The free-living prokaryotic and eukaryotic cells

Abstract

Abstract The last universal common ancestor of cellular life (LUCA) and the last eukaryote common ancestor (LECA) were unique events that committed previously independent replicators to joint vertical transmission by default although horizontal transmission remained possible. Here I develop an explicitly organismal narrative emphasizing that the organizational complexity of simple prokaryote cells and complex eukaryote cells was based on cell closure and division of labor among cellular elements. My arguments reconcile Huxley’s (1912) principle that nothing alive can be functionally homogeneous with Williams’ dictum that genetic homogeneity is a necessary condition for maintaining organizational complexity. Echoing arguments brought forward intermittently since the late 1800s, I question the validity of the cell as machine metaphor because that reductionist approach addresses only proximate aspects of functional causation and cannot explain the self-organized, and self-referential aspects of unicellular maintenance and reproduction as they are shaped by natural selection. I review the substantial insights obtained from studies of societies of bacterial and protist cells. They vindicated inclusive fitness theory and are now being extended to address viral social behavior, but they have no direct relevance for understanding LUCA and LECA as major evolutionary transitions in organizational complexity. Finally, I evaluate the scattered evidence for germline–soma differentiation within unicellular organisms, which increasingly confirm that these domains also have forms of reproductive division of labor and differential rates of aging.