So paper currency (at least in the western world) began existence as promissory notes. The notes would denote some physical value insured by a trusted third party, I don't believe currency could ever begin life without some physical backing (and I don't mean gold, gold is worthless... something like wool, or a meal). Once a currency has been established as representing the good it can be exchanged based on the relative value of that good - a piece of paper worth a good meal might be worth two nights on a cot for instance. Mediums of exchange make all commerce easier so at some point this momentum really gets going, but for a significant life of the currency the exchange unit may only have value if you can say "I'd like two nights on the cot, and I can give you this note for a good meal - or I can run across the street and get you a good meal, since it's easier to hand you a note and since you may not be hungry right now, why don't you accept this note and have the meal when you want it."

So the emergence of TokenA is that someone promises that it's worth some value and that the promise made is trusted by enough people.

If you enjoy a good read, I'd suggest you check out Going Postal and Making Money by Terry Pratchett, they discuss (among much hilarity) the emergence of a postal system and stamp trading, along with the eventual evolution into paper money. There are certainly better academic papers about the technical side but I honestly think those two novels make a much more compelling case because you are given a narrative where people start accepting a fiat currency and everyone is doing so for perfectly logical reasons.