People use to barter or trade their goods for other peoples goods which they wanted, eg. crops.

When metal tools were introduced it started a whole new leaf of barter.

People started not trading things as such but more trading coins for these things and receiving a type of payment for them.

The introduction of metals started this whole new type of barter. Because shells were a very popular form of barter that people used to trade with, it is thought that replicas of shells made out of base metals can be thought of as the original development of metal coins and therefore giving coins their original shapes. Ancient coins would be found with holes in the centers and this was because people used to carry them around with them on types of necklaces.