Thomas Cavalier-Smith (1942–2021)

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.

Lynn Margulis (1938–2011)

Cavalier-Smith was born in London in 1942. He studied at Gonville & Caius College at the University of Cambridge, UK, and completed a PhD at King’s College London in 1967, with the biophysicist John Randall. When Cavalier-Smith turned to electron microscopy, he began to reveal the diversity of protists and algae and the complexity of organelles such as mitochondria and chloroplasts, the products of endosymbiosis.

After postdoctoral studies at the Rockefeller University, New York City, Cavalier-Smith returned to King’s in 1969 as lecturer, and then reader, in biophysics. He moved to the University of British Columbia in Vancouver, Canada, in 1989. Observing that some living protists seemed to lack mitochondria, he suggested that the eukaryotes originated before the endosymbiotic event that led to mitochondrial evolution. He placed these intermediate forms in a new kingdom, the Archezoa, developing a scheme for the stepwise evolution of cellular complexity in eukaryotes.

Another of his hypotheses proposed that a huge branch of eukaryotic life, including large multicellular forms such as the seaweed kelp, were the product of endosymbiosis between one eukaryote and another, much earlier than had been suggested. In the group he designated the chromalveolates, this event left behind a chloroplast — the cellular compartment for photosynthesis. The group contains lineages as divergent as marine phytoplankton (such as diatoms) and the malaria parasite, whose chloroplasts are colourless and non-photosynthetic.

A microbial marriage reminiscent of mitochondrial evolution

These hypotheses represented confident strides towards an evolutionary synthesis that was new in two ways. They were detailed, but spoke of billion-year timescales; and they offered testable predictions. He gave the infant field of evolutionary cell biology a common language and a set of ideas to either work with or to disprove.