Thomas Cavalier-Smith was a biologist who studied cell evolution and genetics. Thomas proposed several new taxonomic branches of the phylogenetic tree of life, which is a model of evolutionary relationships based on physical or genetic characteristics, and clarified the position of its root.

He used genetic sequencing, microscopy and computational analyses to research the development and diversity of molecular structure at a cellular level across the various levels represented within the tree of life, such as kingdoms, phyla, classes and species. He provided the most detailed theory for the origin of eukaryote cells, explaining how they and their probable archaebacterial sisters evolved from a eubacterium.

Thomas discovered hundreds of new protist species and defined many new eukaryote groups, including the kingdom Chromista in 1981, most of its phyla, and many phyla and classes of Protozoa, some still being debated by the wider community. Amongst other awards, he received the 2004 Japan Society for the Promotion of Science International Prize for Biology and the 2007 Frink Medal of the Zoological Society of London.

Professor Thomas Cavalier-Smith FRS died on 19 March 2021.