Origin of John and Jane Doe?

Does anyone know the origin of the term John or Jane Doe for an unknown decest person? I haven’t found a reference anywhere.

Lo, behold the glory of our omniscient prophet, for Cecil came down from the mountain and spoke thusly:

http://www.straightdope.com/classics/a2_168a.html

-Beeblebrox

“I am so amazingly cool you could keep a side of meat in me for a month. I am so hip I have difficulty seeing over my pelvis.”