This delights me, but I don't suppose anyone else would care about it: the American Heritage Dictionary (fourth edition) defines 'EM, as I expected, as a contraction of THEM; but the etymology reveals that it comes to us from Middle English hem, which itself comes from Old English him, also spelled heom, the dative and accusative plural of he.
Looking for the sources of THEM, I learn that it comes from Old English thaem and Old Norse theim, again by way of Middle English. So 'EM is not a contraction of THEM?
I thought I had a fancy bit of detective work that I could present to the usual audience of People Who Don't Care, but it turns out the Oxford American Dictionary steals my thunder: its entry of 'EM spells it out for you without requiring the strenuous detective work of reading two different entries. I quote: "Originally a form of hem . . . now regarded as an abbreviation of THEM." Good ol' Erin McKean.
Still, I'm delighted that this old-as-Chaucer word hem, which I doubt anybody speaking English today would claim as their own, is still among us, but unrecognized. Like a Canadian.