Bartering system economy is a myth and never really existed in the history of humankind. It's just a story Adam Smith made up to examine his theory in his book. However, the first form of money is "debt" as "I owe you one and I will pay you back later" form of mindset. The oldest money recorded in Mesopotamia is the ledger to record that "who owe who". After all, money is a sign of "debt" that people have to each other.