All eukaryotic cells have the capacity to secrete molecules to the extracellular space through the constitutive secretion pathway. Animals also have specialized cell types that express the regulated secretion pathway, which enables the packaging and storage of secretory products for delayed release. Neurons, mucocytes (mucus-secreting cells), and cnidocytes are unified by their ability to temporally decouple synthesis and secretion of their payload. Cued release of neurotransmitters by neurons enabled nuanced communication among cells and facilitates the development of coordinated behaviors among groups of cells. Secretion of a gel-forming mucus by mucocytes created a barrier to foreign bodies and promoted segregation of internal and external environments. Synchronized release of a penetrating harpoon and proteolytic enzymes by cnidocytes allowed for assimilation of energy from novel prey sources. This combination of traits provides a solid platform for the evolutionary increase in body size and longevity, which may have facilitated diversification during the early evolution of multicellular life. Thus, understanding the origin of the secretory vesicle is crucial for modeling the transition from early multicellularity to complex life and for identifying overarching rules for evolutionary innovation.
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