The birth of the synapse.

It has long been held that the rise of neurons as a specialized cell type also marked the onset of the grand evolutionary journey for chemical synapses. Research over recent decades has shown, however, that the most dynamic chapters of synaptic history have been 'written' out of the context of neurobiology and neuronal evolution, dating back to the early metazoa and unicellular living forms. Here, I consider and discuss emerging evidence suggesting the exaptive origin of chemical synapses, via tinkering and neo-functionalization of already existent junctional morphologies and constituents of primeval paracrine signalling. Through combination and collateral use of long-established structures and functions, a remarkable enrichment of regulatory and control mechanisms of complex living organisms was achieved, without large-scale reorganization of the genome, with tremendous impact on the evolution and life on our planet.