It was the year 12XX and a flier was posted and it said the following.

The Adventures Guild Are Looking For New Members To Help Out With The Huge Problems That We Have Seen Around.
Any New Adventurer Will Begin As An F-Rank. Then From There You/Your Group Slowly Grows In Rank And The Better The Rank The Better The Loot.
We'll See You In The Adventurers Guild As Soon As Possible. Your First Mission Will Be Supervised And You Will Be Shown How It All Works. 
GOOD LUCK