Where on Earth

Research into the abiotic synthesis of biomolecules of the living cell has yielded a range ofbuilding blocks that furnish a foundation for prebiotic evolution to begin its journey. Some of the syntheses, however, might require potentially contradictory conditions for optimization. For example, polyP and nucleoside productions are favored by high temperatures, whereas purine synthesis and RNA polymerization are favored by icy conditions. Since a heterotrophic origin of life depended on access to multiple environmental nutrients, the question of where an optimal primordial reactor best equipped for life's emergence might be found arises. There are a number of candidate sites.

(i) Surface Water

In atmospheric amino acid synthesis powered by lightning, the amino acids produced would be washed down by rain on to surface bodies of water, which would represent useful candidate sites as observed by Darwin:71

"If (and oh, what a big if) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present that a protein compound was chemically formed, ready to undergo still more complex changes, at the present day such matter would be instantly devoured or adsorbed, which would not have been the case before living creatures were formed."

In looking to the warm little pond, Darwin was in fact anticipating the lead focus ofastrobiological explorations—always following the water.

(ii) Clays and Minerals

There are many varieties of clay, a class of negatively charged minerals, comprising the major groups of kaolinite, montmorillo-nite-smectite, illite and chlorite. Among them, montmorillonite has been most extensively investigated.72-75 There are likewise numerous positively charged minerals such as the double-layer hydroxide (DLH), e.g., green rust, or Fe(II)2Fe(III)(OH)6. Both types of minerals could be widespread in occurrence and play important roles in prebiotic chemistry through catalysis of chemical transformations and concentration of biomolecules from dilute solutions.

350-400°C

f M

\ Ocean Floor

400°C

/ \ h|/ \

/

\

/

\

/

N.

Figure 9.4. Seafloor hydrothermal vent. After Van Dover.79

Thus DLH could bring about the formation and concentration of tetrose phosphate and pentose phosphate for the production of TNA and RNA.5,75 Montmorillonite catalyses the formation of RNA oligomers up to 50 bases long and introduces sequence-, regio- and homochiral selectivities into the oligomer products, yielding up to 13% ofan isomer where only 0.2% might be expected on a random basis, thereby accelerating the appearance of specific oligomers.76 Pre-vesicular prebiotic systems might also be adsorbed on mineral surfaces58 or enclosed in mineral micro-chambers57 to prevent excessive dilution of reactants and impart individuality to the system to make possible natural selection.

(iii) Hydrothermal and Impact Sites

The first hydrothermal vents were discovered along the Galapagos Rift in explorations induced by detection of ocean temperature anomalies.77,78 These vents are fissures on the Earth's crust often in areas of volcanic activities where tectonic plate movements cause an upwelling of the magma, as in the case of the Mid Atlantic Ridge and East Pacific Rise. At these sites sea water is drawn into the hydrothermal system, and heated by the volcano edifice up to as high as 400°C (Fig. 9.4). When the super-heated water runs into chilly surrounding water, minerals precipitate out to build up chimney structures at a rate as high as 30 cm per day, towering as tall as 60 m. Chimneys that emit a cloud of black material rich in sulphur are called black smokers. White smokers in comparison are lower in temperature and emit lighter color materials rich in barium, calcium and silicon. The vents and comparable deep subterranean hot aquifers support the abiotic syntheses ofa range ofbiomolecules (Section 1.4).

Besides the submarine hydrothermal vents, sites close to impacts of meteorites would benefit from organic compounds brought by the meteorites (Table 6.2) as well as phosphate-yielding schreibersite. Crater formation by asteroids and comets also could result in hydrothermal systems. The shock waves created by the impact could generate amino acids and other organic compounds and the impacting bodies could enrich their surroundings with compounds from space.80

(iv) Icy Pools

Water-ice mixtures facilitate template-directed RNA polymerization, which depends on base pairing and base stacking that are favored by low temperatures. In the presence of Mg(II)/Pb(II) mixtures at slightly below the freezing point, up to 90% quasi-equimolar incorporation ofall RNA monomers into 5 to 17-mers was observed with traces of longer products (Section 10.4). Given the pivotal importance of RNA or RNA-like replicators, water-ice mixtures could be attractive sites.81 Such mixtures would also accelerate purine synthesis.23,24

(v) Composite Reactors

Surface water, clays, minerals, hydrothermal systems and icy pools all have important advantages to offer. So where might the fastest reactor be found? Speed was important, for as Darwin pointed out, life forms that arrived late would be lunch for those who arrived early. Given a heterotrophic origin and the need sooner or later for RNA replication, access to multiple nutrient sources together with optimal conditions for RNA polymerization rank high among the factors enabling a speedy reactor.

These considerations favor the proposal ofa composite reactor in a landscape where multiple nutrient sources converged to optimize prebiotic development.5 Figure 9.5 illustrates a reactor ofthis kind at

a tidal pool in a volcanic sub-polar region, not too cold to be totally frozen but cold enough to have continual or intermittent presence of water-ice mixtures, with rain water mixing with sea water to result in a gradient of ionic compositions suitable for formose reaction and adequate solubility ofphosphates, adorned with facilitator clays and minerals and next to hot springs, fumaroles, asteroid and meteorite craters and possibly even near-shore submarine hydrothermal vents. Organic compounds and condensed phosphates from the different sources would be fed into such a Fire And Ice Reactor (FAIR) via streams, seepage and tide to engage in prebiotic evolution. To-day, volcanic icy landscapes of this description may be found in the northern arc of the Pacific Ring of Fire such as Kamchatka Peninsula, Cook Inlet in Alaska, or in the Northern Atlantic such as Reykjavik (or 'Bay of Smokes' on account of its many hot springs). On primitive Earth, when volcanism was prevalent, FAIRs would be commonplace. In countless FAIRs as well as a multitude of other varieties of composite and noncomposite reactors, prelife assemblages would make continual attempts to surmount the barely surmountable hurdles they faced. It was a life or no-life primordial bingo that eventually accomplished at one of these reactors a success story that is still reverberating to-day across the galaxies.

References

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2. Deamer DW, Fleischaker GR. Origins of life. The central concepts. Jones and Bartlett, 1994.

3. Hazen RM. Genesis. The scientific quest for life's origin. Joseph Henry Press, Washington DC, 2005.

4. Jortner J. Conditions for the emergence of life on the Earth: summary and reflections. Phil Trans R Soc B 2006; 361:1877-1891.

5. Mojzsis SJ, Krishnamurthy R, Arrhenius G. Before RNA and after: geophysical and geochemical constraints on molecular evolution. In: Gesteland RF, Cech TR, Atkins JF, eds. The RNA World, 2nd ed. Cold Sprong Harbor Laboratory Press 1999:1-47.

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