The Emergence of Life on Prebiotic Earth
With even just a single cell as a starting point, Darwinian evolution offers an elegant and powerful algorithm for bringing about the vast diversity, and ranging complexity of the living world. It is not difficult to imagine how, by acting on ‘surplus populations’ of replicating organisms; evolutionary forces such as natural selection would eventually lead to adaptations, niches and diversification. However, as true Darwinian evolution requires genetic replication, with heredity information being passed on to ‘progeny’ in order to enact its divergent processes; it is powerless to account for the emergence of the first life (Darwin, 1859).
The first serious scientific research into how life emerged from the prebiotic environment of the early Earth began in 1953; with the now famous Miller-Urey prebiotic or ‘primordial’ soup experiments, at the University of Chicago. In these experiments, Miller passed sparks, as a substitute for lighting, through a gaseous mix of ammonia, hydrogen, methane and water, which were believed at the time to be the major constituents of the early earth’s atmosphere.
The results of the experiments predicted that over time, the young oceans would have become a rich soup of organic monomers such as amino acids, laying the foundations for abiogenesis (Miller and Schlesinge, 1983). The Miller-Urey prebiotic synthesis model dominated the field until the 1980’s. However, it eventually fell out of favour, after new research showed it was unlikely that the Earths early atmosphere contained the high concentrations of reducing gases required by Millers model. Even while the prebiotic soup theory was at the height of its popularity it faced an insurmountable problem.
For a biological cell to arise from even the most favourable free solution of organic compounds seems unlikely, given that all life is DNA (deoxyribonucleic acid) based. DNA allows the transfer of heredity information to progeny, and codes for the proteins which are so essential to a living organism. However, the replication/production of DNA through templated polymerization, requires the pre-existence of catalytic proteins (e. g. gyrase, helicase and DNA –polymerase), which clearly gives rise to a chicken-and-egg type problem (Alberts, et al. , 2002) The next major advance in the field of origins research was the ‘RNA world’ hypothesis.
While DNA is capable of acting only as an information carrying chemical code; RNA (ribonucleic acid), another nucleotide polymer which acts as a intermediary/temporary information carrier in all present day organisms, is capable of both storing information and, in some forms acting as a catalyst. The RNA world hypothesis provides a much more probable explanation of how heredity developed than the alternative. In which both DNA and catalytic proteins must have arisen separately; the fact that the two molecules work so efficiently in-tandem with each other makes this speculation seem highly implausible (Joyce, 2012).
Even with RNA acting as both the molecule of heredity information and the catalytic machinery of the first cells, it is difficult of see how life could have got this far in the first place. Regardless of how probable the formation of complex organic polymers is from their respective monomer building blocks; in a free solution, even those reactions which lead to complex polymers would become to dilute to further develop into multifarious chemical systems (Pereto, 2005).
This means that the ‘genes first’ approach of the RNA world hypothesis, requires a level of random chemical interaction and prerequisite conditions that effectively render it impossibly improbable. However the discovery of micro-compartments, formed in aggregates deposited at alkali hydrothermal vents, revived the theory; as they provided an environment which overcome the problem of reaction products diffusing into the ocean. These micro-compartments also helped solve a problem faced by both the soup and RNA world theories.
Namely, how a disorganised solution of organic chemical components could ever make the transition to a self-contained, co-ordinated chemical system, capable of concentrating reaction products and other vital molecules within its self. (Dawkins, 1986) (Lane, 2010). Along with solving the problems of ‘compartment & concentration’ the alkali vent environment has become the central focus of present day research into biopoiesis. The hydrothermal reactor hypothesis, built around the vents, proposes a ‘metabolism first’ approach, involving much smaller molecules at much higher concentrations.
The hydrothermal reactor hypothesis is not intended to be an alternative to RNA based life, but instead shows how geology at the vents could have provided the structure and conditions which led to free living RNA and eventually DNA based cells (McKay, 2004) Primarily developed by William Martin from the institute of botany at the University of Dusseldorf, and Michael Russell from the Scottish Universities Environmental Research Centre; the alkali vent reactor hypothesis offer’s a thermodynamically favourable solution to the problem of how complex organic molecules, such as RNA could be arrived at.
In this approach, the first life would not have been in the form of a relatively sophisticated ‘nucleotide based replicator’ but a mineral-based metabolizer. The micro-compartments, provided by vent chimney cavities, where in Russell’s own words “life got started” are lined with metal sulphides, in particular FeS, FeS2 and NiS. Not only are FeS and NiS present as molecular clusters at the heart of enzymes such as CODH and ACS which are crucial to the acetyl-coenzyme-A pathway; they are also capable of catalyzing these same biochemical reactions in the absence of proteins.
This means that the cavities could have acted as inorganic analogues, catalysing a Wood- Ljungdahl type linear acetyl-CoA pathway of CO2 fixation (deDuve, 1991). The vent environment would also have provided the necessary electron donor, in the form of H2 gas which bubbled up along with the alkaline vent fluids. While CO2 which was in abundance in the early oceans, causing them to be mildly acidic, would have acted as an electron receiver; similar electrochemical gradients are used to day in all organisms which perform aerobic respiration to convert free energy into chemical energy.
So in the vent cavities, with catalytic metal sulphide linings, and a high energy potential mix of H2 and CO2 dissolved in sea water which percolated through them, there was more than just the right mix of precursory compounds. In short these 3D micro-compartments would have taken in CO2 and spun out acetate. This reaction was not only thermodynamically inevitable, but was also exergonic, and as such, led to a negative change in the Gibb’s free energy- of the vent system, releasing energy back into the micro-compartments.
In conjunction with concentrated levels of acetate, which is a fundamental intermediate in nearly every biologically produced or, biosynthetic, carbon compound; almost certainly, would have led to the formation of monomer base units such as amino acids and nucleotides (Shock, 1992). Finally the catalytic cavity lining, which when examined with a scanning electron microscope, showed perforations sufficient in size to allow the diffusion of primary reactants like CO2 in and out.
While, small enough to retain reaction products, which would have led to the natural assembly of the monomers into the complex organic polymers needed to reach the point at which the RNA world could begin; From which point evolution could take over, eventually leading to the present day. (Russell and Martin, 2004) The field of origins research is still in its infancy, and both the vent reactor, and RNA world theory are far from conclusive.
The best evidence comes not from the vent environment its self, but from corroborating researcher’s findings with evidence from the fields of evolutionary biology and biochemistry. Many of the metabolic pathways used by present day organisms are believed to be very ancient, diverging only slightly between different taxonomic classes. What has emerged from abiogenesis research is reliable and reproducible evidence, which shows that these metabolic pathways are strongly self organising in a range of environments, including that of the vents.
Furthermore, the products of these simple reactions have been shown to ‘piggy back’ there way upward to increasing levels of complexity. A paper, published in the journal chemistry in August 2005, showed that iron sulphide and amino acids were capable of catalysing the formation of other small organic molecules, such as nucleic acids. These original reaction products began to catalyze new reactions, eventually setting-up feedback loops, and developing into metabolic networks which gradually increased in complexity.
While this does not conclusively prove that life emerged at the vents, it shows that issues, such as RNA synthesis are overcome by the energy rich and selectively concentrative micro compartments. In which organic ‘precursor’ compounds arise almost immediately, leading to formation of complete nucleotides. As such, unless new discoveries prove the vent reactor hypothesis wrong it is the best model we have for understanding how inorganic matter came to give rise to, as Darwin called it, “endless forms most beautiful”