Thomas Cavalier-Smith

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Thomas Cavalier-Smith
Thomas Cavalier-Smith drawing.png
Born(1942-10-21)21 October 1942
London, United Kingdom
Died19 March 2021(2021-03-19) (aged 78)
Alma materGonville and Caius College, Cambridge King's College London
Known forCavalier-Smith's system of classification of all organisms
AwardsFellow of the Royal Society (1998)
International Prize for Biology (2004)
The Linnean Medal (2007)
Frink Medal (2007)
Scientific career
InstitutionsKing's College London, University of British Columbia, University of Oxford
ThesisOrganelle Development in Chlamydomonas reinhardii' (1967)

Thomas (Tom) Cavalier-Smith, FRS, FRSC, NERC Professorial Fellow (21 October 1942 - 19 March 2021), was a Professor of Evolutionary Biology in the Department of Zoology, at the University of Oxford.[1] His research has led to discovery of a number of unicellular organisms (protists) and advocated for a variety of major taxonomic groups, such as the kingdom Chromista, chromalveolata, Opisthokonta, Rhizaria, and Excavata. He was known for his system of classification of all organisms.

Life and career[edit]

Cavalier-Smith was born on 21 October 1942 in London. His parents were Mary Maude (née Bratt) and Alan Hailes Spencer Cavalier Smith.[2] He was educated at Norwich School, Gonville and Caius College, Cambridge (MA) and King's College London (PhD). He was under the supervision of Sir John Randall for his PhD thesis between 1964 and 1967; his thesis was entitled "Organelle Development in Chlamydomonas reinhardii".[3]

From 1967 to 1969, he was a guest investigator at Rockefeller University. He became Lecturer of biophysics at King's College London in 1969. He was promoted to Reader in 1982. In 1989 he was appointed Professor of Botany at the University of British Columbia. In 1999, he joined the University of Oxford, becoming Professor of evolutionary biology in 2000.[4]

Awards and honours[edit]

Cavalier-Smith was elected Fellow of the Linnean Society of London (FLS) in 1980, the Institute of Biology (FIBiol) in 1983, the Royal Society of Arts (FRSA) in 1987, the Canadian Institute for Advanced Research (CIFAR) in 1988, the Royal Society of Canada (FRSC) in 1997, and the Royal Society of London in 1998.[5] He received the International Prize for Biology from the Emperor of Japan in 2004, and the Linnean Medal for Zoology in 2007. He was appointed Fellow of the Canadian Institute for Advanced Research (CIFAR) between 1998 and 2007, and Advisor of the Integrated Microbial Biodiversity of CIFAR.[6] He won the 2007 Frink Medal of the Zoological Society of London.[4]

Taxonomic Style[edit]

Smith was the most prolific source of ideas about, mostly microbial, evolutionary relationships. He was able to draw on an unparalleled wealth of information to suggest novel relationships. The suggestions were translated into taxonomic concepts and classifications with which he associated new names, or in some cases, reuse old names.

Smith pursued a distinctive path in the discipline that combined investigations into evolutionary paths (phylogeny) and the classification of the organisms that had resulted. He worked at a time when there was a pervasive presumption that classification should reflect evolutionary pathways. How this should be done had four major influences. First, was a residue of the ‘traditional’ approach that admitted speculation and which lacked any explicit rigor as to how a particular evolutionary insight should be translated into the arrangement and ranks of taxa. Inherent to this approach were narratives that were frequently dismissed as ‘just so’ statements, associated classifications which included paraphyletic and polyphyletic taxa, and in which subjective views rather than phylogenetic relationships would influence the rank assigned to taxa. The second influence was the philosophy as to how scientific progress was made. It had been articulated by Karl Popper [7] [8] and had shifted the emphasis from verification of ideas to the falsification of hypotheses. Popper saw that science progressed by a process that eliminated unsound hypotheses, so whittling down of the array of possible explanations leading towards the explanations which were more likely correct. To fit into this process, hypotheses needed to be falsifiable. The third influence was ‘cladistics’ – an explicit way of presenting (and then falsifying) evolutionary hypotheses. This was initially articulated by Willi Hennig,[9] and was increasingly baked into how taxonomy should be done.[10][11] Finally, technical advances in sequencing technology led to a massive growth of hypotheses about evolutionary relationships based on the similarities among sequences of compared organisms. Algorithms were used to analyse sequence data, with the results being usually presented in the form of dendrograms. These have a superficial similarity with the diagrams used to present evolutionary relationships (trees), and unfortunately gained the name of ‘phylogenies’. The dendrograms of molecular phylogenetics differed depending on the molecules chosen, the organisms included in an anlysis, and the algorithms used to run comparisons. ‘Molecular phylogenies’ were probabilistic summaries that lacked the exactness that was required of the process of progressing through falsifiable hypotheses. The result of this ‘molecular phylogenetics’ was a plethora of hypotheses which had the ability to hide the signal, and were often uncertain in detail.

At a time when Popperian interpretation of scientific progress was dominant, Smith was courageous in his adherence to the earlier traditionalist style characterized by Charles Darwin, that of relying on narratives. One example was his advocacy for the polyphyletic Chromista that united lineages that had plastids with chlorophylls a and c (primarily chrysophytes and other stramenopiles, cryptophytes, and haptophytes). It was Smith's 'just so' story that there was a single endosymbiotic event by which chlorophyll a c containing plastids were acquired by a common ancestor of all three groups, and that the differences (such as cytological components and their arrangements) among the groups were the result of subsequent evolutionary changes. This interpretation that chromists were monophyletic also required that the heterotrophic (protozoan) members of all three groups had arisen from ancestors with plastids. From a Popperian perspective, this was a hypothesis that was falsified (invalidated) from the by the many differences. The alternative hypothesis was that the three lineages were not closely related (to the exclusion of other lineages) (i.e. were polyphyletic), likely that all were ancestrally without plastids, and that separate symbiotic events established the chlorophyll a/c plastids stramenopiles, cryptomonads and haptophytes. The polyphyly of the chromists has been re-asserted in subsequent studies.[12] Smith’s rejection of a requirement for testable phylogenetic hypotheses, and the absence of an exact methodology that would translate evolutionary insights into taxa and hierarchical schemes, were sources of instability and discord. Many of his taxa requiring his frequent adjustment, as illustrated below. In turn this led to confusion as to the scope of taxa a taxonomic name was applied to. Smith also reused familiar names (such as Protozoa) for innovative taxonomic concepts. This created confusion because Protozoa was and still is used in its old sense, [13] alongside its use in the newer senses. This situation could have been resolved by referring to the context for a name (i.e. Protozoa sensu Goldfuß 1818, vs Protozoa sensu Smith 1999 vs Protozoa sensu Smith 2015), but such nuancing was not widely adopted. Because of Smith’s tendency to publish premature and unfalsifiable insights, the need to frequent changes, and the use of taxonomic names for different concepts, he was as much a source of controversy as of insight.


Tree of life and major steps in cell evolution after Cavalier-Smith, ca 2010, before his 2015 revision

Cavalier-Smith wrote extensively on the taxonomy and classification of all life forms, but especially protists. One of his major contributions to biology was his proposal of a new kingdom of life: the Chromista. He also introduced a new group for primitive eukaryotes called the Chromalveolata (1981), as well as Opisthokonta (1987), Rhizaria (2002), and Excavata (2002). Though well known, many of his claims have been controversial and have not gained widespread acceptance in the scientific community to date. His taxonomic revisions often lead to changes in the overall classification of all life forms.

Eight kingdoms model[edit]

Cavalier-Smith's first major classification system was the division of all organisms into eight kingdoms. In 1981, he proposed that by completely revising Robert Whittaker's Five Kingdom system, there could be eight kingdoms: Bacteria, Eufungi, Ciliofungi, Animalia, Biliphyta, Viridiplantae, Cryptophyta, and Euglenozoa.[14]

In 1993, he revised his system particularly in the light of the general acceptance of Archaebacteria as separate group from Bacteria. In addition, some protists lacking mitochondria were discovered.[15] As mitochondria were known to be the result of the endosymbiosis of a proteobacterium, it was thought that these amitochondriate eukaryotes were primitively so, marking an important step in eukaryogenesis. As a result, these amitochondriate protists were separated from the protist kingdom, giving rise to the, at the same time, superkingdom and kingdom Archezoa. This was known as the Archezoa hypothesis. The eight kingdoms became: Eubacteria, Archaebacteria, Archezoa, Protozoa, Chromista, Plantae, Fungi, and Animalia.[16]

However, kingdom Archezoa is now defunct.[17] He assigned former members of the kingdom Archezoa to the phylum Amoebozoa.[18]

Six kingdoms models[edit]

By 1998, Cavalier-Smith had reduced the total number of kingdoms from eight to six: Animalia, Protozoa, Fungi, Plantae (including Glaucophyte, red and green algae), Chromista and Bacteria.[19] Nevertheless, he had already presented this simplified scheme for the first time on his 1981 paper[14] and endorsed it in 1983.[20]

Table 11 From Eukaryote kingdoms: seven or nine?

Five of Cavalier-Smith's kingdoms are classified as eukaryotes as shown in the following scheme:

The kingdom Animalia was divided into four subkingdoms: Radiata (phyla Porifera, Cnidaria, Placozoa, and Ctenophora), Myxozoa, Mesozoa, and Bilateria (all other animal phyla).

He created three new animal phyla: Acanthognatha (rotifers, acanthocephalans, gastrotrichs, and gnathostomulids), Brachiozoa (brachiopods and phoronids), and Lobopoda (onychophorans and tardigrades) and recognised a total of 23 animal phyla.[19]

Cavalier-Smith's 2003 classification scheme:[21]

Seven kingdoms model[edit]

Cavalier-Smith and his collaborators revised the classification in 2015, and published it in PLOS ONE. In this scheme they reintroduced the division of prokaryotes into two kingdoms, Bacteria (=Eubacteria) and Archaea (=Archebacteria). This is based on the consensus in the Taxonomic Outline of Bacteria and Archaea (TOBA) and the Catalogue of Life.[22]

Rooting the tree of life[edit]

In 2006, Cavalier-Smith proposed that the last universal common ancestor to all life was a non-flagellate negibacterium with two membranes.[23]


  1. ^ "Professor Dr Tom Cavalier-Smith, FRS, FRSC, Professor of Evolutionary Biology and NERC Professorial Fellow in the Department of Zoology, Oxford University". Cavali. Archived from the original on 4 March 2016. Retrieved 11 February 2016.
  2. ^ Marriage of Alan C Smith and Mary Maud Bratt 1st Qtr 1942 Wayland Reg Dist. (GRO Ref 4b/743.) Birth of Thomas C Smith (mother's maiden name Bratt) last Qtr 1942, Greenwich Reg Dist. (GRO Ref 1d/650). Source
  3. ^ Cavalier-Smith, Thomas (1967). Organelle development in Chlamydomonas reinhardii (PhD thesis). University of London. OCLC 731219097.[page needed][non-primary source needed]
  4. ^ a b "Thomas (Tom) CAVALIER-SMITH". Debrett's. Archived from the original on 15 March 2016. Retrieved 11 February 2016.
  5. ^ "Awards and distinctions". Cavali. Archived from the original on 23 July 2016. Retrieved 11 February 2016.
  6. ^ "Thomas Cavalier-Smith". Canadian Institute for Advanced Research. Retrieved 11 February 2016.
  7. ^ Popper, K. 1934 The Logic of Scientific Discovery (as Logik der Forschung, English translation 1959), ISBN 0-415-27844-9
  8. ^ Popper, K. 1963. Conjectures and Refutations: The Growth of Scientific Knowledge, ISBN 0-415-04318-2
  9. ^ Hennig, W. 1966. Phylogenetic Systematics. University of Illinois Press, Urbana.
  10. ^ Wiley, E.O. 1981. Phylogenetics: The Theory and Practice of Phylogenetic Systematics. Wiley Publishers
  11. ^ N. Eldredge & J. Cracraft 1980. Phylogenetic Patterns and the Evolutionary Process. Method and Theory in Comparative Biology. New York: Columbia University Press. ISBN 0 231 03802 X. (1981)
  12. ^ Burki, F. Roger, A.J., Brown, M.W., Simpson, A.G.B. 2020. The new tree of eukaryotes. Trends in Ecology and Evolution, 35: 43-55.
  13. ^ El-Bawab, F. 2020. Invertebrate Embryology and Reproduction, Chapter 3 – Phylum Protozoa. Academic press, pp 68-102.
  14. ^ a b Cavalier-Smith, T. (1981). "Eukaryote kingdoms: Seven or nine?". Biosystems. 14 (3–4): 461–481. doi:10.1016/0303-2647(81)90050-2. PMID 7337818.
  15. ^ Cavalier-Smith, Thomas (1987). "Eukaryotes with no mitochondria". Nature. 326 (6111): 332–333. Bibcode:1987Natur.326..332C. doi:10.1038/326332a0. PMID 3561476.
  16. ^ Cavalier-Smith, T (1993). "Kingdom protozoa and its 18 phyla". Microbiological Reviews. 57 (4): 953–994. doi:10.1128/mmbr.57.4.953-994.1993. PMC 372943. PMID 8302218.
  17. ^ Cavalier-Smith, T.; Chao, E. E. (1996). "Molecular phylogeny of the free-living archezoanTrepomonas agilis and the nature of the first eukaryote". Journal of Molecular Evolution. 43 (6): 551–62. Bibcode:1996JMolE..43..551C. doi:10.1007/BF02202103. PMID 8995052. S2CID 28992966.
  18. ^ Cavalier-Smith, T. (2004). "Only six kingdoms of life". Proceedings of the Royal Society B: Biological Sciences. 271 (1545): 1251–62. doi:10.1098/rspb.2004.2705. PMC 1691724. PMID 15306349.
  19. ^ a b Cavalier-Smith, T. (2007). "A revised six-kingdom system of life". Biological Reviews. 73 (3): 203–66. doi:10.1111/j.1469-185X.1998.tb00030.x. PMID 9809012. S2CID 6557779.
  20. ^ Cavalier-Smith, T. (1983). Schenk, HEA; Schwemmler, WS (eds.). "A 6-kingdom classification and a unified phylogeny". Endocytobiology II: Intracellular Space as Oligogenetic. Berlin: Walter de Gruyter & Co.: 1027–1034. doi:10.1515/9783110841237-104. ISBN 9783110841237.
  21. ^ Cavalier-Smith, Thomas (2003). "Protist phylogeny and the high-level classification of Protozoa". European Journal of Protistology. 39 (4): 338–348. doi:10.1078/0932-4739-00002.
  22. ^ Ruggiero, Michael A.; Gordon, Dennis P.; Orrell, Thomas M.; Bailly, Nicolas; Bourgoin, Thierry; Brusca, Richard C.; Cavalier-Smith, Thomas; Guiry, Michael D.; Kirk, Paul M.; Thuesen, Erik V. (2015). "A higher level classification of all living organisms". PLOS ONE. 10 (4): e0119248. Bibcode:2015PLoSO..1019248R. doi:10.1371/journal.pone.0119248. PMC 4418965. PMID 25923521.
  23. ^ Cavalier-Smith, Thomas (2006). "Rooting the tree of life by transition analyses". Biology Direct. 1: 19. doi:10.1186/1745-6150-1-19. PMC 1586193. PMID 16834776.

External links[edit]