We don't have a single currency in the world today, and not even a single reserve currency (where presumably the network effect of liquidity could reign supreme, unfettered by legal tender laws, etc.). I doubt in the history of civilization we ever really have, except perhaps in some isolated simple economies.
Gold served precisely this purpose for about 4000 years, silver too for more common folks.
We are in a practically unique time in the history of history where people are being convinced that gold is not money (oddly enough, mostly loudly by the folks that are buying it all).