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An inherent characteristic of humans is the tendency to group together objects or beings that have similar characteristics. Primitive man, for example, already distributed living things into two groups: edible and inedible.
The distribution of objects or beings into groups according to their similarities and differences is what is called classification.
The branch of biology that deals with the description, nomenclature and classification of living beings is called systematic or taxonomy.
The attempt to systematize the living world is very old and the criteria employed by naturalists varied widely. Some classified them as flying and non-flying, based on locomotion; others classified them as aquatic, aerial and terrestrial, based on habitat.
These classification systems that use arbitrary criteria are called artificial systems. They do not reflect the fundamental similarities and differences between living things.
Currently, classification systems consider a set of relevant characters, which allow to verify the evolutionary kinship relations and to establish the phylogeny of the different groups, that is, to establish the main lines of evolution of these groups. They are known for natural systems, because they naturally order the organisms, aiming at the establishment of the evolutionary kinship relations between them.
Phylogenetic tree of living beings
Thus within the evolutionary characteristics, when talking about animals and plants, for example, we can use as a classification criterion the type of nutrition: animals are heterotrophic beings; plants autotrophs. When considering bacteria and animals, we can use as a criteria for classification the number and type of cells: bacteria are unicellular and prokaryotes; animals are multicellular and eukaryotic.