[Cell polarity and the innovation of the primary cilium/centrosome organ in Metazoa]

Cell-autonomous polarity in Metazoans is inherited from ancestral unicellular organisms. We assume that permanent polarity in unicellular eukaryotes is required for cell motion and sensory reception and that the integration of these two activities corresponds to an evolutionary constrained cell function. While conserving the ancestral flagellum, Metazoans have co-opted a primary cilium/centrosome organ from it, ensuring similar functions, but in different cells, or in the same cell at different moments. We propose that the remodeling necessary to reach a new higher-level unit of selection in multi-cellular organisms, has been triggered by conflicts among individual cell polarities to reach an organismic polarity. We shall provisionally conclude that beyond critical consequences for embryo development, the conservation of cell-autonomous polarity in Metazoans has far reaching implications for the evolution of individuality.