Is Hyphen correct in the following sentence? If yes, would you explain a little why? Is the phrase adverbial? Does
So mean "Too" in here?
Our humanness must be sustained through social interaction - and fairly constantly so.
That should not be a hyphen (-) but an em dash (—).
The dash here marks what follows it as what The Cambridge Grammar of the English Language calls a supplement: a remark or expansion which is added to the main clause, not an integral part of it.
In this case so plays the role of a pro-form: it may stand for an adjectival, adverbial or verbal entity in the same way that a pronoun stands for a nominal entity.
The specific entity which so stands for here is be sustained through social interaction.
Consequently, the information which the supplement adds is that our humanness must not merely be sustained through social interaction, it must be fairly constantly sustained through social interaction.