The Ancient Origins of Serotonin: How a 3‑Billion‑Year‑Old Molecule Shapes Life Today

This video explores the evolutionary history of serotonin, revealing it as one of biology’s oldest signaling molecules.

I. Overview
Emerging nearly three billion years ago—long before neurons or nervous systems existed—serotonin and its precursor tryptophan first appeared in light‑harvesting microorganisms, where they played essential roles in photosynthesis, oxidative protection, and environmental sensing.

III. Serotonin in Plants: A Conserved Signaling System
Modern plants still rely on serotonin as a morphogenic regulator. Research shows that plant serotonin:
(A) Shapes root architecture.
(B) Influences shoot growth.
(C) Interacts with the hormone auxin.
(D) Helps plants respond to environmental stress.
This ancient signaling function highlights serotonin’s original role as a mediator between organisms and their surroundings.

IV. Light, Mood, and Human Physiology
The connection between serotonin and light persists in humans. Sunlight exposure influences serotonin synthesis, affecting mood, circadian rhythms, and vulnerability to seasonal affective disorder. This continuity reflects serotonin’s deep evolutionary purpose: linking internal biological stability to external environmental cues.

V. Serotonin Across Vertebrates
In animals, dietary tryptophan availability directly shapes serotonin production, influencing:
(A) Stress reactivity.
(B) Aggression.
(C) Social behavior.
(D) Emotional regulation.
These effects demonstrate how a molecule that once supported primitive photoprotection now helps coordinate complex social and psychological processes.

VI. A Universal Homeostatic Regulator
Across plants, animals, and humans, serotonin functions as a master integrator—balancing internal physiology with external conditions. Whether guiding root growth, modulating mood, or shaping social behavior, serotonin’s evolutionary continuity underscores its role as a fundamental regulator of life.

SOURCES:
(1) Azmitia. (2009). Evolution of serotonin: sunlight to suicide. In Handbook of behavioral neuroscience (Vol. 31, pp. 3-22). Elsevier.
(2) Erland & Saxena. (2017). Beyond a neurotransmitter: the role of serotonin in plants. Neurotransmitter, 4(5), 1-12. doi: 10.14800/nt.1538.
(3) Negri et al. (2021). The case of tryptamine and serotonin in plants: a mysterious precursor for an illustrious metabolite. Journal of experimental botany, 72(15), 5336–5355. doi: 10.1093/jxb/erab220.
(4) Höglund, Øverli, & Winberg. (2019). Tryptophan Metabolic Pathways and Brain Serotonergic Activity: A Comparative Review. Front Endocrinol (Lausanne), 10, 158. doi: 10.3389/fendo.2019.00158
(5) Pelagio-Flores et al. (2011). Serotonin, a tryptophan-derived signal conserved in plants and animals, regulates root system architecture probably acting as a natural auxin inhibitor in Arabidopsis thaliana. Plant Cell Physiol, 52(3), 490-508. doi: 10.1093/pcp/pcr006
(6) Russo et al. (2009). Tryptophan as an evolutionarily conserved signal to brain serotonin: molecular evidence and psychiatric implications. World J Biol Psychiatry, 10(4), 258-268. doi: 10.1080/15622970701513764
(7) Tanveer et al. (2024). Unlocking the mysteries: Serotonin receptor networks explored. In Serotonin-neurotransmitter and hormone of brain, bowels and blood. IntechOpen.
(8) Azmitia. (2001). Modern views on an ancient chemical: serotonin effects on cell proliferation, maturation, and apoptosis. Brain Res Bull, 56(5), 413-424. doi: 10.1016/s0361-9230(01)00614-1
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