ABIOGENESIS: MAIN THEORIES - BIOLOGY - 2024

The abiogénesis refers to the number of processes and steps that originated the first forms of life on earth, inert monomeric starting blocks, with the passage of time were able to increase their complexity. In light of this theory, life arose from non-living molecules, under the appropriate conditions.

It is likely that after abiogenesis produced simple life systems, biological evolution acted to give rise to all the complex life forms that exist today.

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Some researchers believe that abiogenesis processes must have occurred at least once in the history of the earth to give rise to the hypothetical organism LUCA or last universal common ancestor (from the acronym in English, last universal common ancestor), about 4 billion ago of years.

It is suggested that LUCA must have had a genetic code based on the DNA molecule, which with its four bases grouped in triplets, encoded the 20 types of amino acids that make up proteins. Researchers trying to understand the origin of life study the abiogenesis processes that gave rise to LUCA.

The answer to this question has been widely questioned and is often shrouded in a haze of mystery and uncertainty. For this reason, hundreds of biologists have proposed a series of theories that range from the emergence of a primordial soup to explanations related to xenobiology and astrobiology.

What does it consist of?

The theory of abiogenesis is based on a chemical process by which the simplest forms of life emerged from lifeless precursors.

It is assumed that the process of abiogenesis occurred continuously, in contrast to the view of the emergence abruptly in a lucky event. Thus, this theory assumes the existence of a continuum between non-living matter and the first living systems.

Likewise, a series of varied scenarios are suggested where the beginning of life could occur from inorganic molecules. These environments are generally extreme and different from current conditions on earth.

These supposed prebiotic conditions are often reproduced in the laboratory in order to try to generate organic molecules, like the famous Miller and Urey experiment.

The origin of life: theories

The origin of life has been one of the most controversial topics for scientists and philosophers since the time of Aristotle. According to this important thinker, decomposing matter could be transformed into living animals thanks to the spontaneous action of nature.

Abiogenesis in the light of Aristotelian thought can be summarized in his famous phrase omne vivum ex vivo, which means “all life proceeds from life”.

Subsequently, a fairly large number of models, theories and speculations have tried to elucidate the conditions and processes that led to the origin of life.

The most outstanding theories, both from a historical and scientific point of view, that have sought to explain the origin of the first living systems will be described below:

Theory of spontaneous generation

In the early 17th century it was postulated that life forms could emerge from lifeless elements. The theory of spontaneous generation was widely accepted by the thinkers of the time since it had the support of the Catholic Church. Thus, living beings could germinate both from their parents and from non-living matter.

Among the most famous examples used to support this theory are the appearance of worms and other insects in decomposed meat, frogs that appeared from mud, and mice that emerged from dirty clothes and sweat.

In fact, there were recipes that promised the creation of live animals. For example, in order to create mice from non-living matter, wheat grains had to be combined with dirty clothes in a dark environment and live rodents appear over the days.

Proponents of this mixture argued that human sweat on clothing and the fermentation of wheat were the directing agents of the formation of life.

Refutation of spontaneous generation

In the seventeenth century, flaws and gaps began to be noticed in the statements of the theory of spontaneous generation. It was not until 1668 that the Italian physicist Francesco Redi devised a suitable experimental design to reject it.

In her controlled experiments, Redi placed finely cut pieces of meat wrapped in muslin in sterile containers. These jars were properly covered with gauze, so nothing could come into contact with the meat. Also, the experiment featured another set of jars that were not capped.

Over the days, worms were only observed in the jars that were uncovered, since the flies could enter freely and lay the eggs. In the case of the covered jars, the eggs were placed directly on the gauze.

Similarly, the researcher Lazzaro Spallanzani developed a series of experiments to reject the premises of spontaneous generation. To do this, he made a series of broths that he subjected to prolonged boiling to destroy any microorganisms that will live there.

However, proponents of spontaneous generation claimed that the amount of heat to which the broths were exposed was excessive and destroyed the "life force".

Pasteur's contributions

Later, in 1864, the French biologist and chemist Louis Pasteur set out to put an end to the postulates of spontaneous generation.

To meet this objective, Pasteur manufactured glass containers known as "gooseneck flasks", as they were long and curved at the tips, thus preventing the entry of any microorganisms.

In these containers Pasteur boiled a series of broths that remained sterile. When the neck of one of them was broken, it became contaminated and microorganisms proliferated in a short time.

The evidence provided by Pasteur was irrefutable, managing to overturn a theory that lasted for more than 2,500 years.

Panspermia

In the early 1900s, Swedish chemist Svante Arrhenius wrote a book entitled "The Creation of Worlds" in which he suggested that life came from space via spores resistant to extreme conditions.

Logically, the theory of panspermia was surrounded by much controversy, besides that it did not really provide an explanation for the origin of life.

Chemosynthetic theory

When examining Pasteur's experiments, one of the indirect conclusions of his evidence is that microorganisms only develop from others, that is, life can only come from life. This phenomenon was called "biogenesis".

Following this perspective, the theories of chemical evolution would emerge, led by the Russian Alexander Oparin and the Englishman John DS Haldane.

This view, also called the Oparin – Haldane chemosynthetic theory, proposes that in a prebiotic environment the earth had an atmosphere devoid of oxygen and high in water vapor, methane, ammonia, carbon dioxide and hydrogen, making it highly reductive.

In this environment there were different forces such as electric discharges, solar radiation and radioactivity. These forces acted on inorganic compounds, giving rise to larger molecules, creating organic molecules known as prebiotic compounds.

Miller and Urey experiment

In the mid-1950s, researchers Stanley L. Miller and Harold C. Urey succeeded in creating an ingenious system that simulated the supposed ancient conditions of the atmosphere on earth following the Oparin – Haldane theory.

Stanley and Urey found that under these "primitive" conditions, simple inorganic compounds can give rise to complex organic molecules, essential for life, such as amino acids, fatty acids, urea, among others.

Polymer formation

Although the aforementioned experiments suggest a plausible way by which biomolecules that are part of living systems originated, they do not suggest any explanation for the polymerization process and increased complexity.

There are several models that try to elucidate this question. The first involves solid mineral surfaces, where the high surface area and silicates could act as catalysts for carbon molecules.

Deep in the ocean, hydrothermal vents are an appropriate source of catalysts, such as iron and nickel. According to laboratory experiments, these metals participate in polymerization reactions.

Finally, in the ocean trenches there are hot pools, which due to evaporation processes could favor the concentration of monomers, favoring the formation of more complex molecules. The “primordial soup” hypothesis is based on this assumption.

Reconciliation of Miller and Pasteur results

Following the order of idea discussed in the previous sections, we have that Pasteur's experiments verified that life does not arise from inert materials, while the evidence from Miller and Urey indicates that it does, but at the molecular level.

In order to reconcile both results, it must be borne in mind that the composition of the earth's atmosphere today is totally different from the prebiotic atmosphere.

The oxygen present in the current atmosphere would operate as a "destroyer" of the molecules in formation. It should also be considered that the energy sources that supposedly drove the formation of organic molecules are no longer present with the frequency and intensity of the prebiotic environment.

All forms of life present on earth are composed of a set of structural blocks and large biomolecules, called proteins, nucleic acids and lipids. With them you can "arm" the basis of current life: cells.

In the cell life is perpetuated, and on this principle Pasteur is based to affirm that every living being must come from another pre-existing one.

RNA world

The role of autocatalysis during abiogenesis is crucial, which is why one of the most famous hypotheses about the origin of life is that of the RNA world, which postulates a start from single-chain molecules with the capacity for self-replication.

This notion of RNA suggests that the first biocatalysts were not molecules of a protein nature but rather RNA molecules - or a polymer similar to it - with the ability to catalyze.

This assumption is based on the property of RNA to synthesize short fragments using annealing that directs the process, in addition to promoting the formation of peptides, esters and glycosidic bonds.

According to this theory, ancestral RNA was associated with some cofactors such as metals, pyrimidines, and amino acids. With the advancement and increasing complexity of metabolism, the ability to synthesize polypeptides arises.

In the course of evolution, RNA was replaced by a more chemically stable molecule: DNA.

Current conceptions of the origin of life

Currently it is suspected that life originated in an extreme scenario: oceanic areas near volcanic vents where temperatures can reach 250 ° C and atmospheric pressure exceeds 300 atmospheres.

This suspicion arises from the diversity of life forms found in these hostile regions and this principle is known as the "hot world theory".

These environments have been colonized by archaebacteria, organisms capable of growing, developing and reproducing in extreme environments, probably very similar to prebiotic conditions (including low oxygen concentrations and high CO ₂ levels).

The thermal stability of these environments, the protection they provide against sudden changes and the constant flow of gases are some of the positive attributes that make the seabed and volcanic vents suitable environments for the origin of life.

Biogenesis and abiogenesis terms

In 1974, the renowned researcher Carl Sagan published an article clarifying the use of the terms biogenesis and abiogenesis. According to Sagan, both terms have been used erroneously in articles related to explanations of the origin of the first living forms.

Among these errors is using the term biogenesis as its own antonym. That is, biogenesis is used to describe the origin of life starting from other living forms, while abiogenesis refers to the origin of life from non-living matter.

In this sense, a contemporary biochemical pathway is considered biogenic and a prebiological metabolic pathway is abiogenic. Therefore it is necessary to pay special attention to the use of both terms.

References

1. Bergman, J. (2000). Why abiogenesis is impossible. Creation Research Society Quarterly, 36 (4).
2. Pross, A., & Pascal, R. (2013). The origin of life: what we know, what we can know and what we will never know. Open Biology, 3 (3), 120190.
3. Sadava, D., & Purves, WH (2009). Life: the science of biology. Panamerican Medical Ed.
4. Sagan, C. (1974). On the terms 'biogenesis' and 'abiogenesis'. Origins of Life and Evolution of Biospheres, 5 (3), 529–529.
5. Schmidt, M. (2010). Xenobiology: a new form of life as the ultimate biosafety tool. Bioessays, 32 (4), 322–331.
6. Serafino, L. (2016). Abiogenesis as a theoretical challenge: Some reflections. Jour nal of theoretical biology, 402, 18–20.