Conservation of a synaptic blueprint across the animal kingdom

Chemical synapses are fundamental computational units of the nervous system that enable rapid and precise information transfer through the nanoscale organization of presynaptic and postsynaptic components. Despite hundreds of millions of years of animal evolution and extraordinary diversity in nervous system architectures, the core physiological principles of chemical synaptic transmission remain remarkably conserved. Studies across Drosophila melanogaster, zebrafish, Caenorhabditis elegans, frogs and mammals reveal recurring principles governing active-zone organization, synaptic vesicle exocytosis, calcium signalling and postsynaptic specialization. Here, we examine the evolution of chemical synapses across the animal kingdom, focusing on the relationship between molecular diversification, nanoscale organization and functional conservation. We propose that the evolutionary stability of synapses reflects, in part, a stringent biophysical constraint imposed by the requirements for rapid chemical communication. We further consider how these conserved synaptic operations are assembled into neural circuits of dramatically different complexities, as well as the ctenophore nervous system as a critical test case for distinguishing molecular conservation from architectural and functional convergence. Across species, the emerging picture is that evolution has adhered to conserving the geometry and organizational principles of chemical synapses more stringently than a precise molecular composition. Synapses thus represent stable yet adaptable information-processing architectures in which conserved nanoscale constraints coexist with molecular flexibility and extensive diversification of neural circuits.