Thomas (Tom) Cavalier-Smith,
FRS,
FRSC,
NERC Professorial Fellow (21 October 1942 – 19 March 2021), was a professor of
evolutionary biology
Evolutionary biology is the subfield of biology that studies the evolutionary processes (natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life ...
in the Department of Zoology, at the
University of Oxford
, mottoeng = The Lord is my light
, established =
, endowment = £6.1 billion (including colleges) (2019)
, budget = £2.145 billion (2019–20)
, chancellor ...
.
His research has led to discovery of a number of unicellular organisms (
protist
A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the e ...
s) 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
Norwich School (formally King Edward VI Grammar School, Norwich) is a selective English independent day school in the close of Norwich Cathedral, Norwich. Among the oldest schools in the United Kingdom, it has a traceable history to 1096 as a ...
,
Gonville and Caius College, Cambridge
Gonville and Caius College, often referred to simply as Caius ( ), is a constituent college of the University of Cambridge in Cambridge, England. Founded in 1348, it is the fourth-oldest of the University of Cambridge's 31 colleges and one of t ...
(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".
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
The University of British Columbia (UBC) is a public research university with campuses near Vancouver and in Kelowna, British Columbia. Established in 1908, it is British Columbia's oldest university. The university ranks among the top thr ...
.
In 1999, he joined the
University of Oxford
, mottoeng = The Lord is my light
, established =
, endowment = £6.1 billion (including colleges) (2019)
, budget = £2.145 billion (2019–20)
, chancellor ...
, 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
A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological s ...
) and the resulting classifications, its distinctive character.
Four major influences
Thomas Cavalier-Smith 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, as described in the following subsections.
Prior representation
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 about character evolution and classifications that included paraphyletic taxa, and in which degree of difference would influence the rank assigned to taxa.
Popperian philosophy of science
The second influence was the philosophy as to how scientific progress was made. It had been articulated by
K. Popper 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.
Cladistics
The third influence was 'cladistics' – an explicit way of presenting (and then 'falsifying', although see
) evolutionary hypotheses. This was initially articulated by Willi Hennig, and was increasingly (but not universally
) accepted by many as how taxonomy should be done. The justification of the cladistic approach in terms of Popperian hypothetico-deductivism was popular in the 1970-80s but is no longer considered defensible (see Sober and reviewed in Rieppel
).
Genetic sequencing
Fourthly, 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.
Cavalier-Smith's narrative style
Cavalier-Smith was courageous in his adherence to the earlier traditionalist style characterized by
Charles Darwin
Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...
, 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 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 chromphytic 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 would translate evolutionary insights into taxa and hierarchical schemes, were often confusing to those who did not follow his publications closely. 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.
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 to change his narratives and taxonomic summaries frequently, his approach and claims were frequently debated.
Cavalier-Smith's contributions
Cavalier-Smith wrote extensively on the taxonomy and classification of all life forms, but especially
protists
A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...
. One of his major contributions to biology was his proposal of a new
kingdom of life: the
Chromista, even though it is not widely accepted to be monophyletic (see above).
He also introduced new taxonomic groupings group for eukaryotes such as the
Chromalveolata (1981),
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
The scientific community is a diverse network of interacting scientists. It includes many " sub-communities" working on particular scientific fields, and within particular institutions; interdisciplinary and cross-institutional activities are als ...
. His taxonomic revisions often influenced the overall classification of all life forms.
Eight kingdoms model
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.
In 1983, he revised his system particularly in the light of growing evidence that Archaebacteria were a separate group from Bacteria, 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. In addition, some protists lacking mitochondria were discovered.
As mitochondria were known to be the result of the
endosymbiosis of a
proteobacterium
Pseudomonadota (synonym Proteobacteria) is a major phylum of Gram-negative bacteria. The renaming of phyla in 2021 remains controversial among microbiologists, many of whom continue to use the earlier names of long standing in the literature. The ...
, it was thought that these amitochondriate eukaryotes were primitively so, marking an important step in
eukaryogenesis
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
. As a result, these amitochondriate protists were given special status as a protozan subkingdom
Archezoa, that he later elevated to kingdom status.
This was later referred to as the
Archezoa hypothesis Archezoa was a kingdom proposed in the 20th century by Thomas Cavalier-Smith (1942–2021), and was believed to encompass eukaryotes which did not have mitochondria (and are therefore ''amitochondriate'') or peroxisomes (e.g. '' Giardia''). The c ...
. In, 1993, the eight kingdoms became: Eubacteria, Archaebacteria, Archezoa, Protozoa, Chromista, Plantae, Fungi, and Animalia.
The kingdom Archezoa went through many compositional changes due to evidence of polyphyly and paraphyly before being abandoned. He assigned some former members of the kingdom
Archezoa to the phylum
Amoebozoa.
Six kingdoms models
By 1998, Cavalier-Smith had reduced the total number of
kingdoms from eight to six:
Animalia
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
,
Protozoa,
Fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...
,
Plantae (including Glaucophyte,
red and
green algae),
Chromista, and
Bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
.
Nevertheless, he had already presented this simplified scheme for the first time on his 1981 paper
and endorsed it in 1983.
Five of Cavalier-Smith's kingdoms are classified as
eukaryotes as shown in the following scheme:
*''Eubacteria''
*
Neomura
Neomura is a possible clade composed of the two domains of life of Archaea and Eukaryota. The group was named by Thomas Cavalier-Smith in 2002. Its name means "new walls", reflecting his hypothesis that it evolved from Bacteria, and one of t ...
**''Archaebacteria''
**Eukaryotes
***Kingdom Protozoa
***
Unikonts (
heterotrophs)
****Kingdom Animalia
****Kingdom Fungi
***
Bikonts (primarily
photosynthetic)
****Kingdom Plantae (including red and green algae)
****Kingdom Chromista
The kingdom Animalia was divided into four subkingdoms:
Radiata (phyla
Porifera,
Cnidaria
Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.
Their distinguishing feature is cnidocytes, specialized cells that ...
,
Placozoa, and
Ctenophora),
Myxozoa,
Mesozoa, and
Bilateria
The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...
(all other animal phyla).
He created three new animal phyla:
Acanthognatha
Gnathifera (from the Greek '' gnáthos'', “jaw”, and the Latin '' -fera'', “bearing”) is a clade of generally small spiralians characterized by complex jaws made of chitin. It comprises the phyla Gnathostomulida, Rotifera, Micrognathozoa ...
(
rotifers,
acanthocephalans,
gastrotrichs, and
gnathostomulids
Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively anoxic environments. They were first recognised and described in 1956. ...
),
Brachiozoa
Brachiozoa is a grouping of lophophorate animals including Brachiopoda and Phoronida. It also includes their ancestors, the extinct tommotiids.
References
Lophophorata
Protostome unranked clades
{{Protostome-stub ...
(
brachiopods and
phoronids), and
Lobopoda
''Lobopoda'' is a genus of comb-clawed beetles in the family Tenebrionidae. The type species is '' Lobopoda striata''. The following subgenera
In biology, a subgenus (plural: subgenera) is a taxonomic rank directly below genus.
In the Int ...
(
onychophorans
Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...
and
tardigrades)
and recognised a total of 23 animal phyla.
Cavalier-Smith's 2003 classification scheme:
*
Unikonts
** protozoan phylum
Amoebozoa (ancestrally uniciliate)
**
opisthokonts
*** uniciliate protozoan phylum
Choanozoa
Choanozoa is a clade of opisthokont eukaryotes consisting of the choanoflagellates (Choanoflagellatea) and the animals (Animalia, Metazoa). The sister-group relationship between the choanoflagellates and animals has important implications for th ...
*** kingdom Fungi
*** kingdom
Animalia
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
*
Bikonts
** protozoan infrakingdom
Rhizaria
*** phylum
Cercozoa
Cercozoa is a phylum of diverse single-celled eukaryotes. They lack shared morphological characteristics at the microscopic level, and are instead defined by molecular phylogenies of rRNA and actin or polyubiquitin. They were the first major euk ...
*** phylum
Retaria (
Radiozoa
The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The ela ...
and
Foraminifera
Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...
)
** protozoan infrakingdom
Excavata
*** phylum
Loukozoa
*** phylum
Metamonada
*** phylum
Euglenozoa
*** phylum
Percolozoa
** protozoan phylum
Apusozoa
The Apusozoa are an Obazoa phylum comprising several genera of flagellate eukaryotes. They are usually around 5–20 μm in size, and occur in soils and aquatic habitats, where they feed on bacteria. They are grouped together based on the prese ...
(
Thecomonadea and
Diphylleida)
** the
chromalveolate clade
*** kingdom
Chromista (
Cryptista
Cryptista is a clade of algae-like eukaryotes. It is most likely related to Archaeplastida which includes plants and many algae, within the larger group Diaphoretickes.
Although it has sometimes placed along with Haptista in the group Hacrob ...
,
Heterokonta, and
Haptophyta)
*** protozoan infrakingdom
Alveolata
**** phylum
Ciliophora
The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different ...
**** phylum
Miozoa
Myzozoa is a grouping of specific phyla within Alveolata, that either feed through myzocytosis, or were ancestrally capable of feeding through myzocytosis.
Many protozoan orders are included within Myzozoa.
It is sometimes described as a ph ...
(
Protalveolata,
Dinozoa, and
Apicomplexa)
** kingdom
Plantae (
Viridaeplantae
Viridiplantae (literally "green plants") are a clade of eukaryotic organisms that comprise approximately 450,000–500,000 species and play important roles in both terrestrial and aquatic ecosystems. They are made up of the green algae, which a ...
,
Rhodophyta and
Glaucophyta)
Seven kingdoms model
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
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
(previously 'Eubacteria') and Archaea (previously 'Archebacteria'). This is based on the consensus in the
Taxonomic Outline of Bacteria and Archaea (TOBA) and the
Catalogue of Life
The Catalogue of Life is an online database that provides an index of known species of animals, plants, fungi, and microorganisms. It was created in 2001 as a partnership between the global Species 2000 and the American Integrated Taxonomic In ...
.
Proposed root of the tree of life
In 2006, Cavalier-Smith proposed that the
last universal common ancestor to all life was a
non-flagellate Gram-negative bacterium
Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...
("negibacterium") with two
membranes
A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...
(also known as
diderm bacterium).
Awards and honours
Cavalier-Smith was elected Fellow of the
Linnean Society of London
The Linnean Society of London is a learned society dedicated to the study and dissemination of information concerning natural history, evolution, and taxonomy. It possesses several important biological specimen, manuscript and literature coll ...
(FLS) in 1980, the
Institute of Biology (FIBiol) in 1983, the
Royal Society of Arts
The Royal Society for the Encouragement of Arts, Manufactures and Commerce (RSA), also known as the Royal Society of Arts, is a London-based organisation committed to finding practical solutions to social challenges. The RSA acronym is used m ...
(FRSA) in 1987, the
Canadian Institute for Advanced Research (CIFAR) in 1988, the
Royal Society of Canada
The Royal Society of Canada (RSC; french: Société royale du Canada, SRC), also known as the Academies of Arts, Humanities and Sciences of Canada (French: ''Académies des arts, des lettres et des sciences du Canada''), is the senior national, bil ...
(FRSC) in 1997, and the
Royal Society of London
The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, r ...
in 1998.
He received the
International Prize for Biology from the Emperor of Japan in 2004, and the
Linnean Medal
The Linnean Medal of the Linnean Society of London was established in 1888, and is awarded annually to alternately a botanist or a zoologist or (as has been common since 1958) to one of each in the same year. The medal was of gold until 1976, and ...
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.
He won the 2007
Frink Medal of the
Zoological Society of London
The Zoological Society of London (ZSL) is a charity devoted to the worldwide conservation of animals and their habitats. It was founded in 1826. Since 1828, it has maintained the London Zoo, and since 1931 Whipsnade Park.
History
On 29 ...
.
References
External links
*
*
*
{{DEFAULTSORT:Cavalier-Smith, Thomas
English biologists
Protistologists
1942 births
British evolutionary biologists
English humanists
English microbiologists
English taxonomists
Fellows of the Royal Society
People educated at Norwich School
Alumni of Gonville and Caius College, Cambridge
Alumni of King's College London
Academics of King's College London
Academics of the University of Oxford
20th-century biologists
21st-century biologists
20th-century British scientists
21st-century British scientists
2021 deaths