Hypothesis: bacteria live on the edge of phase transitions with a cell cycle regulated by a water-clock

A fundamental problem in biology is how cells obtain the reproducible, coherent phenotypes needed for natural selection to act or, put differently, how cells manage to limit their exploration of the vastness of phenotype space. A subset of this problem is how they regulate their cell cycle. Bacteria, like eukaryotic cells, are highly structured and contain scores of hyperstructures or assemblies of molecules and macromolecules. The existence and functioning of certain of these hyperstructures depend on phase transitions. Here, I propose a conceptual framework to facilitate the development of water-clock hypotheses in which cells use water to generate phenotypes by living ‘on the edge of phase transitions’. I give an example of such a hypothesis in the case of the bacterial cell cycle and show how it offers a relatively novel ‘view from here’ that brings together a range of different findings about hyperstructures, phase transitions and water and that can be integrated with other hypotheses about differentiation, metabolism and the origins of life.