Cell size matters: a unifying theory across the tree of life.

Cell size shapes how organisms interact with their environment, yet our understanding of these effects remains fragmented. We synthesise these effects into a Theory of Optimal Cell Size (TOCS), predicting that the size of single-cell organisms, as well as the number and size of cells in multicellular organisms, is subject to strong selection. At the heart of TOCS lies a trade-off between power and energy conservation: cell size acts as a 'double-edged sword', influencing both resource acquisition and maintenance costs. Major evolutionary transitions across the tree of life are connected with innovations in cellular organisation. A unified framework for cell size adaptation is essential for revealing the fundamental principles governing the ecology and evolution of life.