Cavalier-Smith's system of classification

The initial version of a classification system of life by British zoologist Thomas Cavalier-Smith appeared in 1978.^[1][2] This initial system continued to be modified in subsequent versions that were published until he died in 2021. As with classifications of others, such as Carl Linnaeus, Ernst Haeckel, Robert Whittaker, and Carl Woese, Cavalier-Smith's classification attempts to incorporate the latest developments in taxonomy.^[3][4], Cavalier-Smith used his classifications to convey his opinions about the evolutionary relationships among various organisms, principally microbial. His classifications complemented his ideas communicated in scientific publications, talks, and diagrams. Different iterations might have a wider or narrow scope, include different groupings, provide greater or lesser detail, and place groups in different arrangements as his thinking changed. His classifications has been a major influence in the modern taxonomy, particularly of protists. ^{[citation needed]}

Cavalier-Smith has published extensively on the

According to Palaeos.com:

Prof. Cavalier-Smith of Oxford University has produced a large body of work which is well regarded. Still, he is controversial in a way that is a bit difficult to describe. The issue may be one of writing style. Cavalier-Smith has a tendency to make pronouncements where others would use declarative sentences, to use declarative sentences where others would express an opinion, and to express opinions where angels would fear to tread. In addition, he can sound arrogant, reactionary, and even perverse. On the other [hand], he has a long history of being right when everyone else was wrong. To our way of thinking, all of this is overshadowed by one incomparable virtue: the fact that he will grapple with the details. This makes for very long, very complex papers and causes all manner of dark murmuring, tearing of hair, and gnashing of teeth among those tasked with trying to explain his views of early life. See, [for example], Zrzavý (2001)^[5] [and] Patterson (1999).^[6][7][8] Nevertheless, he deals with all of the relevant facts.^[9]

Eight kingdoms model

The first two kingdoms of life: Plantae and Animalia

The use of the word "

By mid-nineteenth century, microscopic organisms were generally classified into four groups:

1. Protozoa (primitive animals),
2. Protophyta
   (primitive plants),
3. Phytozoa
   (animal-like plants & plant-like animals), and

In 1858,

By 1959, Robert Whittaker proposed that fungi, which were formerly classified as plants, be given their own kingdom. Therefore, he divided life into four kingdoms such as:

1. Protista, (or unicellular organisms);
2. Plantae
   , (or multicellular plants);
3. Fungi; and
4. Animalia (or
   multicellular
   animals).

Whittaker subdivided the Protista into two subkingdoms:

1. Monera (bacteria) and
2. Eunucleata (single celled
   eukaryotes).^[15]

1. Plantae,
2. Animalia,
3. Protista (Eunucleata),
4. Fungi, and
5. Monera (the kingdom bacteria).[16]

The sixth kingdom: Archaebacteria

The kingdom Monera can be divided into two distinct groups:

1. Eubacteria (all typical bacteria),
2. Archaebacteria (methanogens), and
3. Urkaryotes (all eukaryotes).[18]

In 1990, Woese introduced domain above kingdom by creating three-domain system such as:

1. Bacteria,
2. Archaea, and
3. Eucarya.^[17]

But Cavalier-Smith considered Archaebacteria as a kingdom.^[19]

The seventh kingdom: Chromista

By 1981, Cavalier-Smith had divided all the eukaryotes into nine kingdoms.^[20] In it, he created Chromista for a separate kingdom of some protists.^[21]

Most chromists are

Based on the addition of Chromista as a kingdom, he suggested that even with his nine kingdoms of eukaryotes, "the best one for general scientific use is a system of seven kingdoms",[20] which includes:

1. Plantae
   ,
2. Animalia
   ,
3. Protozoa,
4. Chromista
5. Fungi
   ,
6. Eubacteria
   , and
7. Archaebacteria
   .

The eighth kingdom: Archezoa

In 1983, Cavalier-Smith adapted the term Archezoa, previously used by Haeckel, Perty, and for protists that lack

Archezoa is now defunct.[40] He now assigns former members of the kingdom Archezoa to the phylum Amoebozoa.^[41]

Kingdom Protozoa sensu Cavalier-Smith

Cavalier-Smith referred to what remained of the protist kingdom, after he removed the kingdoms Archezoa and Chromista, as the "kingdom Protozoa". In 1993, this kingdom contained 18 phyla as summarized in the following table.^[13] The first major division relied on the basis of presence or absence of dictyosomes, although a Gogli Apparatus was subsequently demonstrated in the 'Adictyozoa'.^[42]

The phylum Opalozoa was established by Cavalier-Smith in 1991.^[45]

Six kingdoms models

By 1998, Cavalier-Smith had reduced the total number of kingdoms from eight to six: Animalia, Protozoa, Fungi, Plantae (including

• Eubacteria
• Neomura
  • Archaebacteria
  • Eukaryotes
    • Kingdom Protozoa
    • heterotrophs
      )
      • Kingdom Animalia
      • Kingdom Fungi
    • photosynthetic
      )
      • Kingdom Plantae (including red and green algae)
      • Kingdom Chromista

Eukaryotes are divided into two major groups: Unikont and Bikont. Uniciliates are cells with only one flagellum and unikonts are descended from uniciliates. Unikont cells often have only one centriole as well. Biciliate cells have two flagella and bikonts are descended from biciliates. Biciliates undergo ciliary transformation by converting a younger anterior flagellum into a dissimilar older posterior flagellum. Animals and fungi are unikonts while plants and chromists are bikonts. Some protozoa are unikonts while others are bikonts.

The Bacteria (= prokaryotes) are subdivided into Eubacteria and Archaebacteria. According to Cavalier-Smith, Eubacteria is the oldest group of terrestrial organisms still living. He classifies the groups which he believes are younger (archaebacteria and eukaryotes) as Neomura.

The 1998 model

Kingdom Animalia

In 1993, Cavalier-Smith classified

• Porifera, Cnidaria, Placozoa, and Ctenophora
  ),
• Myxozoa,
• Mesozoa, and
• Bilateria (all other animal phyla).

He created five new animal phyla:

• gnathostomulids
  ),
• phoronids
  ),
• tardigrades
  ),
• Kamptozoa (Entoprocta and Symbion
  ), and
• Nemathelminthes (Nematoda, Nematomorpha, Loricifera, Priapulida, and Kinorhyncha)

and recognized a total of 23 animal phyla, as shown here:

• Kingdom Animaia
  • Subkingdom Radiata
    • Infrakingdom
      Spongiaria
      • Phylum
        Porifera
    • Infrakingdom Coelenterata
      • Phylum Cnidaria
      • Phylum Ctenophora
    • Infrakingdom Placozoa
      • Phylum Placozoa
  • Subkingdom Myxozoa
    • Phylum Myxosporidia
  • Subkingdom Bilateria
    • Branch
      Protostomia
      • Infrakingdom Lophozoa
        • Superphylum Polyzoa
          • Phylum Bryozoa
          • Phylum
            Cycliophora
            )
        • Superphylum Conchozoa
          • Phylum Mollusca
          • Phylum
            Phoronida
            )
        • Superphylum Sipuncula
          • Phylum Sipuncula
        • Superphylum Vermizoa
          • Phylum Nemertina
          • Phylum
            Annelida
      • Infrakingdom Chaetognathi
        • Phylum Chaetognatha
      • Infrakingdom Ecdysozoa
        • Superphylum Nemathelminthes
          • Phylum
            Nematoda and Nematomorpha; Priapozoa, Kinorhyncha and Loricifera
            )
        • Superphylum Haemopoda
          • Phylum
            Tardigrada
            )
          • Phylum
            Arthropoda
      • Infrakingdom Platyzoa
        • Phylum
          Platyhelminthes (incl. Xenacoelomorpha
          )
        • Phylum
          Gastrotricha
          )
    • Branch
      Deuterostomia
      • Infrakingdom Coelomopora
        • Phylum Echinodermata
        • Phylum
          Hemichordata
      • Infrakingdom
        Chordonia
        • Phylum
          Urochorda
        • Phylum
          Chordata
  • Subkingdom Mesozoa
    • Phylum Mesozoa^[44]

Kingdom Protozoa

Under Cavalier-Smith's proposed classification system, protozoa share the following traits:

• they have or are descended from organisms with mitochondria
• they have or are descended from organisms with
  peroxisomes
• they lack collagenous connective tissue
• they lack epiciliary retronemes (rigid thrust-reversing tubular ciliary hairs)
• they lack two additional membranes outside their chloroplast envelope

Organisms that do not meet these criteria were reassigned to other kingdoms by Cavalier-Smith.

The 2003 model

Kingdom Protozoa

In 1993, Cavalier-Smith divided the kingdom Protozoa into two subkingdoms and 18 phyla.

• Unikonts
  • protozoan phylum Amoebozoa (ancestrally uniciliate)
  • opisthokonts
    • uniciliate protozoan phylum Choanozoa
    • kingdom Fungi
    • kingdom Animalia
• Bikonts
  • protozoan infrakingdom Rhizaria
    • phylum Cercozoa
    • phylum
      Radiozoa and Foraminifera
      )
  • protozoan infrakingdom Excavata
    • phylum Loukozoa
    • phylum
      Metamonada
    • phylum Euglenozoa
    • phylum Percolozoa
  • protozoan phylum
    Diphylleida
    )
  • the
    chromalveolate
    clade
    • kingdom
      Haptophyta
      )
    • protozoan infrakingdom
      Alveolata
      • phylum
        Ciliophora
      • phylum
        Dinozoa, and Apicomplexa
        )
  • kingdom
    Glaucophyta
    )

By September 2003, Cavalier-Smith's tree of life looked like this:[46]

In the above tree, the traditional plant, animal, and fungal kingdoms, as well as Cavalier-Smith's proposed kingdom Chromista, are shown as leaves. The leaves Eubacteria and Archaebacteria together make up the kingdom Bacteria. All remaining leaves together make up the kingdom Protozoa.

By 2006, Cavalier-Smith's microbial tree look like this: Cavalier-Smith's Tree of Life, 2006^{[cstol 1]}

Legend:
[A]