Where did the hand gesture, "giving someone the bird" come from?

Question

Although there will probably be plenty of answers , First person with the Correct Answer along with the reference will get the vote for best answer.

Answer 1

"Before the Battle of Agincourt in 1415, the French, anticipating victory over the English, proposed to cut off the middle finger of all captured English soldiers. Without the middle finger it would be impossible to draw the renowned English longbow and therefore be incapable of fighting in the future."
http://members.aol.com/wsa1933/news/bird.htm

However, "Several heralds — both French and English — were present at the battle of Agincourt, and not one of them (or any later chroniclers of Agincourt) mentioned anything about the French having cut off the fingers of captured English bowman."
http://www.snopes.com/language/apocryph/pluckyew.htm

"It is identified as the digitus impudicus ('impudent finger') in Ancient Roman writings [1] and reference is made to using the finger in the Ancient Greek comedy The Clouds by Aristophanes. It was defined there as a gesture intended to insult another. It has been noted that the gesture resembles an erect penis." makes more sense (to me).
http://en.wikipedia.org/wiki/The_finger
http://www.straightdope.com/classics/a980904.html

Answer 2

We had to come up with something in society that says I hate you. Before it was spiting on someone or at least acting like we was. Then it was a push away sign then the bird. I don't know.
But whoever it was it such caught on fast. Is that sign language for sex?