por What is the hydrothermal hypothesis?
The hydrothermal hypothesis It aims to explain the primitive conditions in which the first forms of life on planet Earth originated, proposing as the main setting the thermal water springs located in the depths of the oceans.
A series of hot springs have been located with temperatures reaching 350 °C, where a series of fauna typical of these conditions inhabit, such as bivalves, worms, crustaceans, porifera and some echinoderms (starfish and their relatives).
This evidence suggests that deep ocean environments were likely suitable for the origin of life and the earliest forms of life were chemoautotrophic microorganisms.
In addition, a series of chemosynthetic bacteria inhabit boiling water 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 of hydrothermal systems located in the Galapagos Islands.
Background and alternative theories
For several decades, researchers have proposed dozens of theories that attempt to explain the origin of life and the favorable environment in which it must 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 1900’s 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 space microorganisms from a planet where life already 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.
In addition, it is not very likely that the microscopic entities that colonized prebiotic environments have survived the conditions of space to reach planet Earth.
abiotic models
Abiotic models propose that life originated from «microstructures» as transitional forms between organic molecules and the first forms of life. Among the main defenders of this theory are Oparín, Sydney W. Fox and Alfonso F. Herrera.
According to Oparin and Haldane, coacervates are precursor probionts of life, delimited by a plasmatic 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 imitated «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.
Fox managed 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 on the origin of life postulates the appearance of molecules that contain genetic information as the main event. Various authors defend the origin of life from RNA and argue that this molecule served as a template and 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 is the hydrothermal hypothesis?
The hydrothermal hypothesis proposes these extreme aquatic environments as the suitable 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 Archaean fossils, modern submarine hydrothermal vent systems, and theoretical and experimental observations.
Hydrothermal systems are characterized by high energy fluxes, a highly reducing environment, and abundant clay minerals, which are ideal surfaces for catalytic reactions. In addition, it has high concentrations of CH4, NH3, H2 and different metals.
The hypothesis consists of the sequential conversion of CH4, NH3, H2 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 have found cell-like structures dating to about 3.5 to 3.8 billion years ago in underwater hydrothermal assemblages.
In contrast to the previous hypotheses, the hydrothermal hypothesis proposes heat as the source of energy and not UV rays and electrical discharges, like the «primal soup» model. Furthermore, this model proposes the existence of environmental gradients in terms of temperature, pH and chemical concentration.
Criticism of the hydrothermal hypothesis
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 inconsistency and lack of information regarding the geological models of the Prebiotic Era.
Similarly, the molecules essential for the development of life —such as nucleic acids, proteins, and membranes— would face immediate destruction, due to the extremely high temperatures of hydrothermal environments.
However, it is also likely that the earliest life forms were thermostable, similar to the thermophilic organisms that inhabit extreme environments today.
On the other hand, another drawback arises related to the concentration of the components. It is unlikely that life could have evolved in the vastness of prebiotic oceans, where biomolecules would be highly diluted 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 ocean depths.
Defenders of the hydrothermal theory suggest that life could originate in delimited areas that prevented the dilution of newly formed molecules, such as craters.