Epithelia are layers of apico-basally and planar polarized cells, connected through intercellular junctions and resting on a basal lamina. This organization allows the epithelia to resist to internal and/or external stresses, as well as to define a medium compartimentalization necessary for homeostasis. The formation of epithelia has thus been instrumental for animals to appear and evolve in all the diversity of shapes that we can observe today. The goal of our team is to understand how epithelia were first formed during evolution and how their features are conserved or modified from sponges to human. Since epithelia express many genes involved in either the formation of cell-cell junctions or in the establishment of cell polarity we are investigating their role using state-of-the-art techniques in molecular, cellular and tissue biology. We have shown that many genes essential for building junctions and apico-basal polarity in human and whose mutations are associated with cancers are already present in sponges that appeared between 600 and 800 M years ago. Our work on human intestinal epithelial cells is showing that polarity proteins such as PAR6B or CRB3 are necessary for these cells to organize as single monolayers or to modulate their internal actin cytoskeleton to migrate. We are now investigating the role of these molecules in the transmission of forces between epithelial cells using state-of-the-art physical approaches. In addition, we are deciphering the composition of junctions in both a sponge from the Marseille bay area and in a placozoan to understand how these structures have appeared and evolved during animal evolution. By taking these different approaches we hope to answer some important questions in animal cell biology.