Thomas (Tom) Cavalier-Smith, FRS, FRSC, NERC Professorial Fellow (21 October 1942 – 19 March 2021[1]), was a professor of evolutionary biology in the Department of Zoology, at the University of Oxford.

[2] 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 Chromista, Chromalveolata, Opisthokonta, Rhizaria, and Excavata.

[7][8] The scope of Cavalier-Smith's taxonomic propositions was grand, but the numbers and composition of the components (taxa), and, often, their relations were not stable.

His approach was not universally accepted: Others attempted to underpin taxonomy of protists with a nested series of atomised, falsifiable propositions, following the philosophy of transformed cladistics.

[10] Cavalier-Smith's ideas that led to the taxonomic structures were usually first presented in the form of tables and complex, annotated diagrams.

The richness of his ideas, their continuing evolution, and the transition into taxonomies that gave Cavalier-Smith's investigations into evolutionary paths (phylogeny) and the resulting classifications, its distinctive character.

Cavalier-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 Chromista that united lineages that had plastids with chlorophylls a and c (primarily chrysophytes and other stramenopiles, cryptophytes, and haptophytes) despite clear evidence that the group corresponded to a clade.

This interpretation that chromists were monophyletic also required that the heterotrophic (protozoan) members of all three groups had arisen from ancestors with plastids.

[11] Cavalier-Smith's lack of an objective and reproducible methodology that would translate evolutionary insights into taxa and hierarchical schemes, were often confusing to those who did not follow his publications closely.

Palaeos.com described his writing style as follows: Prof. Cavalier-Smith of Oxford University has produced a large body of work which is well regarded.

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.

[18] In 1983, he revised his system particularly in the light of growing evidence that Archaebacteria were a separate group from Bacteria,[19] to include an array of lineages that had been excluded from his 1981 treatment, to deal with issues of polyphyly, and to promote new ideas of relationships.

[20] 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.

[25] 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.