The evolutionary perspective on the universal roles of compounds known as neurotransmitters may help in the analysis of interactions between organisms in a particular ecosystem (also known as biocenosis) – from microorganisms to plants and animals. This neurotransmitter-mediated phenomenon, which is significant for chemosignaling and cellular endocrinology, is an important consideration in the development of disease as the “living environment” influences every organism in the biocenosis relationships (microorganism–microorganism, microorganism–plant, microorganism–animal, plant–animal, plant–plant, and animal–animal). Non-nervous functions of neurotransmitters (rather “biomediators” on a cellular level) are considered in this review and compared with each other across different kingdoms.