Serotonin is directly involved in many of the behaviors and biological systems that are central to human life and higher brain functions such as consciousness (Azmitia, 1999, 2010). We previously proposed that the actions of serotonin began because it is synthesized from tryptophan combining with molecular oxygen in photosynthetic single cellular organisms such as blue-green algae. Photons with a blue light frequency are converted to biological energy by the tryptophan amino acid and the subsequent production of molecular oxygen results in a potentially dangerous reactive state within the cell. In plant evolution many serotonin-based alkaloids such as ibogaine and psilocybin are produced to further protect the cells from oxidation. Animals lost the ability to synthesize tryptophan and developed many special mechanisms to secure tryptophan from their diets and to carefully conserve its integrity during circulation throughout the body. There was the emergence of multiple receptors and reuptake proteins that permit serotonin actions without utilization of serotonin itself. Serotonin receptors appear as early as the blastula and gastrula stages of embryonic development and continually monitor and regulate cell differentiation as it serves phylogenic evolution. Glial cells are an important component of serotonin's trophic action. An important concept to emerge from an analysis of serotonin evolution is its relation to light. Beginning with the light absorption properties of the indole ring of tryptophan, a direct path can be drawn to the effects of sunlight on photosynthesis and serotonin levels in plants. Progressing further in phylogeny, the effects of sunlight are seen on serotonin levels and on mood, sleep, and suicide ideation in humans. The focus on the evolution of serotonin leads from an awareness of the relationship of the beginning of oxygen-dependent life to the current human struggle to enjoy our dominant position on earth.