A friend of mine and I were talking recently about weddings, especially those in the 16th and 17th century, and the subject came up of the position of ‘Best Man’. I was always under the impression that the position existed to make sure nothing ‘went amiss’ at the wedding, and the Man in question was there to make sure the groom didn’t skip town, and that no one would challenge the validity durring the ceremony. I have no cite for this, just what I’ve heard.
My friend said that the Best Man was also there to marry the bride, if the groom did not show. Sort of an emergency backup groom. This seemed a little dubious to me, especially as most weddings (if not all) of the time were existant to cement family unions, and simply marrying off to another person at the ceremony wouldn’t be doing that. Does anyone know if there’s a precedent for what she’s postulating?
I’m not sure about the accuracy of this information, but at least a few sites have mentioned that the Best Man (and his friends, the Ushers) were there to protect the bride. It’s one of those explanations, though, that initially sounds like a big bowl of apocryphal info.
That said, if any tradition of a best man marrying the bride should the groom not show, prove to be true, my rather unresearched guess is that he’d be marrying her as a proxy, only. Which means that the missing groom would still be legally married to the bride (although they may have needed to consumate the marriage–the bride and groom, not the bride and best man–to fully seal the deal).
I have a wide ass, though, so I’m able to pull a lot out of it; take my guesses with a grain of salt. 
The term “best man” in a wedding sense only appears in the OED in 1814. That doesn’t mean it didn’t occur earlier, but trying to take it back to the 16th century would appear ludicrous. And the idea about substituting for the groom if he didn’t show sounds about as bad.
Jerry Seinfeld says that’s why all the groomsmen wear tuxes … so they can just move down the line if the groom doesn’t show.
…I think your friend was kidding with you. There’s a Seinfeld joke that says the Best Man is named as such because if the groom bails, the bride can marry him (being the next Best man).
I got the impression long ago that the groom would pick the best fighter among his acquaintances in case someone was needed to fend off the bride’s family, other suitors, etc. while he and his new “possession” made their escape after the ceremony.
No cite, of course, but it makes nearly as much sense as the “best man marrying the bride” scanario.
And here, I figured that the Best Man was just one of the two official witnesses needed to sign the certificate.