We explain several theories that address how life could arise on Planet Earth.
How and where did life arise on our planet?
Microbes in deep seas, RNA moleculesor extraterrestrial contaminations are just some of the theories proposed to answer a key question that, to this day, remains unanswered despite the scientific evidence that, little by little, is shedding more light on this question.
In this article we will talk about the main theories of the origin of life: which are its most outstanding authors, what they postulate regarding how the first living organism could arise and under what conditions, or what are the most important scientific evidences that support their hypotheses.
The origin of life
One of the great questions that philosophers, scientists and curious people have tried to answer throughout the history of humanity is to know the origin of life. Currently, there are multiple theories that try to explain how living beings arose on Earth.
We know that life on our planet could have begun more than 3 billion years ago, evolving from the simplest microbes to what we have today: an incredible variety and diversity of living things. But how did the first organisms develop? Under what conditions did they do so? How could life arise from “nothing”?
Initially, the study of the origin of life was focused through the paradigm of spontaneous generation, a concept proposed byAleksandr Oparin, which postulated that living beings arose naturally from inanimate or inert matter.
Despite the somewhat mythical character of this postulate, from these ideas others emerged around the idea of abiogenesis and the ability of certain chemical compounds to give rise to more complex molecules, which in the long run could lead to the first living organisms.
With this breeding ground, in recent decades scientific researchers have been postulating new theories, without today we can ensure with total certainty that any of them is the definitive or the one that best explains this issue. Next we will see what are the main theories about the origin of life.
1. The electric spark theory
The lightning generated in storms could have provided the spark needed for life to emerge. And it is that the electrical sparks that cause these meteorological phenomena can produce amino acids and sugars from an atmosphere loaded with various elements; such as water, methane, ammonia and hydrogen.
However, scientific research has revealed that the Earth’s early atmosphere was quite poor in hydrogen, so the theory would not fully hold, although other studies have suggested that volcanic clouds in the early atmosphere could have contained methane, ammonia and hydrogen, along with a large production of storms and lightning; So there are pros and cons to this theory.
2. The clay theory
The first organic molecules could have been found in clay, according to a theory developed by English chemist Alexander Graham Cairns-Smith. According to this scientist, organic compounds are concentrated on clay surfaces and it is possible that this material has helped these compounds organize themselves into molecules very similar to our current genes.
One of the main functions of DNA is to store information about how other molecules should be organized. The genetic sequences in DNA are essentially instructions on how amino acids should be organized into proteins. According to Cairns-Smith, mineral crystals in the clay could have pushed organic molecules to consolidate into organized patterns. And these, after a while, would have taken over this same task.
This theory does not have much support from the scientific community, because many researchers label it as being continuous with unscientific theories such as spontaneous generation.
3. The theory of hydrothermal vents
Life may have begun in deep water. Specifically, what this theory postulates is that the first living beings could originate in underwater hydrothermal vents, which tend to throw molecules rich in hydrogen and other organic compounds, since they receive a lower intensity of radiation.
Hydrothermal vents are cracks in the earth’s surface from which hot water flows. They usually appear in volcanically active places and rocky corners that could have harbored large concentrations of hydrogen molecules, serving as catalysts to produce chemical reactions critical to the origin of life.
4. The glacial theory
3 billion years ago, Earth’s oceans could have been covered in a large mantle of ice, as the Sun in that early period might have been one-third less luminous than it is now. Apparently, that layer of ice (possibly hundreds of feet thick) could have protected the most fragile organic compounds from destruction by ultraviolet radiation and cosmic rays.
The freezing cold could also have helped these organic molecules have a longer survival span, thus allowing key reactions to occur for the origin of the first living beings.
5. The theory of panspermia
One of the most suggestive theories is that life didnot begin on our planet, but was transported to Earth from another place in space and the known universe. The theory of panspermia or exobiogenesis can be divided into three types:
- Lithopanspermia: This variant of panspermia ensures that life could be transported to our planet by means of meteorites. For example, rocks are regularly ejected from Mars due to cosmic impacts, and several Martian meteorites have already been found on our planet that could have brought back the first microbes.
- Radiopanspermia: This type of panspermia attributes the origin of life on Earth to dust grains driven by the pressure of radiation from stars.
- Directed panspermia: attributes the origin of life to the voluntary transport and sending of microorganisms through space to our planet, by extraterrestrial beings, with the aim of initiating life or seeding other solar systems.
6. RNA theory
RNA theory postulates that it was this molecule that was the first to be essential to originate primitive life. Detractors of this theory argued that no molecule could have replicated withoutenzymes. However, in 1982 Thomas Cech discovered ribozymes, RNA molecules with the ability to create bonds between amino acids and produce proteins, which changed the view of many of the researchers.
As is known, DNA needs proteins to form, and vice versa; So: how could one have formed without the other? The answer would be in RNA, which could have served as a store of information (like DNA) and as an enzyme (like proteins), being useful in the production of both DNA and proteins.
The question of what came first, RNA or proteins, is a question that remains unclear to this day. And while some scientists think the molecule may have arisen spontaneously on Earth, others believe it was unlikely to have arisen.
7. The cell membrane theory
Another theory that tries to explain the origin of life questions whether it was possible thanks to complex molecules such as RNA; Conversely, it is suggested that living things could have appeared from smaller molecules interacting with each otherin reaction cycles.
These molecules could have been contained in simple capsules similar to cell membranes; and over time, the more complex molecules that could more effectively perform the reactions carried out by the simplest molecules could have finally evolved, in what is known as the “metabolism first” scenario, versus the other scenario called “genes first”, in which RNA predominates as the main factor.
References
- Orgel, L. E. (1998). The origin of life—a review of facts and speculations. Trends in biochemical sciences, 23(12), 491-495.
- Pardo, A. (2007). The origin of life and the evolution of species: science and interpretations.
- Rojas, I. (2004). The origin of life on earth. Valparaíso: Pontificia Universidad Católica de Chile.
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