What we consider as currency has been a rather interesting journey IMO. People just started out by bartering, but at least those things had intrinsic value, whether they were mules, clothing, or tools. However, it had problems. Both people had to want what the other had, or the trade fails. Also there was no standard. How many apples equals a goat, or how many apple pies equals a day's labor?
Eventually we went to the gold standard, and paper money was redeemable for gold. Then that was abandoned, but paper money that was silver certificates persisted and you could still redeem them for silver bullion. Then that was dropped as well later on. Even the silver dollar and other silver coins had less silver in them. Then after that we went to fiat currency.
Don't ask me why the government puts "In God We Trust" on currency,