[Embryonic origin of temperament: notochord and early signaling]

It is proposed that the origin of temperament is rooted in events of the third week of gestation, when gastrulation and the notochord organize body axes and morphogenetic gradients, integrated through intracellular pathways with downstream effects on circuit architecture and locomotion. The aim of this article is to propose a theoretical mechanistic model that articulates how gastrulation, the notochord, and prenatal epigenetic programming could contribute—probabilistically rather than deterministically—to the origin of human temperament. A three-phase mechanistic flow is proposed: Phase 1, in which gastrulation and the notochord establish axes and gradients that could modulate the organization of ventral circuits and central pattern generators; Phase 2, in which prenatal epigenetic programming calibrates stress, immune, and affective axes; and Phase 3, in which these levels integrate into profiles of activity, exploration, reactivity, and inhibition, consistent with the distributed polygenic architecture described by personality genomics. The notochord emerges as a plausible morphogenetic axis of temperament. Upon this embryonic scaffold, prenatal epigenetic programming may calibrate allostasis, stress reactivity, and affectivity, contributing to long-term temperamental variation. The model formulates falsifiable hypotheses and suggests translational priorities in perinatal prevention, multimodal biomarkers, and personalized neurodevelopmental medicine, whose short-term clinical feasibility remains uncertain.