Aristippus, following his shipwreck, was able to swim to a nearby shore ; he found geometrical figures traced in the sand, and was overcome with joy, judging that he had come upon a Greek people and not barbarians.
Assume you are alone, and arrive by some accident among an unknown people : if you see a coin, you may be sure you have come upon a civilized nation.
Cultivation of the land requires the use of money. This culture depends on many arts and much knowledge, and we always find the arts, knowledge, and needs progressing together. All together they lead to the establishment of a sign of values.
Mountain streams and forest fires  have revealed to us that metals are in the ground. Once they have been separated from it, it has been a simple matter to put them to use.