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.

Serotonine controls a multitude of things including wake-night-cycle, blood pressure and temperature. It doesn't even only work as a neurotransmitter, but mechanically as well.

Further reading: http://www.ncbi.nlm.nih.gov/pubmed/8768313