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. 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. He was known for his systems of classification of all organisms.
Life and career Cavalier-Smith was born on 21 October 1942 in London. His parents were Mary Maude (née Bratt) and Alan Hailes Spencer Cavalier Smith. He was educated at Norwich School, Gonville and Caius College, Cambridge (MA) in Biology and King's College London (PhD) in Zoology. 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". From 1967 to 1969, Cavalier-Smith 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. From the early 1980s, Smith promoted views about the taxonomic relationships among living organisms. He was prolific, drawing on a near-unparalleled wealth of information to suggest novel relationships. 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. Thomas Cavalier-Smith died in March 2021 following the development of cancer.
Taxonomy Cavalier-Smith was a prolific taxonomist, drawing on a near-unparalleled wealth of information to suggest novel relationships. His suggestions were translated into taxonomic concepts and classifications with which he associated new names, or in some cases, reused old names. Cavalier-Smith did not follow or espouse an explicit taxonomic philosophy but his approach was closest to evolutionary taxonomy. He and several other colleagues were opposed to cladistic approaches to taxonomy arguing that the goals of cladification and classification were different; his approach was similar to that of many others' broad-based treatments of protists. 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. Propositions were often ambiguous and short-lived; he frequently amended taxa without any change in the name. 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. However, this approach is no longer considered defensible. Cavalier-Smith's ideas that led to the taxonomic structures were usually first presented in the form of tables and complex, annotated diagrams. When presented at scientific meetings, they were sometimes too rich, and often written too small, for the ideas to be easily grasped. Some such diagrams made their way into publications, where careful scrutiny was possible, and where the conjectural nature of some assertions was evident. 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's narrative style 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. It was Cavalier-Smith's claim that there was a single endosymbiotic event by which chlorophyll a and 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. The alternative hypothesis was that the three chromophytic 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. Cavalier-Smith's lack of an objective and reproducible methodology that translated evolutionary insights into taxa and hierarchical schemes, was often confusing to those who did not follow his publications closely. Many of his taxa required his frequent adjustment (as illustrated below). In turn this led to confusion as to the scope of taxa that a taxonomic name was applied to. Cavalier-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, alongside its use in the newer senses. Because of Cavalier-Smith's tendency to publish rapidly and frequently change his narratives and taxonomic summaries, his approach and claims were frequently debated.
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