10 THEORIES OF THE ORIGIN OF LIFE - BIOLOGY - 2024

The theories of the origin of life try to explain how living things originated. How life arose as we know it is a question that many philosophers, theologians and scientists have asked for many years, in fact, we could say that almost since man was a man.

Different scientific records establish that the earth was formed about 4.5-5 billion years ago and that the oldest known fossils, corresponding to the remains of cyanobacteria found in Western Australia, date from at least 3.5 billion years ago.

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Although there are no fossil records or older geological evidence, many scientists agree that other life forms may have existed earlier, but that fossils may have been destroyed by heat and the shape changes of many rocks during the Precambrian.

What happened during the almost 2 billion years that elapsed since the origin of the earth and the occurrence of the first fossils? It is the biological events that occurred at that time that made the emergence of life possible and those that are so much debated in the scientific community today.

Next we will find some of the main hypothetical theories put forward by different authors to explain the origin of the first living organisms, from which the most “advanced” forms of life presumably evolved.

What was the earth like before life arose?

The earliest known life forms on Earth are putative fossilized microorganisms, found in hydrothermal vents. They are estimated to have lived 4.28 billion years ago.

Some scientists propose that the "initial" earth was impacted by different types of celestial objects and that the temperatures on this planet were so high that the water was not in a liquid state, but in the form of gas.

However, many agree that Precambrian land may have had temperatures similar to land today, which means that water could be found in liquid form, condensed into oceans, seas, and lakes.

The terrestrial atmosphere of the time, on the other hand, is thought to be strongly reducing (with zero or very little free oxygen), so that after exposure to different forms of energy the first organic compounds could have been formed.

Main theories of the origin of life

- Life by spontaneous generation

Aristotle, forerunner of spontaneous generation

From the Greeks to many scientists in the mid-nineteenth century, the proposal was accepted that living things could arise spontaneously, without other parental organisms, from "non-living" matter.

Therefore, for many centuries, different thinkers were convinced that insects, worms, frogs and other vermin formed spontaneously on mud or on decomposing matter.

These theories were discredited on more than one occasion by the experiments carried out by Francesco Redi (1668) and Louis Pasteur (1861), for example.

Portrait of Francesco Redi (Source: Valérie75, via Wikimmedia Commons)

Redi proved that unless adult insects laid their eggs on a piece of meat, the larvae did not spontaneously emerge on it. On the other hand, Pasteur later showed that microorganisms could only come from pre-existing microorganisms.

Furthermore, it must be said that this theory was also ignored because in different historical contexts "spontaneous generation" referred to two quite different concepts, namely:

- Abiogenesis: the notion of the origin of life from inorganic matter and

- Heterogenesis: the idea that life arose from dead organic matter, just as worms "appeared" on decaying meat.

Darwin and Wallace, a little earlier, in 1858, independently published their theories on evolution by natural selection, by means of which they made it clear that the most complex living beings had been able to evolve from more “simple” unicellular beings.

Thus, the theory of spontaneous generation disappeared from the scene and the scientific community began to wonder how those "simpler single-celled beings" emerged that evolutionists spoke of.

- Theory of primary broth and gradual chemical evolution

Alexander Oparin in his laboratory (right).

In 1920, scientists A. Oparin and J. Haldane proposed, separately, the hypothesis about the origin of life on earth that today bears their names and by means of which they established that life on earth could have arisen " step-by-step ”from non-living matter, through“ chemical evolution ”.

Grand prismatic Spring in Yellowstone. This high-temperature environment is thought to be similar to the primeval environment of the Earth's seas. Source:

Both researchers suggested that the "initial" earth must have had a reducing atmosphere (poor in oxygen, in which all molecules tended to donate electrons), a condition that could perfectly explain some events:

- That some inorganic molecules react with each other to form the organic structural "blocks" of living beings, a process directed by electrical energy (from rays) or light (from the sun) and whose products accumulate in the oceans forming a "primary broth".

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- That said organic molecules are subsequently combined, assembling more complex molecules, formed by fragments of simpler molecules (polymers) such as proteins and nucleic acids.

- That said polymers were assembled into units capable of replicating themselves, either in metabolic groups (Oparin's proposal) or inside membranes that formed “cell-like” structures (Haldane's proposal).

- Panspermia

Illustration of a bacterium on a comet. Source: Silver Spoon Sokpop / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

In 1908, a scientist named August Arrhenius proposed that "life-bearing seeds" were scattered throughout cosmic space and that they fell on the planets and "germinated" when conditions were favorable there.

This theory, also known as the theory of panspermia (from the Greek pan, which means "everything" and sperma, which means "seed"), was supported by different scientists and we can also find it referred to in some texts as "the extraterrestrial origin of life".

- Life by electricity

Image by FelixMittermeier from Pixabay.com

Later, part of the scientific community suggested that the origin of life proposed by Oparin and Haldane could have started on earth thanks to an electrical "spark" that provided the energy necessary for the "organization" of the fundamental organic compounds from of inorganic compounds (a form of abiogenesis).

These ideas were experimentally supported by two North American researchers: Stanley Miller and Harold Urey.

Through their experiments, both scientists demonstrated that, from inorganic substances and under some special atmospheric conditions, an electrical discharge was capable of forming organic molecules such as amino acids and carbohydrates.

This theory proposed, then, that with the passage of time the most complex molecules that today characterize living beings could have been formed; which is why this was very supportive of Oparin and Haldane's "primeval stock" theories a few years earlier.

- Life under the ice

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Another theory, perhaps a little less known and accepted, proposes that life arose in deep ocean waters, whose surface was presumably covered by a thick and thick layer of ice, since the Sun of the initial earth probably did not affect so strongly on the surface as now.

The theory proposes that the ice could have protected whatever the biological phenomenon occurred in the sea, allowing the interaction of the different compounds that originated the first living forms.

- Life from organic polymers

Protein

After it could be demonstrated in a laboratory that organic compounds such as amino acids could form from inorganic matter under certain conditions, scientists began to wonder how the polymerization process of organic compounds occurred.

Let's remember that cells are made up of large and complex types of polymers: proteins (polymers of amino acids), carbohydrates (polymers of sugars), nucleic acids (polymers of nitrogenous bases), etc.

Sidney Fox

In 1950, the biochemist Sidney Fox and his work group discovered that, under experimental conditions, if a set of amino acids was heated in the absence of water, they could join together to form a polymer, that is, a protein.

These findings led Fox to suggest that in the "primitive broth" proposed by Oparin and Haldane, amino acids could have formed that, when in contact with a hot surface, promoting the evaporation of water, could form proteins.

Ribonucleic acid and life on clay

Organic chemist Alexander Cairns-Smith later proposed that the first molecules that made life possible could be found on clay surfaces, which not only helped to concentrate them, but also promoted their organization into defined patterns.

These ideas, which came to light in the 1990s, affirmed that clay could serve as a “catalyst” in the formation of RNA (ribonucleic acid) polymers, acting, in turn, as a catalyst support.

- The "genes first" hypothesis

Taking into account the ideas of the “spontaneous” formation of essential organic polymers, some authors set out to imagine the possibility that the first life forms were simply self-replicating nucleic acids, such as DNA (deoxyribonucleic acid) or the RNA.

Therefore, it was suggested that other important elements, such as metabolic networks and membrane formation, for example, be added later to the "primeval" system.

Given the reactivity characteristics of RNA, many scientists support the notion that the first auto-catalytic structures were formed by this nucleic acid (evident as ribozymes), hypotheses known as "the world of RNA".

Accordingly, RNA could potentially have catalyzed the reactions that allowed its own copying, making it capable of transmitting genetic information from generation to generation and even evolving.

- The "metabolism first" hypothesis

On the other hand, different researchers rather supported the notion that life took place first in "protein-like" organic molecules, establishing that the initial forms of life could have consisted of "self-sustaining" metabolic networks prior to nucleic acids.

The hypothesis implies that "metabolic networks" may have formed in areas near hydrothermal vents, which maintained a continuous supply of chemical precursors.

Thus, the earlier, simpler pathways may have produced molecules that acted as catalysts for the formation of more complex molecules, and eventually metabolic networks may have been able to form other, even more complex molecules, such as nucleic acids and large proteins.

Finally, these self-sustaining systems could have been "encapsulated" inside membranes, thus forming the first cellular beings.

- The origin of life by "necessity"

Some researchers belonging to the Massachusetts Institute of Technology (MIT, USA) have contributed to the formulation of a theory that explains the origin of the first living beings by "necessity", somehow "following the laws of nature" and not by "Chance" or "chance".

According to this theory, the emergence of life was an inevitable matter, since it was established that matter generally develops in "systems" that, directed by an external source of energy and surrounded by heat, are more efficient in dissipating energy.

Experiments related to this theory have shown that when a population of random atoms is exposed to a source of energy, they organize themselves to dissipate the energy more efficiently, suggesting that this "re-modeling" would eventually end the formation of life..

The alternate source of energy could easily have been the sun, although other possibilities are not entirely ruled out.

- Creationism

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Creationism is another of the theories supported by an important part of today's societies, mainly by act of faith. According to this current of thought, the universe and all the life forms that are in it were created from "nothing" by a God.

It is a theory that is interestingly opposed to modern theories of evolution, which seek to explain the origin of the diversity of living forms without the need for a God or any other "divine power" and, many times, simply by "chance. ”.

There are two types of creationists: the biblical and the "old earth." The former believe that everything stated in the Genesis chapter in the Bible is literally true, while the latter consider that a creator made everything that exists, but without affirming that the story of Genesis is a literal story.

However, both types of creationists believe that changes in organisms can imply changes in a species, and they also believe in changes "downward", such as negative mutations, for example.

However, they do not believe that these changes could have led to the evolution of a "lower" species into a "higher" or much more complex species.

Creationism and evolutionism have been the subject of debate and dispute since the publication of the first evolutionary theories and, even today, both views seem to be mutually exclusive.

References

1. Andrulis, ED (2012). Theory of the origin, evolution, and nature of life. Life, 2 (1), 1-105.
2. Choi, C. (2016). Live Science. Retrieved April 26, 2020, from www.livescience.com/13363-7-theories-origin-life.html
3. Horowitz, NH, & Miller, SL (1962). Current theories on the origin of life. In Fortschritte der Chemie Organischer Naturs
4. TN & EL Taylor. 1993. The Biology and Evolution of Fossil Plants. Prentice Hall, New Jersey.
5. Thaxton, CB, Bradley, WL, & Olsen, RL (1992). The mystery of life's origin. na.
6. The Editors of Encyclopaedia Britannica. (2017). Encyclopaedia Britannica. Retrieved April 26, 2020, from www.britannica.com/topic/creationism