The Serial Endosymbiotic Theory explains the origin of nucleated eukaryotic cells by a merging of archaebacterial and eubacterial cells. The paradigmatic change is that the driving force behind evolution is not ramification but merging. Lynn Margulis describes the symbiogenetic processes in the language of mechanistic biology in such terms as “merging”, “fusion”, and “incorporation”. Biosemiotics argues that all cell-cell interactions are (rule-governed) sign-mediated interactions, i.e., communication processes. As the description of plant communication demonstrates, the biosemiotic approach is not limited to the level of molecular biology, but is also helpful in examining all sign-mediated interactions between organisms on the phenotypic level. If biosemiotics also uses the notions of “language” and “communication” to describe non-human sign-mediated interactions, then the underlying scientific justification of such usage should be critically considered. Therefore, I summarize the history of this discussion held between 1920 and 1980 and present its result, the pragmatic turn.