ORIGIN OF LIFE:
Life as We Don't Know
Theories of the origin of life on Earth fall into two general categories.
Prebiotic broth theories postulate a protracted origin by the self-assembly
of high-molecular weight structures, such as RNA, proteins, and vesicles,
in a cold prebiotic broth of preaccumulated modules (1).
More recently, theories based on a hydrothermal origin have gained ground.
For example, the theory of a pressurized iron-sulfur world (2)
suggests a fast origin by an autotrophic metabolism of low-molecular weight
constituents, in an environment of iron sulfide and hot magmatic exhalations.
Cody et al.'s (3) results on page 1337
of this issue provide key support for the latter theory and greatly strengthen
the hope that it may one day be possible to understand and reconstruct
the beginnings of life on Earth.
Pyruvic acid, CH3-CO-COOH, is one of the most crucial
constituents of extant intermediary metabolism. It occurs in numerous metabolic
pathways, notably the reductive citric acid cycle and the pathways that
produce amino acids and sugars. It has been suggested that pyruvic acid
or its anion pyruvate formed primordially by double carbonylation (4
). Cody et al. provide experimental support for this suggestion.
They show that pyruvic acid forms from formic acid in the presence of nonylmercaptane
and iron sulfide at 250°C and 200 MPa. Water is initially absent and
forms only by the dehydration of the formic acid. This result poses fascinating
thermodynamic and kinetic questions. Pyruvic acid is an extremely heat-sensitive
compound that decomposes at its boiling point of 165°C. It appears
paradoxical that at the very high temperature required for dehydration
of formic acid, the relatively unstable pyruvic acid can form and exist
at detectable concentrations. Moreover, it is astonishing that acetic acid
is formed at a lower yield than pyruvic acid. The explanation may well
lie in the very high pressure.
The work is particularly exciting because experience with organic
synthesis in the high-pressure/high-temperature regime is very limited.
The experiments require a combination of 200 MPa (corresponding to a rock
depth of about 7 km or a 20-km water column) and 250°C, in addition
to high CO pressure in the absence of water. It remains to be established
whether such conditions are geophysically possible.
The new finding, if it holds, fills a critical gap in the experimental
picture of the iron-sulfur world (see the figure). All individual reaction
steps for a conversion of carbon monoxide 1 to peptides 8 have now been
demonstrated: formation of methyl thioacetate 4 (4),
of pyruvate 6 (1), of alanine 9 by reductive amination
of pyruvate 6 ( 5), and of peptides 8 by activation
of amino acids with CO/H2S (6). The challenge
will now be to overcome the discrepancies in the reaction conditions and
to establish the right conditions for autocatalysis (reproduction) and
evolution. This may involve a primitive version of the citrate cycle in
which (methyl) thioacetate and pyruvate participate (3,
) and/or ligand (notably peptide) feed back to the catalytic metal center
Reactions in the iron-sulfur world. Reaction conditions are given
in the table below. The dotted arrow represents ligand feedback.
|CONDITIONS FOR REACTIONS IN THE FIGURE
The reaction scheme in the figure is in substantial agreement with
extant metabolism in terms of overall metabolic patterns, reaction pathways,
and catalysts. The newly demonstrated formation of pyruvic acid by double
carbonylation, however, has no analog in extant metabolism. It may have
disappeared because of metabolic takeover, first by a reverse pyruvate-formate-lyase
reaction and later, after the advent of thiamine pyrophosphate, by carboxylation
with pyruvate oxidoreductase.
Cody et al.'s results support the view that the primordial
organisms were autotrophs feeding on carbon monoxide. But more importantly,
the reactions shown in the figure can still occur today because the required
conditions are in general still available on Earth, albeit at a lesser
frequency. They may thus be a source for geoorganics today; these geoorganics
may serve as food for extant heterotrophs, and primitive microbes feeding
on CO might still be tracked down in hot pressurized spaces previously
inaccessible to exploration.
The reaction conditions chosen by Cody et al. are a compromise
between the requirements of geochemical modeling and the requirements of
the experimental technique. CO gas cannot be used at these very high pressures
without extreme danger. Decomposition of formic acid was therefore used
as a source for CO. This requires a temperature of 250°C and the absence
of water. But in the real world, the temperature may well have been lower,
as may have been the pressure. On early Earth, outgassing (the release
of gases by volcanic activity) must have been massive and omnipresent,
with a wide spectrum of physical conditions, which only later became restricted
to vents and volcanoes because of a thickening crust.
It is occasionally suggested that experiments within the iron-sulfur
world theory demonstrate merely yet another source of organics for the
prebiotic broth. This is a misconception. The new finding drives this point
home. Pyruvate is too unstable to ever be considered as a slowly accumulating
component in a prebiotic broth. The prebiotic broth theory and the iron-sulfur
world theory are incompatible. The prebiotic broth experiments are parallel
experiments that are producing a greater and greater medley of potential
broth ingredients. Therefore, the maxim of the prebiotic broth theory is
"order out of chaos." In contrast, the iron-sulfur world experiments are
serial, aimed at long reaction cascades and catalytic feedback (metabolism)
from the start. The maxim of the iron-sulfur world theory should therefore
be "order out of order out of order."
I. Fry, The Emergence of Life on Earth (Rutgers Univ. Press,
New Brunswick, NJ, 2000) [publisher's
G. Wächtershäuser, Prog. Biophys. Mol. Biol.
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G. Wächtershäuser, Proc. Natl. Acad. Sci . U.S.A.87,
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W. Heinen and A. M. Lauwers, Origins Life Evol. Biosphere 26,
The author is at Tal 29, 80331 Munich, Germany. E-mail:
289, Number 5483, Issue of 25 Aug 2000, pp. 1307-1308.
© 2000 by The American Association for the Advancement of Science.