Evolutionary transition from single-cell to multi-cell organisms: the role of surface waters and their hydrodynamics.

The evolutionary transition from unicellular to multicellular organisms exhibited a sharp increase in metabolic rates, yet the physical conditions supporting this transition remain poorly understood. Here we propose that wide-spread hydrodynamic enhancement of nutrient delivery to the microorganisms in the Neoproterozoic Era constituted a necessary enabling condition for early multicellularity, complementing the effect of rising oxygen levels occurring at the same time. Along with conceptual arguments in support of this hypothesis, we define three testable hydrodynamic mechanisms (viscous-diffusive, viscous-convective, and inertial-convective) controlling nutrient uptake and largely applicable for both bed-attached and free-floating microorganisms. We also highlight a flow-bed interface in surface streams as potentially important hotbed for multicellularity breeding, complementing the mainstream perception on the defining role of early oceans. Overall, our results emphasize hydrodynamics as under-represented physical component in existing evolutionary frameworks and generate testable predictions linking organism size, flow conditions, and habitat type.