There's an urban legend about the origin of the middle finger. According to lore, the French would cut off the middle finger of Englishman after defeat. The English longbow, made from the yew tree, was the ultimate weapon of the time. Once the middle finger was removed, the English would be unable to pluck the bow. However, as the story goes, when an Englishman defeated a Frenchman, they would stand above them, with middle finger extended. "I can still," he would declare, "pluck yew. PLUCK YEW!"
The middle finger has evolved to become one of the most recognizable and versatile non-verbal words in our language (and I'd assume other languages as well, though I speak none of them). The middle finger can communicate the entire spectrum of emotional responses. Perfect for most every situation.
Here's a list of the many underused uses of our most versatile finger: