THE PRIMEVAL SOUP THEORY

Alexander Oparin speculated that the atmosphere of the early Earth may have been “chemically reducing” in nature, composed primarily of methane, ammonia, water, hydrogen sulfide, carbon dioxide or monoxide, and phosphate, with molecular oxygen and ozone either rare or completely absent. In such a reducing atmosphere, electrical activity like lightning (or possibly impact shocks or ultraviolet light) could catalyze the creation of certain basic small molecules or monomers such as amino acids and other simple organic compounds.

These compounds then accumulated in a “primeval soup” and could become chemically bonded to other monomers to form more complex organic polymers, such as the long, chain-like molecules (such as proteins and nucleic acids) which are essential for building living creatures. In order for cells to accomplish self-replication, the cooperative action of both proteins and nucleic acids is required, and the complex information detailing the specific structure of the proteins inside living things is stored in nucleic acids like RNA and DNA.

(Click for a larger version) Schematic of the Miller-Urey experiment of 1953 (Source: Wikipedia: http://en.wikipedia.org/ wiki/Miller-urey_experiment)

In an attempt to prove Oparin's hypothesis, the famous Miller-Urey experiments of Stanley Miller and his professor Harold Urey at the University of Chicago in 1953 demonstrated the feasibility of producing basic organic monomers such as amino acids in conditions which attempted to simulate the conditions believed to have prevailed on the primeval Earth. This involved the simulation of an atmosphere consisting of a highly reduced mixture of gases (methane, ammonia and hydrogen), the presence of pools of liquid water and sporadic sources of energy (with electricity simulating lightning storms).

In 1961, John Oró showed that the nucleic acid purine base, adenine (a chemical component of DNA and RNA), could be formed by heating aqueous ammonium cyanide solutions under conditions which may have been similar to those of primitive Earth. In the 1950s and 1960s, the American biochemist Sidney Fox demonstrated that amino acids could spontaneously form small peptide structures under conditions that might plausibly have existed early in Earth's history, and that these amino acids and small peptides could also be encouraged to form closed spherical membranes called "microspheres", which are similar to primitive cells.

Many commentators have argued that the spontaneous development of life runs counter to the Second Law of Thermodynamics, which rules that entropy and disorder inexorably increases over time and that order and organization always declines. The work of the Russian-born Belgian chemist Ilya Prigogine in the 1960s and 1970s, however, showed that many systems spontaneously organize themselves if they are forced away from thermodynamic equilibrium (such as by radiation or other unknown influences). Besides, the law specifically applies to isolated systems, whereas living systems are necessarily open and interactive systems. There are many other examples of apparent spontaneous increases in order (such as the growth of crystals from featureless liquids, the development of large scale structures in the universe, etc), but the growth of order in one place is always at the price of entropy generated elsewhere (such as the production of heat or other types of radiation).

Encouraging though these early experiments may perhaps be, critics point out that the Miller-Urey experiments were far from conclusive, especially as they also resulted in several other substances that might well cross-react with the amino acids (and potentially terminate the peptide chain). Neither did it explain how the relatively simple organic building blocks went on to polymerize and form more complex structures, interacting in consistent ways to form a protocell.