Fluid Mechanics and the Evolution of Biological Complexity

A fundamental issue in biology is the nature of evolutionary transitions from unicellular to multicellular organisms: What are the advantages of being physically larger and more complex? Green algae are models for this transition, as they span from unicellular species to multicellular ones with tens of thousands of cells, all of which swim through the action of flagella that are closely related to the cilia found throughout the human body. A paradigm for understanding this transition is the phenomenon of phototaxis, directed motion toward light. In this talk I will describe experimental and theoretical work on the physical mechanism of algal phototaxis, spanning 3 orders of magnitude in cell number, that reveals an evolutionarily conserved response which raises interesting questions in fluid mechanics, dynamical systems, and cellular decision-making.