Until the advent of the internet currencies trading was really limited to interbank activity on behalf of their clients. Gradually, the banks set up proprietary desks to trade for their own accounts and this was followed by large multinational corporation’s hedge funds and high net worth individuals.
The forex trading is possible today mainly on account of the centralized format of commodities trading and specialized bourses which started around mid sixteenth century, since then, trading or bartering one good/services/currency/stock for another, at the preferred rate has become the norm. You might want to check out the BBC series on big finance…