We explain what the biological kingdoms are and the history of these sets of species. In addition, the characteristics of each one.
What are the biological kingdoms?
In biology, and especially taxonomy, each of the the large groups into which known species of living beings are classified, attending to their evolutionary kinship, that is, to their place of origin in the long history of life. It is the second level of classification of living beings, below the domains and above the phyla (or phylum).
Throughout the history of science, human beings have made efforts to understand the dynamics of origin and change inherent to life, and for this they have developed these classification systems, which in turn have varied a lot over time.
As scientists understand more and more in detail the characteristics of living things, new possibilities for classification appear and old assumptions are considered obsolete. For this reason, there are various classification systems in biological kingdoms, not always coincident with each other.
The latest and most widely accepted system is the proposed by Anglo-Canadian Thomas Cavalier-Smith in 2015Although there is still debate about it in the specialized scientific community.
History of the biological kingdoms
The first systems for classifying life date back to ancient times, when ancient philosophers proposed approaches to life by distinguishing between its basic observable properties. Thus, we have:
- Two kingdom system. Attributed to the Greek philosopher Aristotle (IV BC), he divided living beings into two large categories, based on what the theorist called “vegetative soul” and “sensitive soul.” In the first case, it translated into the ability to grow, nurture and reproduce, while in the second this also included desire, movement and perception. This system was inherited a long time later by the famous Swedish scientist and naturalist Carlos Linnaeus, who proposed in 1735 a classification system of two kingdoms proper: Vegetabilia Y Animalia.
- Three kingdom system. A third kingdom would emerge for the first time in 1858, when the English biologist Richard Owen realized the difficulty of classifying certain microorganisms based on the two kingdoms of Linnaeus, and proposed a third: Protozoa, composed of microscopic beings made up of nucleated cells. . This new kingdom was renamed Proctista in 1860 by the also English John Hogg, although in his considerations he also proposed the existence of a “mineral kingdom”, which was later discarded by Ernst Haeckel, father of protistology, who in 1865 baptized the third kingdom as Protista and included in it all forms of microscopic life with animal, vegetable and mixed characters, but distinguishing for the first time between unicellular and multicellular organisms.
- Four kingdoms system. As microbiology progressed, the three-kingdom system required rethinking, as the distinction between prokaryotic (without a cell nucleus) and eukaryotic (with a cell nucleus) organisms became more evident and important. And to distinguish between nucleated and non-nucleated microorganisms, Herbert Copeland in 1938 proposed a system of four kingdoms: Animalia, Plantae, Protoctist and a new group for enucleated bacteria: Monera.
- Five kingdoms system. The fifth kingdom arose in 1959 when Robert Whittaker found that the fungi were a totally different group from the plant, and in 1969 he proposed a system of five kingdoms that included the Fungi (mushrooms), and kept all four of Copeland. This was one of the most popular systems in history.
- Six kingdoms system. The advancement of DNA and RNA study and exploration techniques in the second half of the 20th century revolutionized many of the assumptions of biology, allowing Carl Woese and G. Fox to reinvent the system and propose six different kingdoms: Bacterium, Archaea, Protista, Plantae, Animalia Y Fungi. These six kingdoms are divided, in turn, into two domains: Prokaryota (Bacteria and Archea) Y Eukaryota (the rest). In many places this is the accepted system.
- Seven kingdoms system. The work of the Canadian Cavalier-Smith and later developers, he proposed the creation of the kingdom Chromista to distinguish diatoms, oomycetes and similar algae, and recovering the name Protozoa for the rest of the eukaryotic microorganisms. Thus, the seven kingdoms would be the two of prokaryotes: Archaea Y Bacterium, and five from eukaryotes: Protozoa, Chromista, Plantae, Fungi, Animals.
Kingdom bacterium
One of the two kingdoms of prokaryotic organisms, that is, without a cell nucleus and with much simpler and smaller cell structures, encompasses the most abundant and diverse single-celled microscopic beings on the planet, that lead a photosynthetic, saprophytic and even parasitic existence, in practically all habitats of the world. They have a peptidoglycan wall that allows them to be classified into two types: Gram negative (they present double wall) or Gram positive (they present single wall).
Follow in: Monera Kingdom
Kingdom archaea
This is the other type of known prokaryotes, lacking peptidoglycan cell walls, non-pathogenic and present in very extreme habitats, since its nutrition is based on chemosynthesis, that is, the use of specific chemical reactions in anaerobic environments (in the absence of oxygen). The existence of archaea or archaebacteria was known since the 18th century, but their difference from bacteria was not understood until the 20th century.
Kingdom protozoa
This kingdom is considered the basal group of eukaryotes, that is, the first to emerge, from which the others would later have emerged. It is a paraphyletic group, that is, they includes the first common ancestor but not all of their descendants.
Here we can find, then, eukaryotic unicellular microorganisms, usually flagellated, without a cell wall and that do not form tissues, dedicated to heterotrophic nutrition, either saprophytic or predating other microorganisms, such as bacteria and other protists.
Kingdom chromist
It is a eukaryotic kingdom of organisms without too many common characteristics, but which can be summarized in various types of algae, which were traditionally classified within the plant or fungal kingdom, since may or may not have chlorophyll or additional pigments. Many chromists, in fact, can lead a parasitic life. This group includes unicellular and multicellular algae, oomycetes and apicomplexes.
Kingdom plant
A group of photosynthetic multicellular eukaryotes, that is, that carry out the synthesis of sunlight, absorbing atmospheric CO2 and releasing oxygen in return. This group it is indispensable for the support of life as we know it, especially land plants. They are characterized by cells with a cellulose wall, by their immobile life and by their sexual or asexual reproduction depending on the species and the given conditions.
Follow in: Kingdom plantae
Kingdom fungi
The kingdom of fungi, that is, of Aerobic and heterotrophic multicellular eukaryotic organisms, unable to synthesize their nutrients and therefore dedicated to a saprophytic or parasitic existence: they either act as decomposers of organic matter, or they infect the bodies of other living organisms. They have a cell wall like plants, but made of chitin instead of cellulose, and they reproduce by means of spores, both sexually and asexually.
Kingdom animalia
The last kingdom is that of animals, that is, that of eukaryotic multicellular organisms. endowed with their own mobility and a heterotrophic metabolism, supported by respiration: the consumption of oxygen and organic matter from other living beings, for their oxidation and obtaining chemical energy and expulsion of CO2. Animals are an enormously diverse kingdom, spread across aquatic, terrestrial, and even air habitats, which can be divided into two groups: vertebrates and invertebrates, depending on whether or not they have backbones and endoskeletons.