Two themes emerge -
The proliferation of transmitters may reflect "evolutionary
"garbage", the accumulation of a lot of things that were
tried, worked well enough, and became enshrined in the
genome because the cost of eliminating the inefficiencies
exceed the benefit.
The proliferation of transmitters is widespread in the
animal kingdom.
This just suggests that the experimentation (and the
accumulation of garbage) happened a long time ago!
It seem more and more like evolution spent a very long
time working out the details on how to build a complex
animal body, including most of the developmental
controls, the cell signalling apparatus (including
neurotransmitters) etc (the biochemistry and molecular
biology had been done long before animals).
Then, when it all worked (garbage and all) perhaps a
half billion years ago or so, a tremendous variety of
organisms used that bag of tricks in different ways to
produce a proliferation of body types that we now call
the animal kingdom.
