The archaea are a group of unicellular organisms like bacteria, but so different from those that are classified in their own taxonomic kingdom.

Classification of living beings, broadly speaking

There is a more or less consensual scientific criterion that viruses are not living beings , since they lack the necessary enzymes to replicate, so they must necessarily parasitize another organism. They have their own genetic material, but no way to replicate it without resorting to a host cell.

Thus, excluding viruses due to their metabolic rarity, living beings are divided into two large groups: those that have a cell nucleus (called eukaryotes , a word that derives from the Greek term “eukaryon”, which means “true nucleus) and those that lack of the. Eukaryotic organisms can be unicellular or multicellular, depending on their number of cells.

Eukaryotic organisms make up a taxonomic domain, which is the largest unit of classification of organisms, and in turn the eukaryotic domain is divided into three kingdoms: animals, plants, and fungi. On the other hand, there are two groups of prokaryotic organisms (without a nucleus): bacteria and archaebacteria or archaea. Each of these groups is its own domain and kingdom.

Thus, living beings are classified into three domains and five kingdoms.


Archaea , also called archaea , were originally classified as bacteria. This was due to the fact that both bacteria and archaea lack a cell nucleus, as well as the majority of organelles present in eukaryotic cells. But the metabolic differences between the two groups are so great that they ended up being classified into different domains.

The most striking characteristic of archaea is that many live in extreme environmental conditions, such as salty lakes or hot water fumaroles on the ocean floor . Organisms that inhabit these conditions are known as Extremophiles.

The metabolic pathways of archaea are quirky and unique among living things. Some are capable of obtaining energy from ammonia, by sulfur or by using hydrogen. Archaea capable of obtaining energy from sunlight and others capable of fixing their carbon from the CO2 present in the air are known. But, unlike what happens to plants when they photosynthesize, there is no known archaea capable of both at the same time (fix carbon using sunlight).

Archaea began to be studied in the 18th century in Italy, when methane bubbling was observed in Lake Maggiore. This phenomenon was due to the fact that the archaea at the bottom of the lake synthesized methane gas, which rose to the surface. It wasn’t until 1936 that the species that did this were identified, but they were initially believed to be bacteria.

In 1977, the first taxonomic classification was made that included archaea as a separate group from bacteria , thanks to a classification of ribosomal RNA carried out by Woese and Fox. Ribosomes are cellular organelles that carry out the transcription of DNA into proteins. The ribosomes of bacteria are smaller in size than those present in archaea and eukaryotes.

Originally only extremophilic archaea were known, but in recent years species of archaea have been discovered that inhabit environments that are not so aggressive for life.

Image 1 Hot springs in Yellowstone Park, USA. Archaea can inhabit this type of extreme environments, where other forms of life are not conceivable.

Evolution of archaea

Studying the origin of life is complicated, mainly because no one was there to take notes and analyzing the samples that are left from that time is extremely difficult. If it is already difficult to study the evolution of animals because the former lacked shells, bones or other hard parts, it is more difficult to study the fossil remains of bacteria and archaea. But it can be done.

The most widely accepted hypothesis is that archaea evolved from primitive bacteria , and that this evolution was due to a group of bacteria beginning to synthesize antibiotics. Antibiotics are a multitude of compounds that affect metabolic pathways or other processes necessary for life, causing cell death.

Different living things are susceptible to different antibiotics due to metabolic differences. This is why some antibiotics are effective against a certain bacterial strain and others are not. This is also why the antibiotics we take do not affect us directly, which would mean that they would kill us (although they can affect our gut microbiome, the bacteria that live in our gut).

It is speculated that archaea evolved to become resistant to the antibiotics that some bacteria generated because many genes that distinguish archaea and bacteria are associated with resistance to antibiotics of bacterial origin. This would also explain why archaea evolved to adapt to extreme environments, as they were looking for ecological niches that were not occupied by bacteria.

The main differences between archaea and bacteria , apart from resistance to antibiotics, are the structure of the cell membrane, the size of their ribosomes and their flagella. The cell membrane of archaea is different from that of bacteria and eukaryotes, having a unique chemical composition. Their flagella (organelles that single-celled organisms use to move around in liquid media) are also different.

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