The Peclet innovation and its lasting imprint on metazoan evolution

Motile cilia are a hallmark of eukaryotic life, driving low-velocity but highly organized flows essential to processes ranging from embryonic development to human organ function. However, motile cilia are a highly conserved functional design inherited from an offshoot lineage originating within the Archaean domain. Extant Archaea possess a very different body filament that acts as a propulsive organelle, the archaellum, and the introduction of cilia represented an evolutionary novelty within the Archaea. That introduction had the effect of shifting material transport from diffusion to advection dominated fluid regimes around cells. By controlling advective flows past the cell body, cells acquired the ability to direct fluids and their contents, a trait that supported a variety of other eukaryotic roles and has eventually radiated throughout the animal kingdom. We term this foundational shift in adaptive potential the Peclet innovation in reference to the Peclet number (Pe) which is an index of the relative importance of advective to diffusive mechanisms of material transport in a fluid. Here we highlight how a microscopic fluid dynamic novelty reshaped the cellular adaptive landscape, enabling diverse processes that became critical throughout the animal kingdom.