Gregorian Chant is often sung in fourths or fifths, an harmony technique called 'Organum', which dates back at least to the 9th century.

Its origins were quite simple - some monks had higher/lower voices than others, and they sang in the pitch that felt more comfortable to them, and discovered they liked the resulting harmony.

Our modern ear finds this technique a little strange, because we're more accustomed to hearing a mix of intervals.