The history of taxonomy stretches back to the 4th century BC, to the times of Aristotle and before. Since the first classification systems emerged, dividing the world of life into various groups with various relationships, scientists have grappled with the task of keeping classification in sync with scientific evidence.
The sections that follow provide a summary of the changes that have taken place at the highest level of biological classification over the history of taxonomy.
Two Kingdoms (Aristotle, during 4th century BC)
Classification system based on: Observation (phenetics)
Aristotle was among the first to document the division of life forms into animals and plants. Aristotle classified animals according to observation, for example, he defined high-level groups of animals by whether or not they had red blood (this roughly reflects the division between vertebrates and invertebrates used today).
- Plantae - plants
- Animalia - animals
Three Kingdoms (Ernst Haeckel, 1894)
Classification system based on: Observation (phenetics)
The three kingdom system, introduced by Ernst Haeckel in 1894, reflected the long-standing two kindoms (Plantae and Animalia) that can be attributed to Aristotle (perhaps before) and added third kingdom, Protista that included single-celled eukaryotes and bacteria (prokaryotes).
- Plantae - plants (mostly autotrophic, multicelluar eukaryotes, reproduction by spores)
- Animalia - animals (heterotrophic, multicelluar eukaryotes)
- Protista - single celled eukaryotes and bacteria (prokaryotes)
Four Kingdoms (Herbert Copeland, 1956)
Classification system based on: Observation (phenetics)
The important changed introduced by this classification scheme was the introduction of the Kingdom Bacteria. This reflected the growing understanding that bacteria (single-celled prokaryotes) were very much different from single-celled eukaryotes. Previously, single-celled eukaryotes and bacteria (single-celled prokaryotes) were grouped together in the Kindom Protista. But Copeland elevated Haeckel's two Protista phyla to the level of kindom.
- Plantae - plants (mostly autotrophic, multicelluar eukaryotes, reproduction by spores)
- Animalia - animals (heterotrophic, multicelluar eukaryotes)
- Protista - single-celled eukaryotes (lack tissues or extensive cellular differentiation)
- Bacteria - bacteria (single-celled prokaryotes)
Five Kingdoms (Robert Whittaker, 1959)
Classification system based on: Observation (phenetics)
Robert Whittaker's 1959 classification scheme added a fifth kingdom to Copeland's four kingdoms, the Kindom Fungi (single and multi-celluar osmotrophic eukaryotes)
- Plantae - plants (mostly autotrophic, multicelluar eukaryotes, reproduction by spores)
- Animalia - animals (heterotrophic, multicelluar eukaryotes)
- Protista - single-celled eukaryotes (lack tissues or extensive cellular differentiation)
- Monera - bacteria (single-celled prokaryotes)
- Fungi (single and multi-celluar osmotrophic eukaryotes)
Six Kingdoms (Carl Woese, 1977)
Classification system based on: Evolution and molecular genetics (Cladistics/Phylogeny)
In 1977, Carl Woese extended Robert Whittaker's Five Kingdoms to replace Kingdom bacteria with two kingdoms, Eubacteria and Archaebacteria. Archaebacteria differ from Eubacteria in their genetic transcription and translation processes (in Archaebactera, transcription and translation more closely resembled eukaryotes). These distinguising characteristics were shown by molecular genetic analysis.
- Plantae - plants (mostly autotrophic, multicelluar eukaryotes, reproduction by spores)
- Animalia - animals (heterotrophic, multicelluar eukaryotes)
- Eubacteria - bacteria (single-celled prokaryotes)
- Archaebacteria - prokaryotes (differ from bacteria in their genetic transcriptionand translation, more similar to eukaryotes)
- Protista - single-celled eukaryotes (lack tissues or extensive cellular differentiation)
- Fungi - single and multi-celluar osmotrophic eukaryotes
Three Domains (Carl Woese, 1990)
Classification system based on: Evolution and molecular genetics (Cladistics/Phylogeny)
In 1990, Carl Woese put forth a classification scheme that greatly overhauled previous classification schemes. The three-domain system he proposed is baseed on molecular biology studies, and resulted in the placement of organisms into three domains.
- Bacteria
- Archaea
- Eukarya
