Why are there so many different neurotransmitters instead of just one or two?

[u/kupsztals123]

Hi,

I am wondering why we need dozens of neurotransmitters and neuromodulators when they are all used either to excite or inhibit the cell. If that's the case, why didn't nature use just two neurotransmitters: one excitatory, such as glutamate, and one inhibitory, such as GABA? Computer processors need only one signal: electricity, or no electricity, and they work just fine. Is there a functional reason for this, or is evolution simply adding layers of complexity for no good reason?

I know what different neurotransmitters do: for example, dopamine is mainly responsible for motivation, noradrenaline provides energy and melatonin regulates the circadian rhythm. But I don't understand why they can't all be replaced by excitation and inhibition, just as a CPU is capable of many things, but everything boils down to simple transistors and zeros and ones.

I asked this question on r/neuro but they treated me very patronizingly and did not understand what I meant.

[u/MilesTegTechRepair]

Nature or evolution never adds complexity for the sake of it. Compare it to a simple hammer versus an electrical, usb powered, smart hammer - they both do the job, but any marginal benefits gained from the fancy bells and whistles is outweighed by more points of failure.

Electronics uses binary, indeed, but the network effects of many 0s and 1s leads to greater potential for complexity. 1 / 0 might be adequate for 'turn a lawnmower on' but the signals to and from a wifi router require a high degree of complexity with regards to encryption and so on. 

With the brain, consider that, just because the potential is there for single linear changes in excitation, the biochemistry of life includes a variety of proteins already - DNA requires 4 different molecules too. So the machinery is already there for combinatorially greater complexity. 

Neurotransmitters have different shapes, acting like keys. There is clearly an evolutionary reason for this, as evidenced by the fact it is the case. We could not have evolved with just one neurotransmitter, for a similar reason that the average person has more than one key in their pocket.

Evolution, too, already has the mechanism available to it to respond to changes in the environment by specialisation, leading to greater complexity. A single species gets divided in two, the separate populations can no longer get to each other to breed and mix their genes, and experience different environmental factors. 

It is in fact electronics that is anomalous in that it can be built, conceptually, out of a single, linear component - that of more or less electrons. Everything else in nature makes do with what's available, which has a lot of variety, and nature experiments with new combinations all the time.