PREBIOTIC EVOLUTION: WHERE IT HAPPENED AND WHAT IS NECESSARY - BIOLOGY - 2025

The term prebiotic evolution refers to the series of hypothetical scenarios that seek to explain the origin of life starting from non-living matter in an environment under primitive conditions.

It has been suggested that the conditions of the primitive atmosphere were strongly reducing, which favored the formation of organic molecules, such as amino acids and peptides, which are the building blocks of proteins; and purines and pyrimidines, which make up nucleic acids - DNA and RNA.

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Primitive conditions

Imagining how the first forms of life emerged on Earth can be a challenging - and even almost impossible - question if we don't place ourselves in the right primitive environment.

Thus, the key to understanding life from abiotic molecules suspended in the famous "primitive soup" is the atmosphere in that remote environment.

Although there is no total agreement regarding the chemical composition of the atmosphere, since there is no way to confirm it completely, the hypotheses range from reducing compositions (CH ₄ + N ₂, NH ₃ + H ₂ O or CO ₂ + H ₂ + N ₂) to more neutral environments (with only CO ₂ + N ₂ + H ₂ O).

It is generally accepted that the atmosphere lacked oxygen (this element increased its concentration significantly with the arrival of life). For the efficient synthesis of amino acids, purines, pyrimidines and sugars, the presence of a reducing environment is necessary.

If the actual atmosphere at that time did not have these prebiotic chemical conditions, the organic compounds had to have come from dust particles or other space bodies such as meteorites.

Where did prebiotic evolution occur?

There are several hypotheses in relation to the physical space on Earth that allowed the development of the first biomolecules and replicators.

A theory that has gained a significant following in the initial formation of biomolecules in hydrothermal vents in the ocean. However, other authors find it unlikely and discredit these regions as important agents in prebiotic synthesis.

The theory proposes that chemical synthesis occurred through the passage of water within a term gradient from 350 ° C to 2 ° C.

The problem with this hypothesis arises because organic compounds decompose at high temperatures (350 ° C) instead of being synthesized, which suggests less extreme environments. So the hypothesis has lost support.

What is needed for prebiotic evolution?

To carry out a study related to prebiotic evolution, it is necessary to answer a series of questions that allow us to understand the emergence of life.

We must ask ourselves what type of catalytic process favored the origin of life and where the energy that favored the first reactions was taken from. In answering these questions, we can go further and ask whether the first molecules to appear were membranes, replicators, or metabolites.

We will now answer each of these questions to gain an understanding of a possible origin of life in a prebiotic environment.

Catalysts

Life, as we know it today, requires a series of "moderate conditions" to develop. We know that most organic beings exist where the temperature, humidity and pH are physiologically acceptable - with the exception of the Extremophilic organisms, which, as their name implies, live in extreme environments.

One of the most relevant characteristics of living systems is the ubiquity of catalysts. The chemical reactions of living beings are catalyzed by enzymes: complex molecules of a protein nature that increase the speed of reactions by several orders of magnitude.

The first living beings must have had a similar system, probably ribozymes. In the literature, there is an open question whether prebiotic evolution could have occurred without catalysis.

According to the evidence, in the absence of a catalyst biological evolution would have been highly unlikely - as the reactions would have taken monumental time intervals to occur. Therefore, their existence is postulated during the early stages of life.

Energy

The energy for the prebiotic synthesis had to appear from somewhere. It is proposed that certain inorganic molecules, such as polyphosphates and thioesters, may have played an important role in the production of energy for reactions - in times before the existence of the famous energy "currency" of cells: ATP.

Energetically, the replication of the molecules that carry the genetic information is a very costly event. For an average bacterium, such as E. coli, a single replication event requires 1.7 * 10 ¹⁰ ATP molecules.

Thanks to the existence of this extraordinarily high figure, the presence of an energy source is an unquestionable condition for creating a probable scenario in which life originated.

Likewise, the existence of “redox” type reactions could have contributed to abiotic synthesis. Over time, this system could become important elements of the transport of electrons in the cell, linked to the production of energy.

Which of the cellular components originated first?

There are three basic components in a cell: a membrane, which delimits the cell space and turns it into a discrete unit; replicators, which store information; and metabolic reactions, which occur within this system. The functional integration of these three components gives rise to a cell.

Therefore, in the light of evolution, it is interesting to ask the question of which of the three arose first.

The synthesis of membranes seems to be simple, since lipids spontaneously form vesicular structures with the ability to grow and divide. The vesicle allows the storage of replicators and keeps the metabolites concentrated.

Now, the debate focuses on the leadership of replication versus metabolism. Those who give more weight to replication, argue that ribozymes (RNA with catalytic power) were able to replicate themselves, and thanks to the appearance of mutations a novel metabolic system could arise.

The opposite view highlights the importance of the generation of simple molecules - such as the organic acids present in the tricarboxylic acid cycle - to combustion under moderate heat sources. From this perspective, the first steps of prebiotic evolution involved these metabolites.

References

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