I've done a couple of Bulgarian folk singing workshops and the woman who runs them explained that singing folk songs and especially in harmony (or the harmonies that the average punter used ) supressed when the orthodox christian church became promonent. This would suggest that in eastern europe, at least, harmony singing was commonplace quite a long time ago. They tend to sing in thirds, with the emphasis on the two thirds under the melody, quite often with the odd drone thrown in. Bulgaria being where it is, their tradition also has quite a middle eastern sound at times with some solo songs that wouldn't sound out of place in the arab states.
My point being, I find it hard to beleive that "traditional songs" were never sung in harmony. It seems to be an awfully widespread practise if such was the case.Look at 'rounds', although the effect is produced by staggering the melody the effect is a harmony! I do believe that there are distinct styles of songs, some which obviously lend themselves to large amounts of harmonies and others that sound best unacompanied.
Also, historically, instruments were prohibitively expensive for many people. So a voice would be the obvious instrument for most people to play.
People who enjoy singing, to a certain extent, can't help but improvise around a melody, if only to see what it sounds like!
TTFN M'Lady P.