Once upon a time there

was.... or "once upon a time there were...."
Vanessa's request for her Glenogle and Bell Advent calendar today is for the "most memorable opening line".
Well, there you have it. The line that opens so many stories.  
I  wonder who first used it? What story did they tell? To whom did they tell the story - and why?
We usually think of stories as something to entertain us but they often have another, more serious side. Before literacy was widespread, before scrolls and then books were widely available, people relied on oral storytellers.
In my teens I used to go to a camp in the summer. It was run by the Girl Guides for children with disabilities. In the early evenings we would sit around "the campfire" and someone would read a story to the children. It met with varying reactions for a variety of reasons - not least because the children ranged from six to fourteen. 
One night a friend who taught drama came and told a story instead. It was more than just relating what happened. Her hands moved. Her voice changed for each character. Her words were simple but said all they needed to say.
There was absolute silence until, at the end of it, there was a collective sigh.
And yes, she began with "Once upon a time..."