deserve attention in an evolutionary context are those that remain constant on the
evolutionary relevant, geological timescale. This consideration discounts the evolutionary
importance of occasional energy inputs such as impact bombardment,
atmospheric electric discharges, and shock waves. The primordial atmosphere on
Earth is assumed to be dominated by carbon dioxide [25,93-100]. Hence, energy
was initially needed to reduce CO2 to organic compounds that could participate
in prebiological syntheses [34]. Currently, the fixation of CO2 by living organisms
is supported by two energy fluxes: the communities at the Earth’s surface
depend, via photosynthesis and its products, on solar light [101], whereas the
biotopes at the sea floor can also exploit the redox potential difference between
the reduced hydrothermal fluids and oxygenated ocean waters [102]. Accordingly,
some scholars have considered solar radiation to be the driving force of abiogenesis
[5,85,103-112]. Others have hypothesized that chemical or redox disequilibria
at the sea-floor hydrothermal vents [113-123] or at the surface of sea-floor
iron minerals [124-130] could have driven the emergence of the first organisms.
As argued in more detail elsewhere [85], a direct analogy between primordial life
and modern deep-sea biotopes is not possible, since the redox energy span of > 1
eV between the reduced compounds of hydrothermal fluids and the sea-dissolved
oxygen became exploitable only after the ocean waters – only 2 Ga ago – were saturated
by oxygen, a by-product of cyanobacterial photosynthesis [101,131,132].
deserve attention in an evolutionary context are those that remain constant on theevolutionary relevant, geological timescale. This consideration discounts the evolutionaryimportance of occasional energy inputs such as impact bombardment,atmospheric electric discharges, and shock waves. The primordial atmosphere onEarth is assumed to be dominated by carbon dioxide [25,93-100]. Hence, energywas initially needed to reduce CO2 to organic compounds that could participatein prebiological syntheses [34]. Currently, the fixation of CO2 by living organismsis supported by two energy fluxes: the communities at the Earth’s surfacedepend, via photosynthesis and its products, on solar light [101], whereas thebiotopes at the sea floor can also exploit the redox potential difference betweenthe reduced hydrothermal fluids and oxygenated ocean waters [102]. Accordingly,some scholars have considered solar radiation to be the driving force of abiogenesis[5,85,103-112]. Others have hypothesized that chemical or redox disequilibriaat the sea-floor hydrothermal vents [113-123] or at the surface of sea-flooriron minerals [124-130] could have driven the emergence of the first organisms.As argued in more detail elsewhere [85], a direct analogy between primordial lifeand modern deep-sea biotopes is not possible, since the redox energy span of > 1eV between the reduced compounds of hydrothermal fluids and the sea-dissolvedoxygen became exploitable only after the ocean waters – only 2 Ga ago – were saturatedby oxygen, a by-product of cyanobacterial photosynthesis [101,131,132].
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