oxygen veryearly in the earth’s history” (Brinkman, 1969). Davidson has also stated thatthere is no evidence that the atmosphere ever differed greatly from that of thepresent (Davidson, 1965). There is also no geochemical evidence supporting thepresence of methane within the atmosphere at any given time (Abelson,1966),providing some weighted evidence against chemical evolution and theOparin-Haldane theory.
The final hypothesis I am going to evaluate is the Deep OceanHydrothermal Vents hypothesis, that life originated from hydrothermal ventsalong the seafloor. In 1977, scientists discovered biological communitiesliving within the proximity of a hydrothermal vent discovered in the EastPacific Rise mid-oceanic ridge (Martin et al., 2008). These biologicalcommunities were surviving on a chemical soup rich in hydrogen, carbon dioxideand sulphur far away from sunlight. The idea proposed that these hydrothermalvents provided the necessary conditions for microbial life to flourish,suggesting the potential for life to have originated within these chemicallyreactive environments.
In 2000, a new type of vent system was discovered, dubbed the LostCity hydrothermal field (LCHF) (Kelley, 2005). The LCHF provides insights intopast mantle geochemistry and presents a better understanding of the chemicalcompositions that existed during the evolutionary transitioning, fromgeochemical 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 thefact 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 beremoved in order to form a polymer (Da Silva, 2015)The hypothesis has sound evidence towards life being able to besustained within the proximity of hydrothermal events, and helps to disprovethe chemical evolution theory, those complex organic molecules were synthesisedfrom vents, not from ultraviolet light penetrating the ocean surface. All threehypotheses have their validities and invalidities, but neither has beencompletely disproven – there is potential that particular species on Eartharose from ancestors with different points of origin. After reviewing thesethree hypotheses I personally believe that the origin of life is indeed acomplicated process.
However, given the potential of evidence for and againstthe theories, there is a possibility that the process for the origin of lifemay have occurred more than once, from arising on Earth from multiple origins.