The earliest form of currency was not coins or tokens, says Kabir Sehgal, author of the book Coined. In the book, Sehgal writes about the evolution, psychology, and history of contemporary currency
The first form of money was food, says Sehgal—in other words, energy. Coins and tokens, a more abstract form of exchange, came later. Four thousand years ago, the ancient Mesopotamians, Sehgal explains, would use barley and silver in exchange for goods and services. They would barter with one another and make loans. The Mesopotamians would keep track of these loans on a tablet (a clay one, not one from Apple). Coins as we know them didn't appear until later.