Chemical neurotransmission is a highly specialized feature of the nervous system, but many neurotransmitters and receptor architectures are evolutionarily much older than neurons themselves. This Mini Review considers the possibility that neuronal signaling developed largely through the recruitment and reorganization of pre-existing systems for metabolism, nutrient sensing, environmental chemical detection, and intercellular communication. Glutamate, glycine, GABA, and catecholamines illustrate different forms of evolutionary continuity. Importantly, conservation of a signaling molecule does not necessarily imply conservation of its receptor or physiological function. Ancient chemical sensing mechanisms may have been progressively incorporated into increasingly organized forms of cell-to-cell communication and, eventually, synaptic transmission. From this perspective, the nervous system did not create chemical signaling de novo, but transformed ancient molecular components into rapid, directional, and plastic information-processing systems.

Keywords: Evolution of Neurotransmission; Chemical Sensing; Receptor Evolution; Intercellular Signaling; Synaptic Plasticity

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