Thomas Cavalier-Smith (1942–2021).
Polymath of cellular evolution who shaped understanding of the tree of life.

Since Charles Darwin first proposed his theory of evolution, biologists have struggled to fit all life forms — from the tiniest bacterium to the blue whale — onto a tree of life that explains their ancestry.
The tree, it turned out, was more of a web.
Branches were fused by the demonstration that endosymbiosis can lead to the integration of a microbial cell into another cell to form a discrete compartment passed from generation to generation over hundreds of millions of years.
Thomas (Tom) Cavalier-Smith played a crucial part in understanding major transitions in evolution, including the role of endosymbiosis.
He has died, aged 78. Cavalier-Smith’s aim was to understand the rise of the eukaryotes — organisms with complex, compartmentalized cells, including plants, animals and fungi.
His passion was the huge diversity of single-celled eukaryotes — the protists.
His ideas were based on the thesis that we cannot grasp evolutionary history without understanding how all dimensions of a cellular system — function, structure, biochemistry, economy and spatial organization — arose.
How this network varies across the tree, he argued, defines the tree. Historically, the study of microbial forms focused on interpretations gleaned from light microscopy.
With his second wife and colleague Ema Chao, Cavalier-Smith rationalized the comparative study of protists.
He and Chao combined light and electron microscopy with genetic analysis to construct a new systematics for the eukaryotes and to pursue a unified taxonomy for all life.