The note is said to have been composed by an Italian monk named Guido d'Arezzo around 1026, and the full name of the note is interesting.

Do – Dominus – God
Re – rerum – matter
Mi – miraculum – miracle
Fa – familias planetarium – solar system
Sol – solis – the sun
La – lacteal via – milky way
Si– siderae – sky

He composed the names of the notes in order of the first letters of the lines of the prayer dedicated to St. John.
Also, the choir singers of that time held hands and sang with their hands, without a conductor.