Central Europeans of the Bronze Age used bronze rings, ribs and axe blades that were roughly standardised in their shape and weight as an early form of coins, a new study says.
The paper by researchers at Leiden University in the Netherlands appeared in the journal Plos One on Wednesday.
Money is a defining aspect of modern life and standardisation of currency is one of its key features.
But because ancient people lacked precise forms of measurement, archaeologists disagreed about whether bronze items found in "hoards" in modern day Germany, Austria, the Czech Republic and other places were really currency, or just metal blocks for melting into other products.
These objects go back about 4,000 years ago, when Central Europeans lived in farming societies that used bronze tools, weapons, armour, building materials and more.
The research was based on a new methodology he developed "that is more attuned to how people would weigh items in the Bronze Age", lead author Maikel Kuijpers, an assistant professor in European prehistory, told AFP.
Mr Kuijpers and his co-author, Catalin Popa, studied more than 5,000 objects from about 100 ancient hoards.
Rather than measuring the items with scales, they compared their weight using a psychology principle known as the "Weber fraction".
This quantifies the concept that if objects are similar enough in mass, people weighing them by hand cannot tell the difference.
They found that about 70 per cent of rings, which averaged about 195 grams, were similar enough in weight to have been indistinguishable by hand.
These pieces are not the world's oldest money. People in Mesopotamia developed coins about 5,000 years ago, but Mr Kuijpers said the methodology could have wider use for investigating objects of the ancient world.
As to how much each object might be worth in today's money and what items they might have been used to buy, such as cattle or weapons, it is impossible to know for certain.
But "it's a material that they valued a lot, that much is clear", Mr Kuijpers said.