This maps nicely to the two separate locations where the body produces most serotonine: In the gut and in the brain. (Serotonine isn't able to cross the blood-brain-barrier).

There are even two different genes TPH1 and TPH2 which encode the enzyme information for this. This split is apparently very old and evolved before vertebrates did.

This is also why SSRIs have so many side-effects. They target receptors in the brain, but at the same time jumble serotonine-levels in the body, which are involved in appetite, temperature, blood-pressure, day/night cycle and many other systems.