The Earth was formed ~4.6 billion years ago (Fig.1). PrimordialEarth was highly inhospitable during this period due to its composition of hotmolten magma and sparse oxygen. The atmosphere was thought to have formed fromthe gases emitted by volcanoes, with the early atmosphere consisting of carbondioxide (CO2), methane (CH4), ammonia (NH3)and water.
As the Earth cooled, the water vapour released by the Earth’s crustcondensed, forming the Earth’s oceans. Itis here, where life is predicted to have originated, with the first fossilevidence of life being obtained ~3.5 billion years ago (Fig.1). Thereare numerous theories about how life originated on Earth, although no matterhow many experiments are performed, we will never know the true reason, butsimply that such things are possible (Lane, 2010). The terrestrial origins of life can be sub-divided intoinorganic and organic origins.
The clay hypothesis supports the inorganicorigin of life theory, insinuating that life evolved through the naturalselection of crystals (Cairns-Smith, 1965). It is thought that these crystalswere synthesised shortly after the creation of the Earth’s oceans, due tosilicate minerals initiating pyrogenic activity (Ponnamperuma, Shimoyama, , 1982). Certain areas of the clays lattice imperfections would have requiredreplication, in order to form this crystalline structure.
During thisself-replication, the clay would have been responding to environmental selectionpressures, mutating, and evolving, leading to the development of increasinglycomplex organic materials (Davis & McKay, 1996). It is commonly believedthat RNA was most probably the first macromolecule; and due to recentexperiments, it has been illustrated that clay catalyses the chemical reactioninvolved in the formation of RNA oligomers, further corroborating with thishypothesis. Furthermore, the only environmental conditions necessary for thishypothesis is the presence of an aqueous environment and clay minerals, both ofwhich have a high probability of being present at the origin of life.The Oparin-Haldane model is a representation of aterrestrial organic origin of life theory, being the most investigated of allthe hypotheses.
In 1953, Miller et al.,demonstrated the abiotic chemical reactions which synthesised simple organicmaterials. He achieved this by simulating electrical discharges into a pre-bioticsoup mixture of the early atmospheric gases.
Miller found that these dischargesproduced amino acids and compounds such as hydrogen cyanide (HCN) and formaldehyde.When the correct environmental conditions are present, a cascade ofcondensation reactions, solely involving formaldehyde, results in the formationof the ribose sugars that form nucleotides. Moreover, NH3 and HCNare readily combined in thermodynamically favourable conditions to form adenine(Or?,1961). This illustrates that through the combination of simple organicmaterials, the building blocks for DNA, and therefore the possibility for life,can be produced.
However, it was established that the organic building blockswould have been readily hydrolysed in the pre-biotic soup unless they wereformed on solid materials surface such as clay (Ferris et al., 1996). Whentrialled in the lab, the clay mineral montmorillonite was used, acting as acatalyst to adhere the building blocks together creating polynucleotide chainsup to 50 nucleotides long. Therefore, if this reaction occurred repeatedly inthe environment, there is a possibility that self-replicating primordial formsof these polymers would be produced. Conversely, this hypothesis has faced numerous criticisms. Forexample, a better understanding of the chemical reactions of the atmospherehave led scientists to now believe that the early atmosphere comprised ofcarbon dioxide instead of methane and nitrogen instead of ammonia (Kasting,1983).
This is important as carbon dioxide and nitrogen are unable tosynthesise these organic molecules efficiently (Freeman and Herron, 2007).Following on from this, the fact that Ferris required a clay mineral for thebuilding blocks to adhere to, disregarding this model and further corroboratingwith the clay hypothesis previously mentioned. The RNA world hypothesis is alsoan organic terrestrial origin of life hypothesis. This theory proposes that anRNA based living system was the primordial form: replicating, mutating, and evolvingto form DNA and proteins that our modern day living systems consist of (Freemanand Herron, 2007). RNA stores geneticinformation, as well as a catalysing its own self-replication, therefore, RNAshould theoretically be able to evolve over time by natural selection (Joyce& Orgel, 1993).
Beaudy and Joyce (1992) tested this theory using a ribozymeknown as Tetrahymena. The resultscorroborate that with each replication, the probability of a mutation occurringincreased, providing the substrate for evolution to occur. Conversely, themajority of examples show RNA being replicated by protein enzymes, which wouldnot have been present in the RNA world.
Thefinal and most probable organic terrestrial origin of life hypothesis is thatof hydrothermal vents. There are two types of hydrothermal vents, obtaining aworldwide distribution (fig.2).
Hydrothermal vents known as the ‘black smokers’are located primarily along the mid-Atlantic ridge, pumping ~400?C hydrogensulphide rich smoke into the surrounding cold ocean waters. This smoke isconjured up from the magma surface below and is immensely acidic, obtaining a pHof ~2-3. The chimneys of these vents contains autotrophic sulphurbacteria which can extract the hydrogen from this black smoke to attach tocarbon dioxide in order to form organic molecules. One early hypothesis for theorigin of life from these black smokers is that the hydrogen sulphide reactswith the oxygen in the seawater releasing the energy required to convert CO2into organic matter. However, the problem with this is the lack of oxygenduring the early years of the Earth.
2001 saw the discovery of the firstsubmarine lost city hydrothermal field by Kelley et al (2008). Thesecond type of hydrothermal vents, are the alkaline vents. These vents are locatedfar away from the mid-Atlantic ridge, on the smooth surface of the ocean floorand produce hot smoke (~200?C). An abundance of hydrogen isavailable in these vents due to an exothermic process known as serpentinisation,in which ultramafic minerals from the upper mantle are transformed intohydrogen (Herschy et al., 2014). Thesevents form a network of interconnected micro-pores which are bound by thininorganic walls.
The difference in pH inside these walls (~10-11)compared to the ocean waters (~5-6) creates a natural occurringproton gradient. This allows hydrogen and CO2 to undergo protontransfer, being catalysed by the iron nickel sulphate (Fe(Ni)S) walls. Thisreaction gives rise to acetyl thioester which can react with CO2 toproduce pyruvate, a key molecule involved in the Krebs cycle. Similarly, whenphosphate reacts with another acetyl thioester, acetyl phosphate is producedwhich can enter the reverse Krebs cycle which in turn, produces increasinglycomplex organic molecules. The extra-terrestrial origins of life, proposed by Arrheniusin 1908, are known as interstellar, interplanetary, and directed Panspermiahypotheses. Interstellar panspermia suggests that life originated in anothersolar system before travelling to Earth through interstellar space.
For this tobe the case, these spores would need a significant amount of radiation pressureto escape the gravitational field of their home star, as well as obtaining carbonaceousshield to avoid radiation damage (Secker etal., 1994). Interplanetary panspermia suggests that life evolved on anotherplanet within our solar system, before travelling to Earth. One theory is thatlife could have evolved in hydrothermal vents on Jupiter’s moon, Europa, due tonumerous volcanoes and an icy surface.
It is believed that these microbes couldbe dislodged and carried through space on debris, before landing on Earth via ameteor impact. However, this poses the same problems as the interstellarpanspermia: in which this microbe must be encased in a shield to protect itagainst radiation. Finally, directed Panspermia, hypothesised by Crick andOrgel (1973), suggests microbes were sent to Earth aboard an extra-terrestrialspacecraft in the aim to immerse the Earth with life. Crick and Orgel arguethat if our technical and medical advances carry on the way they are, we ourselvescould launch a similar mission, and therefore it is conceivable to believe thatanother civilisation on another planet or solar system did so ~4 billionyears ago. As of yet, there is no evidence of this theory, however, absencedoesn’t equate to non-existence.
In ~4.6billion years of the Earth’s existence it is viable to believe the origin oflife occurred more than once due to the early Earth composing primarily ofliquid oceans. As illustrated by the six hypotheses presented, it is relativelyeasy to synthesise simple organic molecules. However, the ability for thesesimple molecules to mutate and evolve to form the Last Universal CommonAncestor (LUCA) is highly unlikely (Herschyet al., 2014). This is due to themagnitude of components present in the early environment that could have causedharm to this early life form, i.e.
the pH levels and the temperatures of theoceans. Despite the numerous hypotheses presented about theorigin of life, the hydrothermal vents theory provides a much more feasible,natural explanation for the origins of life with little-to-no drawbacks, orquestionable processes, unlike the additional five hypotheses explored.