"Primordial soup" is a term introduced by the Soviet biologist Alexander Oparin. In 1924, he proposed a theory of the origin of life on Earth through the transformation, during the gradual chemical evolution of particles that contain carbon in the primordial soup.
Biochemist Robert Shapiro has summarized the "primordial soup" theory of Oparin and Haldane in its "mature form" as follows:
- Early Earth had a chemically reducing atmosphere.
- This atmosphere, exposed to energy in various forms, produced simple organic compounds ("monomers").
- These compounds accumulated in a "soup", which may have been concentrated at various locations (shorelines, oceanic vents etc.).
- By further transformation, more complex organic polymers – and ultimately life – developed in the soup.
A reducing atmosphere
Whether the mixture of gases used in the Miller–Urey experiment truly reflects the atmospheric content of early Earth is controversial. Other less reducing gases produce a lower yield and variety. It was once thought that appreciable amounts of molecular oxygen were present in the prebiotic atmosphere, which would have essentially prevented the formation of organic molecules; however, the current scientific consensus is that such was not the case.
The first atmosphere would have consisted of gases in the solar nebula, primarily hydrogen. In addition, there would probably have been simple hydrides such as those now found in the gas giants (Jupiter and Saturn), notably water vapor, methane and ammonia. As the solar nebula dissipated, these gases would have escaped, partly driven off by the solar wind. (We know that Venus lost its hydrogen to space because its deuterium/protium (D/H) ratio is very high but Earths deuterium/protium (D/H) ratio is 100 times smaller than at Venus).
Outgassing from volcanism, supplemented by gases produced during the late heavy bombardment of Earth by huge asteroids, produced the next atmosphere, consisting largely of nitrogen plus carbon dioxide and inert gases. A major part of carbon-dioxide emissions soon dissolved in water and built up carbonate sediments.
Oxygen would have been produced in the interior of the Earth by the "smelting" of iron oxide during the iron catastrophe. But it is unclear how much, if any, of the oxygen that was produced would have reached the surface.
One of the most important pieces of experimental support for the "soup" theory came in 1953. A graduate student, Stanley Miller, and his professor, Harold Urey, performed an experiment that demonstrated how organic molecules could have spontaneously formed from inorganic precursors, under conditions like those posited by the Oparin-Haldane Hypothesis. The now-famous "Miller–Urey experiment" used a highly reduced mixture of gases—methane, ammonia and hydrogen—to form basic organic monomers, such as amino acids. This provided direct experimental support for the second point of the "soup" theory, and it is around the remaining two points of the theory that much of the debate now center
Apart from the Miller–Urey experiment, the next most important step in research on prebiotic organic synthesis was the demonstration by Joan Oró that the nucleic acid purine base, adenine, was formed by heating aqueous ammonium cyanide solutions. In support of abiogenesis in eutectic ice, more recent work demonstrated the formation of s-triazines (alternative nucleobases), pyrimidines (including cytosine and uracil), and adenine from urea solutions subjected to freeze-thaw cycles under a reductive atmosphere (with spark discharges as an energy source).
The spontaneous formation of complex polymers from abiotically generated monomers under the conditions posited by the "soup" theory is not at all a straightforward process. Besides the necessary basic organic monomers, compounds that would have prohibited the formation of polymers were formed in high concentration during the Miller–Urey and Oró experiments. The Miller experiment, for example, produces many substances that would undergo cross-reactions with the amino acids or terminate the peptide chain.
However, polymerization would be unnecessary if the first self-reproducing structure was a liquid Crystal instead.
- ↑ Shapiro, Robert (1987). Origins: A Skeptic's Guide to the Creation of Life on Earth. Bantam Books. p. 110. ISBN 0-671-45939-2.
- ↑ 2.0 2.1 Zahnle, K.; Schaefer, L.; Fegley, B. (2010). "Earth's Earliest Atmospheres". Cold Spring Harbor Perspectives in Biology 2 (10): a004895. doi:10.1101/cshperspect.a004895.
- ↑ Miller, Stanley L. (1953). "A Production of Amino Acids Under Possible Primitive Earth Conditions". Science 117 (3046): 528–9.
- ↑ Oró, J. (1961). "Mechanism of synthesis of adenine from hydrogen cyanide under possible primitive Earth conditions". Nature 191 (4794): 1193–4.
- ↑ "Synthesis of pyrimidines and triazines in ice: implications for the prebiotic chemistry of nucleobases". Chemistry 15 (17): 4411–8. 2007.
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