Classification
Even back to Antiquity the two clades of animals and plants were recognized. They were given the taxonomic rank of Kingdom (biology) by Linnaeus. Though he included the fungi with plants with some reservations, it was later realized that they are quite distinct and warrant a separate kingdom, the composition of which was not entirely clear until the 1980s. The various single-cell eukaryotes were originally placed with plants or animals when they became known. The German biologist Georg A. Goldfuss coined the word protozoa in 1830 to refer to organisms such as ciliates and this group was expanded until it encompassed all single-cell eukaryotes, and given their own kingdom, the Protista by Ernst Haeckel in 1866. The eukaryotes thus came to be composed of four kingdoms:
- Kingdom Protista
- Kingdom Plantae
- Kingdom Fungi
- Kingdom Animalia
The protists were understood to be "primitive forms", and thus an evolutionary grade, united by their primitive unicellular nature. The disentanglement of the deep splits in the tree of life only really got going with DNA sequencing, leading to a system of domains rather than kingdoms as top level rank being put forward by Carl Woese, uniting all the eukaryote kingdoms under the eukaryote domain. At the same time, work on the protist tree intensified, and is still actively going on today. Several alternative classifications have been forwarded, though there is no consensus in the field.
A classification produced in 2005 for the International Society of Protistologists, which reflected the consensus of the time, divided the eukaryotes into six supposedly monophyletic 'supergroups'. Although the published classification deliberately did not use formal taxonomic ranks, other sources have treated each of the six as a separate Kingdom.
| Excavata | Various flagellate protozoa |
| Amoebozoa | Most lobose amoeboids and slime moulds |
| Opisthokonta | Animals, fungi, choanoflagellates, etc. |
| Rhizaria | Foraminifera, Radiolaria, and various other amoeboid protozoa |
| Chromalveolata | Stramenopiles (or Heterokonta), Haptophyta, Cryptophyta (or cryptomonads), and Alveolata |
| Archaeplastida (or Primoplantae) | Land plants, green algae, red algae, and glaucophytes |
However, in the same year (2005), doubts were expressed as to whether some of these supergroups were monophyletic, particularly the Chromalveolata, and a review in 2006 noted the lack of evidence for several of the supposed six supergroups.
Read more about this topic: Eukaryote