User:MonroeShindelar/Primordial Soup

“Primordial Soup” is a term introduced by Soviet biologist Alexander Oparin and English scientist John Burdon Sanderson Haldane. In 1924, they proposed a theory for the origin of life through transformation, during the gradual chemical evolution of molecules that contain carbon in the primordial soup.The Primordial Soup hypothesis explains that the earliest form of life began in the oceans when the earth was in its earliest stages and the atmosphere was reducing, and consisting mainly of methane. hydrogen, water vapor, and ammonia. Oparin and Haldane both theorized that the electrical energy, coming in the form of lightning strikes and UV radiation, coupled with the combination of gases in the atmosphere could spark spontaneous formation of amino acids, the building blocks for proteins, in the oceans of an early forming earth. They theorized that the amino acids would form an oily film that would float on top of the oceans, which would be made up of self replicating molecules that would eventually develop into the original cell.

History
Primordial Soup was first hypothesized by Alexander Oparin. He used his own research on coacervates in order to explain how the oceans on primeval earth could have separated into layers. Oparin's original hypothesis was that it may have been possible for the primitive oceans to separate into different coacervates, once of which could have led to the generation of life. Alexander Oparin's hypothesis was not translated into english until 1936, however a similar theory was proposed in Europe by John Burdon Sanderson Haldane in 1929, in an article published in Rationalist Annual called “The origin of life”. Haldane is credited for introducing the modern model of the chemical origin of life. In the article, he detailed the process of organic compounds forming in a “hot dilute soup” under an atmosphere that was completely devoid of oxygen, but which contained carbon dioxide, ammonia, and water. He suggested that the radiation from the sun as well as electrical energy from lighting may have been responsible for a chemical reaction to occur, where a thin oily layer would form over the oceans of primeval earth that contained organic compounds, or monomers.

A Reducing Atmosphere
The Primordial Soup hypothesis is entirely dependent on the atmosphere of early earth being reducing. For Primordial Soup to be accurate, the composition of the atmosphere would have to be completely devoid of oxygen or any oxidizing gases or vapors. The presence of oxygen in earth's early atmosphere would have prevented the formation of any organic molecules. It was previously debated whether or not early earth contained any oxygen, but the current consensus is that oxygen or any oxidizing gases or vapors were not present until the Great Oxygenation event, which is where cyanobacteria began introducing oxygen into the atmosphere as a byproduct of photosynthesis.

Monomer Formation
The most vital piece of research came in 1953 when graduate student Stanley Miller and his professor Harold Urey performed an experiment that attempted to demonstrate how organic molecules could have formed from inorganic precursors. The Miller-Urey experiment took a sealed mixture of water, ammonia, methane, and hydrogen and heated it to evaporate the water to make water vapor. This mixture was a simulation of the atmosphere that would have existed on early earth. Electricity was passed through the mixture in order to simulate the lighting. Miller and Urey analyzed the mixture after a week and found that amino acids were present in the mixture. This experiment was the first direct evidence to support Oparin and Haldane's Primordial Soup hypothesis, as it proved that it was possible for monomers to form in a reducing atmosphere. Another groundbreaking experiment was conducted by Joan Oro, a biochemist From Catalonia. Oros discovered through the synthesis of nucleobase adenine from hydrogen cyanide, that amino acids can be formed by adding ammonia to hydrogen cyanide (HCN) in an aqueous solution.

Criticisms
The Miller-Urey experiment shows that it could have indeed been possible for amino acids to be formed in the oceans of the earth in the way that was originally proposed by Oparin and Haldane in ideal conditions, but critics of the theory, like evolutionary biologist William Martin, senior author of “On the Origin of Biochemistry at an Alkaline Hydrothermal Vent” in BioLife, thinks that the experiment does not provide any driving force to sustain any kind of reaction necessary to create life, saying "Despite bioenergetic and thermodynamic failings the 80-year-old concept of primordial soup remains central to mainstream thinking on the origin of life, But soup has no capacity for producing the energy vital for life." [Citation]. Many other scientists behind the theory that life started with the help of hydro thermal vents at the bottom of the ocean floor provide a similar argument, that the energy required by the Miller-Urey experiment required a constant source of energy that would not likely be present during the early stages of earth.The Miller-Urey experiment also produced compounds that would have prohibited the formation of any polymers in high concentrations. It produced many compounds that would have undergone a cross reaction with the amino acids that were already produced or terminate the peptide chain. Another criticism with the theory is that it is possible that the atmosphere may not have contained as much methane and ammonia as previously thought, and that may inhibit the creation of amino acids through chemical reaction. In the paper "Earths Earliest Atmosphere", Kevin Zanhle, Laura Schaefer, and bruce Fegley discuss the composition of the earths earyl atmosphere, saying “However, photochemical studies showed that any methane or ammonia in the atmosphere would quickly be destroyed. Meanwhile geologically based arguments, which treat the atmosphere as outgassed from the solid Earth, were taken as strongly suggesting that Earth’s original atmosphere was composed mostly of H2O, CO2, and N2, with only small amounts of CO and H2, and essentially no CH4 or NH3...Miller-Urey-type experiments performed in the more oxidized mixtures of modern volcanic gases generate relatively little of prebiotic interest, especially when CO2 is abundan. New work suggests that spark yields of ammonia, HCN, and amino acids in CO2-N2-water mixtures can be less disappointing if the water is allowed to become acidi. Nevertheless, the contrast between methane and ammonia on the one hand and carbon dioxide and dinitrogen on the other led many prebiotic chemists, Miller and Urey prominent among them, to regard the presence of life on Earth as providing a strong boundary condition on the nature of Earth’s early atmosphere."