The form of descant sung today, a soprano countermelody above a congregational hymn tune, emerged and grew rapidly following the introduction of the 1906 English Hymnal, which some consider Ralph Vaughan Williams' magnum opus (he might not agree). The volume's majestic arrangements of obscure and forgotten tunes curated by Vaughan Williams invited such treatment, as if merely adding an 'amen' to the end paid insufficient homage to this new standard of composition.
As you might expect, however, the practice and use of the descant had an earlier form, as the term itself implies: descant - 'against the song,' from dis-, a Latin cognate implying opposition, and from cantus, 'song;' a word that lives today as the word chant. In its early practice, it was often extemporized, and frequently sung in a different style or rhythm. It was often melismatic, hence the alternate name of the soprano voice, treble.
Image: boy singers from Cantoria, a series of reliefs featuring young singers and musicians, Lucca della Robbia, Florence, 14thC.