Serotonine is very old. There are two different genes TPH1 and TPH2 which encode the enzyme information for serotonine-production, which  happens at two sites in the body. This split is apparently very old and evolved before vertebrates did.<p>Serotonine controls a multitude of things including wake-night-cycle, blood pressure and temperature. It doesn&#x27;t even only work as a neurotransmitter, but mechanically as well.<p>Further reading: <a href="http:&#x2F;&#x2F;www.ncbi.nlm.nih.gov&#x2F;pubmed&#x2F;8768313" rel="nofollow">http:&#x2F;&#x2F;www.ncbi.nlm.nih.gov&#x2F;pubmed&#x2F;8768313</a>