Our genes have to build a brain [to take day-to-day decisions for them]….The reason why they cannot manipulate our puppet strings directly is…time lag.  Genes work by controlling protein synthesis, but it is slow.  The whole point about behaviour, on the other hand, is that it is fast.  It works on a time-scale not of months but of seconds and fractions of seconds. Something happens in the world, an owl flashes overhead, a rustle in the grass betrays prey, and in milliseconds nervous systems crackle into action, muscles leap, and someone’s life is saved – or lost.  Genes don’t have reaction times like that…..[they] can only do their best  in advance  by building a fast executive computer for themselves, and programming it in advance with rules and ‘advice’ to cope with as many eventualities as they can ‘anticipate’.  But life….offers too many different possibilities for all of them to be anticipated.  …The genes have to ‘instruct’ their survival machines not in specifics but in the general strategies and tricks of the living trade.
R Dawkins (1976) The Selfish Gene. Oxford University Press

[It has been] suggested that peptide molecules evolved very early and that the endocrine and nervous systems may express similar peptides….because they evolved from primitive cells that had already evolved these substances for use in intracellular communication……This notion does not explain why common messenger molecules were retained during the period that the unicellular organisms evolved into specialized cells that apparently no longer needed the molecules for intracellular communication.
I Kupfermann (1991) Functional studies of cotransmission.  Physiological Reviews  vol 71 pp 683-732

It was during this term that I began to realize that Sebastian was a drunkard in quite a different sense to myself. I got drunk  often, but through an excess of high spirits, in the love of the moment, and the wish to prolong and  enhance it; Sebastian drank to escape……….
Julia used to say, ‘Poor Sebastian. Its something chemical in him.’
That was the cant phrase of the time, derived from heavens knows what misconception of popular science.  ‘There’s something chemical between them’ was used to explain the over-mastering hate or love of any two people. It was the old concept of determinism in a new form. I do not believe there was anything chemical in my friend.
Evelyn Waugh (1945) Brideshead Revisited.

Why was there jealousy – not just for him, but for lots of people?  Why did it start up?  It was related to love in some way, but that way wasn’t quantifiable or comprehensible.  Why did it suddenly start wailing in his head, like a ground-warning system in an aircraft: six and a half seconds, evasive action now.  

That was what it felt like sometimes, inside Graham’s skull.  And why did it pick on him?  Was it some kind of fluky chemistry?  Was it all dished out at birth?  Did you get given jealousy the way you got given a big bottom or poor eyesight, both of which Graham suffered from?  If so, maybe it wore off after a while; maybe there was only enough jealousy chemical in that soft box up there for a certain number of years.  

Perhaps, but Graham rather doubted it: he’d had a big bottom for years, and that showed no signs of easing up.
Julian Barnes (1983) Before She Met Me.   Picador, London.