Structural Biochemistry/The Oparin-Haldane Hypothesis

The Oparin-Haldane Hypothesis

The Oparin-Haldane hypothesis is a hypothesis independently developed by two scientists: Russian chemist A.I. Oparin and British scientist John Haldane.

Both independently suggested that if the primitive atmosphere was reducing (as opposed to oxygen-rich), and if there was an appropriate supply of energy, such as lightning or ultraviolet light, high temp of around 1073 K then a wide range of organic compounds might be synthesized.

Oparin came up with the hypothesis in 1924 that Earth’s atmosphere was extremely reducing in its early stages of development. This means that the atmosphere had an excess of negative charge and could cause reducing reactions by adding electrons to compounds. Oparin suggested that these organic compounds could have undergone a series of reactions leading to more and more complex molecules. Under these circumstances, Oparin hypothesized that organic molecules could have formed from simple inorganic molecules.He proposed that the molecules formed colloid aggregates, or 'coacervates', in an aqueous environment. These coacervates were able to absorb and assimilate organic compounds from the environment in a way reminiscent of metabolism. They would have taken part in evolutionary processes, eventually leading to the first lifeforms. Similarly, in 1929, before Haldane read about Oparin’s theory of a reducing atmosphere, Haldane also hypothesized that the early stages of Earth’s atmosphere was reducing, which could catalyze reactions that would form more complicated organic molecules from simpler molecules. Haldane hypothesized that the oceans served as a huge cooking pot where powered by the sun or lightning, chemical reactions could occur in an aqueous environment to form a huge diversity of organic compounds. Haldane proposed that the primordial sea served as a vast chemical laboratory powered by solar energy. The atmosphere was oxygen free, and the combination of carbon dioxide, ammonia and ultraviolet radiation gave rise to a host of organic compounds. The sea became a 'hot dilute soup' containing large populations of organic monomers and polymers. Haldane envisaged that groups of monomers and polymers acquired lipid membranes, and that further developments eventually led to the first living cells.

Haldane coined the term 'prebiotic soup' or 'prebiotic atmosphere' that consisted of an abundance of methane, ammonia, and water. This term became a powerful symbol of the Oparin-Haldane view of the origin of life. Miller-Urey Experiment

In 1953, two scientists set out to test Oparin and Haldane's hypothesis. Harold Urey and his student Stanley Miller tried to calculate the chemical constituents of the atmosphere of the early Earth. They based their calculations on the view that the early atmosphere was reducing. In order to do this, they simulated early earth atmospheric conditions by creating a closed system which contained water, methane gas, ammonia, and hydrogen gas in 2:1:2 ratio along with water vapours and along with it kept a high temperature of 1073 K.Urey suggested that his student, Miller should attempt to synthesize organic compounds in this type of atmosphere.

Miller carried out an experiment in which he passed a continuous spark discharge at 75,000 volts through a flask containing the gases identified by Urey along with water. Furthermore, this electrical current was run through the laboratory set up to simulate the catalytic source of lightning that was present in the early atmosphere.

Miller found that after 18 days of circulating the mixture, most of the ammonia and much of the methane had been consumed. The main gaseous products were carbon monoxide (CO) and nitrogen (N2). In addition, there was an accumulation of dark material in the water. Few of the specific constituents of this could not be identified, but it was clear that the material included a large range of organic polymers. From the results of their experiment, they found that up to 15% of the carbon in the system was inorganic compounds that had formed in the system. This conclusion proved that organic molecules could be formed from inorganic molecules in Earth’s early atmosphere. In addition, out of the organic molecules produced, Miller and Urey showed that some of the organic compounds were amino acids, which are necessary for living organisms.

Analysis of the aqueous solution showed that the following had also been synthesized:

1. 25 amino acids (the main ones being glycine, alanine and aspartic acid)

2. Several fatty acids

3. Hydroxy acids

4. Amide products.

Aftermath of the Miller-Urey Experiment

The Miller-Urey experiment was immediately recognized as an important breakthrough in the study of the origin of life. It was received as confirmation of the Oparin-Haldane hypothesis in that several of the key molecules of life could have been synthesised on the primitive Earth in the kind of conditions envisioned by Oparin and Haldane. These molecules would then have been able to take part in prebiotic chemical processes, leading to the origin of life.

Other similar experiments have been done to mimic early Earth conditions in an attempt to find other ways organic molecules could have formed. One experiment was done to mimic deep underwater volcano conditions. At these underwater volcanoes, catalytic heat as well as many minerals were constantly supplied. This provided an ideal system for organic molecules to be formed. This system was also found to produce amino acids, which is essential for living organisms to process into proteins.

Since the Miller-Urey experiment, a great deal of effort has been spent investigating prebiotic chemistry. It has become apparent that organizing simple molecules into assemblies capable of reproducing and evolving is a far greater task than was generally realized during the excitement that followed the experiment. In addition, the view that the early atmosphere was highly reducing was challenged towards the end of the twentieth century, and is no longer the consensus view.

Although the significance of specific details of the Miller-Urey for the origin of life may now be in question, it began the new scientific discipline of prebiotic chemistry, and has been enormously influential in the development of ideas about the origin of life.

References:

"Origin Of Life: Twentieth Century Landmarks." Origin Of Life: Oparin-Haldane Hypothesis. N.p., n.d. Web. 28 Oct. 2012. .

"The Miller/Urey Experiment." The Miller/Urey Experiment. N.p., n.d. Web. 28 Oct. 2012. .

Reece, Jane B., and Neil A. Campbell. Campbell Biology. Harlow: Pearson Education, 2011. Print.