The music and lyrics of "O Come, O Come Emmanuel" started life as completely separate entities. The Latin lyrics can be traced back to 12th century, but they were most likely around much earlier. John M. Neale, an Anglican priest and prolific translator of the mid nineteenth century, translated the first English version of the words.
The music was first linked to the lyrics in 1851, when Thomas Helmore published Hymnal Noted which paired the tune with Neale's translation. In the publication, Helmore attributed the song to a "French Missal in the National Library, Lisbon", however this was not proved and some even suggested that Helmore had written it himself.
The mystery wasn't solved until 1966 when musicologist Mary Berry (not to be confused with the renowned TV baker) discovered a 15th century transcript of the melody in the National Library of France.
Over the years the song became a celebration of collaboration, with verses being changed and added up until the early 20th Century.