Solfeggio was credited to Guido de Arezzzo (or Guido D'arezzo), but didn't start out with Do but Ut
The origin came from a Latin prayer to Saint John the Baptist by the monk and historian Paulus Diaconus (720-800AD)
Ut Queant Laxis
Roughly translated means; "So that we your servants may freely sing of the wonders of your deeds, cleanse the guilt from our polluted lips, Oh Saint John"
Later, after Guido there was a 7th note added to the Sol-fa system.
Guido used the ascending hexachord chant for the basis for his teaching system.