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 * 1) Prebiotic soup (Class: S)

Primordial soup, or prebiotic soup (also sometimes referred as prebiotic broth), is the hypothetical set of conditions present on the Earth around 4.2 to 4.0 billions of years ago. It is a fundamental aspect to the heterotrophic theory of the origin of life, first proposed by Alexander Oparin and John Burdon Sanderson Haldane during the first part of the 20th century.

According to the heterotrophic theory, organic compounds were synthesized in the primitive Earth under prebiotic conditions. It is important to make the distinction between prebiotic and abiotic processes. While an abiotic process refers to anything that occurs without the presence of life, a prebiotic process refers to something that happens in the atmospheric and chemical conditions that the primitive Earth had about 4.2 billion years ago, and that preceded to the origin of life on the planet.

In 1936 the russian biochemist Alexander Ivanovich Oparin published the book "The Origin of Life". This writing, whose English translation was published in 1938, is the mature version of Oparin's first approach (1924) to the theme of the origin of life. In his work, impregnated by a Darwinian thought that involved a slow and gradual evolution from the simple to the complex, Oparin proposed a heterotrophic origin, result of a long process of chemical and pre-biological evolution, where the first forms of life should have been microorganisms dependent on the molecules and organic substances present in their external environment. That external eviroment was the primordial soup.

The idea of a heterotrophic origin was based, in part, on the universality of fermentative reactions, which, according to Oparin, should have first appeared in evolution due to its simplicity. This was opposed to the idea, widely accepted at that time, that the first organisms emerged endowed with an autotrophic metabolism, which included photosynthetic pigments, enzymes and the ability to synthesize organic compounds from CO2 and H2O; for Oparin it was impossible to reconcile the original photosynthetic organisms with the ideas of Darwinian evolution

From the detailed analysis of the geochemical and astronomical data known at that date, Oparin also proposed a primitive atmosphere devoid of O2 and composed of CH4, NH3 and H2O; under these conditions it was pointed out that the origin of life had been preceded by a period of abiotic synthesis and subsequent accumulation of various organic compounds in the seas of primitive Earth (Oparin, 1938). This accumulation resulted in the formation of a primordial broth containing a wide variety of molecules.

There, according to Oparin, a particular type of colloid, the coacervates, were formed due to the conglomeration of organic molecules and other polymers with positive and negative charges. Oparin suggested that the first living beings had been preceded by pre-cellular structures similar to those coacervates, whose gradual evolution gave rise to the appearance of the first organisms (Oparin, 1938).

Like the coacervates, several of Oparin's original ideas have been reformulated and replaced; this includes, for example, the reducing character of the atmosphere on primitive Earth, the coacervates as a pre-cellular model and the primitive nature of glycolysis. In the same way, we now understand that the gradual processes are not necessarily slow, and we even know, thanks to the fossil record, that the origin and early evolution of life occurred in short geologic time lapses.

However, the general approach of Oparin's theory had great implications for biology, since his work achieved the transformation of the study of the origin of life from a purely speculative field to a structured and broad research program (Lazcano, 2010). Thus, since the second half of the twentieth century, the theory of the origin and early evolution of Oparin's life has undergone a restructuring that accommodates the experimental findings of molecular biology, as well as the theoretical contributions of evolutionary biology.

A point of convergence between these two branches of biology and that has been perfectly incorporated into the heterotrophic origin theory is found in the RNA world hypothesis.

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