HYDROTHERMAL HYPOTHESIS: WHAT IT IS, BACKGROUND AND CRITICISM - BIOLOGY - 2024

The hydrothermal hypothesis aims to explain the primitive conditions in which the first forms of life originated on planet Earth, proposing as the main setting the hot springs located in the depths of the oceans.

A series of thermal water sources have been located with temperatures that reach 350 ° C, where a series of fauna typical of these conditions inhabit, such as bivalves, worms, crustaceans, porifers and some echinoderms (starfish and their relatives).

This evidence suggests that deep ocean environments were probably suitable for the origin of life and the earliest forms of life were chemoautotrophic microorganisms.

Furthermore, boiling water is home to a series of chemosynthetic bacteria that extract their energy from sulfur substances, which are abundant in this type of environment.

Chemosynthetic bacteria have producer functions in ecosystems, being the base of the food chain, analogous to the role of plants in typical ecosystems.

Ideas related to the hydrothermal hypothesis began to emerge in early 1977, when researcher Corliss made direct observations in hydrothermal systems located in the Galapagos Islands.

Background and alternative theories

For several decades, researchers have proposed dozens of theories that seek to explain the origin of life and the favorable environment in which it should have developed. How life originated has been one of the oldest and most controversial scientific questions.

Some authors support the primary origin of metabolism, while their opponents support the genetic origin.

Panspermia

In the mid-1900s the renowned scientist Arrhenius proposed the theory of panspermia or the cosmological theory. This idea raises the origin of life thanks to the arrival of spatial microorganisms from a planet where life previously existed.

Logically, the cosmological theory does not provide ideas that solve the problem, since it does not explain how extraterrestrial life originated on said hypothetical planet.

Furthermore, it is not very likely that the microscopic entities that colonized prebiotic environments have survived the conditions of space until they reached planet Earth.

Abiotic models

Abiotic models propose that life originated apart from "microstructures" as transitional forms between organic molecules and early life forms. Among the main defenders of this theory are Oparín, Sydney W. Fox and Alfonso F. Herrera.

According to Oparin and Haldane, coacervates are probionts precursors of life, delimited by a plasma membrane that allows interaction with their environment. According to the authors, they originated before the molecules that transmit genetic information: DNA or RNA.

For their part, Stanley Miller and Harold Urey managed to build an ingenious system that mimicked "the primitive atmosphere of life." The components present in the hypothetical atmosphere, very different from the current one, were able to synthesize organic molecules essential for life (such as amino acids) when heat and voltage were applied to it.

Fox was able to obtain microspheres of a size similar to bacteria, subjecting the amino acids to a heat source.

In the same way, other researchers have achieved the synthesis of organic molecules using inorganic molecules as raw material, thus explaining the origin of life from an abiotic environment.

RNA world

Another position of the origin of life postulates as the main event the appearance of the molecules that contain genetic information. Various authors defend the origin of life from RNA and argue that this molecule served as a template and a catalyst at the same time.

The greatest evidence is the existence of ribosomes, RNA molecules capable of catalyzing reactions and, at the same time, storing information in their nucleotide sequence.

What does it consist of?

The hydrothermal hypothesis proposes these extreme aquatic environments as the proper place for the synthesis of organic compounds that led to the origin of living organisms on Earth.

The authors of this theory are based on Archean fossils, modern underwater hydrothermal vent systems, and theoretical and experimental observations.

Hydrothermal systems are characterized by high energy fluxes, a highly reducing environment, and abundant mineral clays, which are ideal surfaces for catalytic reactions. In addition, it has high concentrations of CH ₄, NH ₃, H ₂ and different metals.

The hypothesis consists of the sequential conversion of CH ₄, NH ₃, H ₂ into amino acids, these into proteins and then into more complex polymers, until reaching a structured metabolism and living organisms.

Examining fossils in Precambrian rocks has found cell-like structures dating from about 3.5 to 3.8 billion years ago in undersea hydrothermal assemblages.

In contrast to the previous hypotheses, the hydrothermal hypothesis proposes heat as a source of energy and not UV rays and electric discharges, as the model of the "primal soup". Furthermore, this model proposes the existence of environmental gradients in terms of temperature, pH and chemical concentration.

Criticisms of the theory

Although the hydrothermal hypothesis has several valid arguments, it is not universally accepted. One of the criticisms of the origin of life in a hot spring is the incongruity and lack of information regarding the geological models of the Prebiotic Era.

Likewise, the essential molecules for the development of life - such as nucleic acids, proteins and membranes - would face immediate destruction, due to the high temperatures of hydrothermal environments.

However, it is also likely that the first life forms were thermostable, similar to the thermophilic organisms that today inhabit extreme environments.

On the other hand, another problem arises related to the concentration of the components. It is unlikely that life could have evolved in the vastness of the prebiotic oceans, where biomolecules would be highly dilute and dispersed.

For an environment to be suitable for the origin of life, it must foster interactions between molecules, so that they form more complex entities; not dilute them, as would happen in the deep ocean.

Proponents of the hydrothermal theory suggest that life could have originated in delimited areas that prevented the dilution of newly formed molecules, such as craters.

References

1. Chatterjee, S. (2016). A symbiotic view of the origin of life at hydrothermal impact crater – lakes. Physical Chemistry Chemical Physics, 18 (30), 20033–20046.
2. Corliss, JB, Baross, JA, & Hoffman, SE (1981). An hypothesis concerning the relationships between submarine hot springs and the origin of life on earth. Oceanologica Acta, Special issue.
3. Holm, NG (1992). Why are hydrothermal systems proposed as plausible environments for the origin of life ?. In Marine Hydrothermal Systems and the Origin of Life (pp. 5–14). Springer, Dordrecht.
4. Jheeta, S. (2017). The Landscape of the Emergence of Life. Life, 7 (2), 27.
5. Lanier, KA, & Williams, LD (2017). The Origin of Life: Models and Data. Journal of Molecular Evolution, 84 (2), 85–92.