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Functionally, there are 2 requirements for life: 1) elements requiring modest energy to cohere reactive substrates that can energetically perpetuate for metabolism, and 2) readily reproducible compounds for replication.
As the organic building blocks of life are readily built in space, it seems inevitable life would revolve around nucleobases and amino acids, as they demonstrate flexible complexity at a low energy cost.
The crust under the ocean is a rich reservoir of all possible ingredients for life, including rare earth elements. There exists the most extensive, concentrated library of chemical possibilities.
Hydrogen, a randy reductant, is a rich potential energy source. Hydrogen-consuming bacteria are known, though now largely confined to deep-sea hydrothermal vents where H2 is still readily available.
Thermal vents on the ocean floor and subsea are places where life originating would have been most amenable. The structural similarity between the minerals precipitated at hydrothermal vents and the most ancient enzymes shows that only a petite push of coherence would be sufficient for life to take hold.
Geologic recycling of the Earth’s crust leave scant evidence of atmospheric conditions during the first 650 million years of the planet’s history, particularly regarding which form of carbon predominated early on: CO (carbon monoxide), CO2 (carbon dioxide), or CH4 (methane). By 4.1 bya, when life originated on Earth, CO2 was the predominant atmospheric gas.
Whereas reactions in carbon dioxide are low-yield and of limited variety, reactions in methane yield abundant diverse organic compounds. This difference in energy potential was a significant factor in where early life formed.
The presumed requisite conditions for the emergence of life include water and a sufficiently stable environment. Those conditions would have been largely met near Earth’s surface by aerosols, volcanic and interplanetary dust particles, and organic films; and at depth by hydrothermal minerals, chemical precipitates, and vesicular structures of mineral and organic compounds.
Likewise, the necessary energy sources for organic compound production were available via sunlight on the surface, or in the ocean or underneath it by chemical disequilibria between hydrothermal fluids and seawater.
The reducing power of oceanic, dissolved iron was available in both environments. Iron plays a significant role in cellular respiration.
