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A10. Hydrothermal Vents or Primoridal Soup?

The case for the origin of life in deep sea hydrothermal vents and not in a primordial "Campbell's" soup has been argued convincingly by Lane et al (2010).   What's needed for life are reasonably complex molecules and an energy source to drive unfavorable reactions.  It's the latter on which that Lane et al focus. In an early primordial world that was low in oxygen, exergonic oxidation reaction of organic molecules would provide little energy.  This can be surmised from the low energy yield (compared to aerobic respiration)  achieved in present day glycolytic (fermentative) pathways from all major domains of life, archaea, bacteria, and eukaryotes.   

Background:  Based on rRNA sequences, a primordial cell evolved into two different types of cells, one that became bacteria, and another that split further into archaea (single cells, similar to bacteria) and eukaryotic cells (complex cells with internal organelles that eventually formed multicellular organisms).  Bacterial and archaea are collected called prokaroytic cells.

In addition, these pathways required the evolution of up to dozen different enzymes to produce their relatively meager energy yield which ultimately depends on the oxidation of an organic molecule by another organic molecule instead of by a powerful oxidant like dioxygen.  An anisotropic arrangement of molecules in a concentrated soup could lead to transient chemical potential fluctuations but these would be inefficient and impermanent sources of energy.  Effectively the primordial soup would be at equilibrium and hardly expected to provide the energy for synthesis of RNA enzymes and replicators.  UV light leads to photo-damage and photolysis not replication of complex molecules.  What is needed is a way to drive the synthesis of molecule with high chemical potential energy (like sulfur esters and phosphoanhydrides) compared to their lytic products.  These could then provide an energy sources to drive ATP synthesis, for example. 

A detailed look a the bioworld shows that the earliest organisms used energy from the collapse of the proton gradient (chemisomotic principle elucidated by Peter Mitchell).  All present autotrophs (organisms that can fix CO2 and form complex organic molecules) and many heterotrophs (use complex organic molecules of other organisms for fuel) use redox complexes in membranes coupled to membrane gradients.   These complexes would take reduced molecules and pass electrons from them to oxidizing agents (electron acceptors), including O2, CO2, and Fe3+ to form H2O, CH4, and Fe2+.  Fermentors also use ATPase membrane enzymes to transport nutrients.  Yet genomic analysis of bacteria and archaea show that enzymes involved in fermentation differ significantly, suggesting that they evolved separately towards a convergent function.  Structure in common include DNA, RNA, ribosomes and membrane ATPases, which Lane et al suggest were in a the Last Universal Common Cell (LUCA). 

All autotrophs produce their energy source by fixing CO2 using either H2 directly or indirectly using H2O and H2S.  All of the are available in nonhydrothermal deep sea vents. Volcanic vents, however, are extremely hot (not optimal for organic molecule synthesis), very acidic, and lack hydrogen gas.  A different type of nonvolcanic vent, an alkaline hydrothermal one, might produce more conducive as a site of the origin of life.  In these vents, water chemically reacts with minerals in the crust (such as olivine) leading to their hydroxylation and subsequent fracture, with promotes more water entry into the crust.  It has been reported that there is more water found as hydroxylated minerals in the crust, that there is liquid water in the oceans.  These processes result in temperatures up to 200 degrees Celsius and release of hydrogen gas into a moderately alkaline vents into the sea water at temperatures more conducive (70 degrees C)  to the origin of life.  

Figure: Alkaline Vent

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