Once upon a time it was salt. It was (still is) useful; needed to preserve meat, etc. So ancient people used salt as currency.
Then, around 800 BC, people had the bright idea to mint metal coinage, mostly bronze and silver; gold for the very rich.
Trust the Chinese to introduce paper money.
Paper money in the USA was once backed by gold and silver, redeemable in the real thing.
That got phased out by bankers and the politicians in their pockets.
Now it's just paper, based on nothing. Just like toilet paper but with fancy inking.
And the way the Fed is printing new money (another trillion) based on nothing, you might as well start using one dollar bills to... well, you know what.