system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Where does "Do, Ray, Me, Fa, So, La, Ti, Da" Come from when you hear singers practice scales?
gpt
The use of a seven-note diatonic musical scale is ancient, though originally it was played in descending order.

In the eleventh century, the music theoristGuido of Arezzo developed a six-note ascending scale that went as follows: ut, re, mi, fa, sol, la, and named the Aretinian syllables after him. A seventh note, "si" was added shortly after.The names were taken from the first verse of the Latin hymn Ut queant laxis, where the syllables fall on their corresponding scale degree.
Ut" was changed in the 1600s in Italy to the open syllable Do, at the suggestion of the musicologue Giovanni Battista Doni, and Si (from the initials for "Sancte Iohannes") was added to complete the diatonic scale. In Anglophone countries, "si" was changed to "ti" by Sarah Glover in the nineteenth century so that every syllable might begin with a different letter. "Ti" is used in tonic sol-fa and in the song "Do-Re-Mi".

Source: Wikipedia. 
Thanks OP for asking the question, I've learnt something new today as well!