The earth was formed4.54 billion years ago (Ga) and life followed once a hospitable environmentarose after the ‘heavy bombardment’ in the Hadean eon. Life is defined asthe ability to store information in a genotype while expressing it in aphenotype and be able to transfer this information. From the start of theArchean eon, 4 Ga, prokaryotic life forms established, with the first fossilevidence dating to 3.5 Ga (Schopf et al.
, 2002). There are multiplehypotheses as to how these life forms originated. This evaluation of thosehypothesis focuses on those with scientific credibility and not those relatedto religion.
In the first half of the 20thcentury, Alexander Oparin and J. B. S. Haldane independently published seminalpapers that provided coherent arguments for the origin of life (McCollom, 2013).They suggested that the prebiotic earth had a reducing atmosphere containingmethane, water, hydrogen and ammonia (McCollom,2013). Their hypothesis proposed that life originated in three stages.
Firstly,energy forms synthesised organic molecules from inorganic precursors which thenaccumulated in a ‘prebiotic soup’. Secondly, the monomers reacted with oneanother to form polymers. The last step occurred when polymers formed complexunits that were capable of self-replication. There have been studies which bothconfirm and contradict this hypothesis. The classicalabiogenesis study undertaken by Stanley Miller and Harold Urey in 1953 mimickedthe suggested reducing atmosphere and oceans in flasks to determine if theseconditions perform abiogenesis (Fig.
1). The spark discharge shown in Fig. 1represented a lighting strike on the prebiotic earth (McCollom, 2013). The results in conjunction withother experiments concluded that a reducing atmosphere, with an influx ofenergy, can synthesise amino acids, nucleotides and other organic molecules,confirming to stage one (Oro, 1961; McCollom, 2013). Furthermore, in 1958 Foxand Kaoru endorsed the second stage of the hypothesis by producing protenoidsthrough polymerisation. Their experiment was plausible as it resembledprebiotic pools where temperatures would reach 140-180oC;dried-up lagoons, hot springs and pressurized volcanic magma (Fox and Harada, 1958). Fox certified the thirdstep as he created microspheres under prebiotic condition.
These microshpereshave multiple similarities to a cell; they are uniformly symmetrical, metabolicand able to divide by binary fission (Fox, 1980). These steps provide credibleevidence to the origin of life under reducing conditions. In 1993, Kasting analysed ancient rock compositions and as aconsequence abandoned the notion of a reducing prebiotic atmosphere andproposed a more neutral one (Kasting, 1993) This discovery undermined the Oparin-Haldane hypothesis. Although,there have been successful experiments that have produced amino acids underneutral conditions (Cleaves, et al.
, 2008). The ability to accumulatehigh concentrations of molecules in prebiotic pools has also been criticiseddue to the low yields of monomers in experimental studies (McCollom, 2013). The Oparin-Haldane hypothesisprovides a process for the origin of life which is evidenced in experiments,however under natural conditions is not as credible. The RNA World hypothesis was proposedindependently in the 1960s by Frances Crick, Carl Woese and Leslie Orgel(Robertson and Joyce, 2012). It proposes that RNA is the precursor to DNA andproteins.
The direct synthesis of RNA monomers in prebiotic conditions iscontroversial (Ferris, 2006). Nucleotides have been synthesised in laboratoriesunder reducing conditions, however, there is speculation as to the success ofsynthesis in the natural prebiotic world (Oro, 1961). It has been suggestedthat simpler nucleotide precursor existed in a Pre-RNA World (Orgel, 2000). A seminal paper by Ferris describes the process for constructing RNA oligomers using clay mineral montmorillonites (Ferris, 2006). Montmorillonites are formed by the accumulation of volcanic ash, hence likely to be present on the prebiotic earth (Ferris, 2006).
The most reliable characteristic of this hypothesis is that the RNA oligomers are able to catalyse their own replication. The discovery of the ribozyme in 1982 showed that RNA can store genetic material and act as a biological catalyst (Robertson and Joyce, 2012). This self-replicating RNA provides greater evidence for life than protein microspheres as it is still utilised in all living organisms. Moreover, the mutability and error in replication would provide the raw material for evolution in the RNA world (Robertson and Joyce, 2012).
The ribozyme further has the ability to form peptide bonds between amino acids, indicating the role of the RNA in later protein production (Robertson and Joyce, 2012). The RNA World is supported by extensive ribozyme evidence however due to the dependency on a reducing atmosphere for the synthesis of monomers, as with the Oparin-Haldane hypothesis, it cannot be qualified alone. ThePanspermia Hypothesis suggests that life originated in space and then travelledto earth. The most plausible theory from this hypothesis is that the buildingblocks of life were generated in space and later assembled on earth. Organicmolecules, including amino acids, were found in the Murchison meteorites in Australia(Kvenvolden, et al., 1970). The amino acids discovered are found in modern organisms,indicating the first prokaryote may have developed from these monomers(Kvenvolden, et al., 1970).
Furthermore, NASA was able to synthesize thenucleobase uracil under an astrophysical environment (Nuevo, et al., 2009).These experiments provide evidence that organic molecules could have been depositedon earth by meteorites. The synthesis of organic molecules in space provides asolution to the RNA World hypothesis and together can create a valid origin to life.
The Panspermia Hypothesis is eccentrichowever it provides explanations and evidence where the RNA World does not. The hydrothermal vent theory provides a credible hypothesis for the origin of life. The recent discovery of alkaline hydrothermal vents presents a promising explanation for the origin of life through metabolic pathways. The vents have microporous labyrinths with inorganic barriers containing iron-sulphur minerals (Wächtershäuser, 1990). Due to exothermic reactions, there is a highly reducing, alkaline (pH9-11), warm fluid inside the labyrinth, creating electrochemical gradients. The movement of protons through the catalytic walls across this gradient provides energy for abiogenesis and is extremely comparable to an autotrophic cell (Fig.2) (Sojo, et al., 2016; Martin and Russell, 2003).
There is evidence that the metal catalyst in the pore encourages chemical changes which sustain rudimentary metabolic pathways and carbon fixation (Wächtershäuser, 1990). This hypothesis suggests a natural mechanism for the formation of life from geochemical interactions rather than the organic molecule first hypothesis previously stated.These hydrothermal vents also provide supporting evidence for the formation of organic molecules (Martin and Russell, 2003). With high concentrations of ammonium and methane, the Urey-Miller experiment can be facilitated.
These organic molecules are transferred to cooler waters by hydrothermal fluids where clay minerals could assist in polymerisation and hence form a protocell. The hydrothermal vent hypothesis is the most credible as it describes the formation of life through the interaction of metabolic pathways with organic molecules, unlike the other singularly molecular hypothesis. Moreover, these processes occurred in natural conditions unlike the RNA world or Oparine-Haldane hypothesis.The origin of life is a complex process which requires multiple conditions to be perfectly aligned. The probability of such an alignment is low at any one point in space and time. However, based on the hypothesis that the universe is infinite (Borchardt, 2007) in both space and time dimension then the probability that life will be created more than once becomes a certainty. It is less the complexity involved in the formation of life which dictates if it has only happened once but more the size of the space in which such an event may occur which governs the probability of life being a singular or multiple occurrence.
The recent analysis of Jupiter’s moon Europa indicates that there is sufficient energy to create life, evidencing the possibility for new life (Chyba, 2000). Onearth, evidence suggests that complex life may have evolved on multipleoccasions. The conditions required for complex life were available from 2.4-2Ga (Kipp et al., 2016). These findings indicate that complex life couldhave existed before the first known fossilised eukaryote 1.75 Ga (Kipp et al., 2016).
Life’s complexity makes it a hard process to replicate, however it ispossible. There are several scientifichypotheses that suggest ways in which prokaryotes could have originated around3.8 Ga. The Oparin-Haldane hypothesis provides evidence for theproduction of a prebiotic soup, although under conditions not replicating thoseof the prebiotic world. There are faults in this hypothesis which makes it lesscredible. The RNA World is a plausible hypothesis if used in conjunction withthe Panspermia as a way of acquiring nucleotides. The dynamic alkaline ventspresent the most convincing evidence for the origin of life, presenting bothmetabolic and molecular evidence in a natural environment. The mechanism forthe origin of life will continue to be a mystery but the alkaline vents hypothesisposes a suitable candidate.