Ancient “Covers”

When viewing images of written ancient sheet music, I knew it would appear different than sheet music we use today. Truthfully, I don’t know too much about sheet music but with my minimum knowledge, I know that modern sheet music has notes instructing for instruments as well as words to sing. Ancient sheet music only had what I believe to be the words written, which makes me wonder if the people back then had a different approach to music as a whole. It can be explained with time comes development, so perhaps being able to have both notes and words on the sheet was a musical development that improved the world of music. However, if back then the sheet music only had the words, did that allow more flexibility in what style of music was being played? Were people able to alter the instrument portion to their desire, perhaps dependent on their mood? I view this possibility as ancient “covers”. How nowadays an artist can redo a popular upbeat song into a slower version using different acoustics, not including exact instrumental directions may have been an indication for the first sign of singing “covers” to songs.