Serotonin, or 5-HT,  predates the nervous system and vertebrates.  Yes, it was here first! Serotonin is found and has profound roles in plants, single-celled organisms, multi-celled organisms, worms, flukes, insects, seafood (I mean ocean creatures…I must be hungry), amphibians, reptiles, mammals etc. etc. Serotonin has regulatory functions in the human central and peripheral nervous system, reproductive system, cardiovascular system, endocrine system etc etc. Serotonin is just not a mood regulator. 

Dr. Fred Clary the founder of Functional Analysis Chiropractic Technique begins a series on the role of the phylogenetically ancient monoamine in our lives and our collective history.