『Abstract
Not only have submarine hydrothermal systems been responsible
for a variety of mineral deposits, they may also have contributed
to the emergence of life in the Hadean. Sulfide deposits can be
precipitated where metal-bearing hydrothermal solutions invade
bacteriogenic H2S-bearing wet sediments and
the overlying seawater or brine. Similarly, life might be viewed
as a complex organic product that emerged when and where hydrothermal
H2 and other reduced chemical species reacted
with CO2 and other mildly oxidized molecules
dissolved in the Hadean ocean. Semipermeable and semiconducting
FeS barriers or membranes would have precipitated spontaneously
where H2 and HS--bearing alkaline
waters at ≦110℃ seeped into the cool mildly acidic Fe-bearing
Hadean ocean at a submarine hydrothermal mound on a ridge flank
or on the deep ocean floor. The mound, consisting of Mg-rich clays,
ephemeral carbonates, green rust, as well as the sulfides, acted
as a natural, self-restoring hydrothermal reactor. In particular,
the sulfide barriers, composed of mackinawite and greigite, prevented
the immediate titration of the two fluids and controlled their
interactions. Metal sulfides were, and in tiny amounts are still,
vital to all cells. Among other reactions they help catalyze the
reduction of CO2 in autotrophic bacteria,
including photosynthetic organisms. And the structure of greigite
(NiFe5S8) is remarkably
similar to the active sites (e.g., NiFe4S5) of the enzyme prompting the early metabolic
pathway that generates acetate (CH3COO-)
and H2O from CO2, H2, and a methyl group (-CH3).
So clusters of greigite, sequestered in a simple organic envelope,
could have acted as a protoenzyme, catalyzing the synthesis of
acetate in the hydrothermal mound in the same way.
Although, like “spent” ore fluid, most of the acetate and all
of the water would have been lost to the Hadean ocean, an acetate
fraction retained in microcavities within the mound could have
combined to form the simple organic building blocks of life. Hydrothermal
ammonia and minor cyanide also would have contributed to the synthesis
of amino and nucleic acids. Traces of phosphorylated organic molecules,
such as RNA (ribonucleic acid), would have adhered to mineral
surfaces in the membranous barriers. Once their phosphates were
bonded to such a surface, short RNA strands could have polymerized
and provided a crude code for the assembly of variable sequences
of amino acids (incipient proteins) generated in the same milieu.
Alternatively, they could have replicated further RNA. Amino-acid
sequences were a significant component of the first membranes
and would have influenced membrane and cell survival. Once RNA
codes for successful amino-acid sequences were passed on to daughter
cells then life could be said to have emerged and evolution to
have begun. Bacteria have flourished around hot springs ever since
and on occasion have been responsible for the deposition of giant
base metal sulfide deposits at or below the sea floor.
Hence the study of life may be best begun by the study of those physico-chemical phenomena which result from the contact of two different liquids.
Stephane(最初のeの頭に´) Leduc, 1911, p.xiv.』
Introduction
The early Earth
Two classes of submarine hot springs
High-temperature systems (≧350℃) in mafic and ultramafic
hosts
Low-temperature systems (85゜-115℃) in ultramafic hosts
Ocean water, the hot spring sink
The alkaline hydrothermal mound - A natural flow reactor
Formation of the first compartments
The onset of life
Organic syntheses
The organic takeover
From catalysts to protoenzymes
Polymerization
Organic takeover of the membrane
Crude coding of the peptides
Evolutionary assays in the hydrothermal mound
The beginning of Darwinian evolution
Life away from the mound
Tests and expectations
Conclusions
References