We’ve just seen that many preadaptations for the evolution of neurons were in place. Calcium toolkit for secretion of neurotransmitters? Check. Post-synaptic scaffold? Check. Circuitry for neurogenesis? Check. But what about the neurotransmitters themselves?
Now, in principle, just about any molecule could serve as a neurotransmitter, as you simply need something that specifically binds to a receptor that triggers the opening of an ion channel. In fact, some amino acids, like glutamate, can moonlight as neurotransmitters.
Nevertheless, I decided to check up on two important neurotransmitters – acetylcholine and epinephrine. These neurotransmitters play key roles in your autonomic nervous system, the ancient system that maintains homeostasis through the regulation of all your body’s organs. What’s more, acetylcholine is the neurotransmitter used to stimulate muscle contraction.
The hypothesis of front-loading neurons would lead us to expect that these neurotransmitters exist in organisms that do no possess neurons. And sure enough, they do.
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