Cell-cell junctions
Without the ability for cells to adhere to one another all life on this planet would be destined to be unicellular. To achieve multicellularity life had to evolve a solution to mechanically connect the two daughter cells that form after cytokinesis. Interestingly many of the proteins that assemble to form the contractile ring during cell divisions are also found at cell-cell junctions, the structures that adhere cells together, even though the functions of these cellular structures are very different. The contractile ring functions to cut a cell in two while cell-cell junctions functions to mechanically link cells together and allow tissues to form barriers; without them multicellularity on the level of a human being could not exist. Some organism such as Choanoflagellates can live as a unicellular or multicellular organism. They achieve multicellularity not with specialized cell-cell junctions but by skipping the last step of cytokinesis, remaining connected via and intracellular bridge and sharing cytoplasm \cite{Dayel2011}. Again highlighting the intimate link between cell division and adhesion. To achieve mulicellularity on a larger scale cells had to evolve specialized adhesion solutions so that tissues could be both rigorous enough to maintain tissue and barrier integrity and dynamic enough to maintain tissue homeostasis through the flux of cell renewal and cell death. Cell-cell junctions emerged 600 million years ago but the ultrastructure of cell-cell junctions was first identified only a few decades ago in a seminal electron microscopy study by Farquhar and Palade \cite{13944428}. The authors described the apical junctional complex in vertabrates being composed of tight junctions (or zonula occludens), where the space between epithelial cells is almost completely obliterated, adherens junctions (or zonula adherens), located just basal to the tight junction where the cell membranes are brought in close proximity (~20 nm apart), as well as “conspicuous bands of dense material located in the subjacent cytoplasmic matrix”, which we now know to be junctional actomyosin \cite{13944428}. The apical junctional complex in vertebrates plays the critical role of sealing the paracellular space and adhering epithelial cells to one another \cite{Hartsock_2008,Van2014}