It seems to me that harmony singing might naturally occur when the lead singer has pitched the melody where others can't reach certain notes.

My former band (Out of the Rain) was known for harmony singing. We had a distictive sound and we did not resort to the typical you're a tenor, you're a bass, you're a soprano rules. We approached each song individually and harmonies were worked out according to how the notes sounded by ear rather than by such rules as singing a third above the melody line.

The history of harmony singing as discussed in this thread is interesting. It's refreshing to have a truly musical thread going. This is what the Forum should be for.