Thomas Cavalier-Smith
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Thomas Cavalier-Smith is a professor of evolutionary biology at the University of Oxford, and is winner of the International Prize for Biology 2004 and one of the most notable researchers concerning the relationships, development, and classification of living things.
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Prof. Thomas Cavalier-Smith has published many works which organize and systematize the classification of the living world, taking a bold yet detailed approach on the basis of his special expertise in cell biology, electron microscopy, and molecular biology, backed by his knowledge of the latest developments in every field of biological science. Focusing on the evolution of cells by endosymbiosis, he has helped create a more natural classification system, primarily by proposing the "six kingdom theory," which added the kingdom Chromista to the five kingdoms (the Monera, Protista, Plantae, Fungi, and Animalia) that had been generally accepted for some time. These many notable contributions by Prof. Cavalier-Smith make him a worthy recipient of the International Prize for Biology 2004.
Since the earliest days of taxonomy, which forms the basis for the study of biological diversity, in addition to the need to accurately describe each of the myriad forms of life (said to number several million species, or more), an issue of key importance has been how to divide organisms into major subgroups. The division of all organisms into "animals" and "plants" probably came about spontaneously, before the advent of taxonomy; as the biological sciences developed, other categories of life such as fungi, protists, and bacteria were generally recognized as independent subdivisions, and by the 1970s and 1980s, the "five kingdom theory" was widely accepted. This explained the evolution of life on the basis of a classification into the kingdom Monera, containing bacteria with no nucleus (prokaryotes), the kingdoms Plantae, Animalia, and Fungi, containing multicellular eukaryotes (organisms composed of cells with nuclei), and the kingdom Protista, which forms a link between the latter and the prokaryotes. This system came under challenge, however, because the phylogenetically very heterogeneous nature of the kingdom Protista was not consistent with current principles of taxonomy based on phylogeny. In 1981, Prof. Cavalier-Smith postulated that only those groups of organisms which acquired chloroplasts by "primary endosymbiosis" and which have a double chloroplast envelope, namely, the three plant divisions Chlorophyta, Rhodophyta, and Glaucophyta, should constitute the true plant kingdom. He proposed classifying as a sixth kingdom, independent of the plants, those algae whose chloroplasts, acquired by "secondary endosymbiosis," possess three or four bounding membranes; this new kingdom is known as the Chromista. Prof. Cavalier-Smith's "six kingdom theory" was far more reflective of phylogenetic relationships than earlier theories, and today it has the support of many researchers. He later proposed combining the Chromista and the Alveolata, which include dinoflagellates, to form a new category known as chromalveolates. He also suggested that the endosymbiotic acquisition of chloroplasts was a single event in the case of primary endosymbiosis, and also a single event in each of chlorophytan and rhodophytan lineages in the case of secondary endosymbiosis.
With his coworkers, Prof. Cavalier-Smith has also investigated the origin of eukaryotes, focusing on the structural evolution of genes as well as molecular phylogenetic analysis, and has developed a model of unprecedented simplicity to trace the stages of cellular evolution from prokaryotes to eukaryotes. In particular, he has concluded that, while archaebacteria have a great many characters in common with eukaryotes, the two are sister groups rather than representing an evolutionary pathway from the archaebacteria to the eukaryotes.
Through this work, which marks a very significant advance in our understanding of the evolution and diversity of life, Prof. Cavalier-Smith has made a major contribution with far-reaching implications for research in many areas of biology.