Dear Geoff: my dictionary says: a vocation or occupation requiring advanced education and training, and involving intellectual skills, as medicine, law, theology, engineering, teaching, etc.
I have seen in print claims that practitioners of commercial sex were the earliest, that money changing was the earliest.
The dictionary definition above fails to list many that are now recognized, from soldier all the way to IT consultant. Anyone with special skill and knowledge is entitled to be called a professional - even gamblers.
Excluding only the amateurs and dilettantes, who do not earn their living from their knowledge.