Once upon a time in this forum there was a thread about how new DID/MPD is and why it would all of a sudden "show up" in this day and age. I tried to find the thread, but there aren't many pages of the forum kept.
Anyway from what I am studying at this time it looks like the first *documented* case was in 1811 (Mary Reynolds). Just thought I'd share my findings. I am certain there were more prior to that time - but it probably took people a little time to figure out what was going on. And then more time to write about it.
=)