While the present seems fixed (who among the baby boomers could have envisioned a life without the Soviet Union?), history is fluid indeed. Coins, bills and currency change rapidly as well.
In physical terms, money has gone from bartered objects to monetary metals to paper to electrons.
In ownership terms, money has been produced by those who wanted a convenient way to transmit value, but difficulty arose as soon as there was more than one person producing money. His coins weighing 3 units of 20% pure silver were used to buy her fish which she purchased with her 2 units of her currency with 40% pure silver. How does he purchase the fish? In this hypothetical, there is an agreement that his three units can close the deal. If he didn't trust her money, then he would have to find a third party who did trust her and make a trade with that third party who would then trade with her; and the third party would charge a fee, hence the beginning of exchange rate transaction costs.
Now, nations have near monopolies on currencies and there are about 200 of them, but last year there were more and next year there will be fewer.
The links here are for the sections Past to Present showing significant events on the journey, thus far, toward leading toward a Single Global Currency, and the second Proposed Chronology Toward Goal shows the hoped for, planned schedule for implementing the new currency.