I was just thinking about written music and it got me thinking...what was the first instrument where it was decided that "This will be A" and "This will be C"?
There must've been a first instrument that all other written music was based off, so what was it?
Sorry if this is a silly/unsuitable question for this sub.
That's a totally suitable question for this sub, no worries. :-)
I'm at work right now, so I'll give this a better, more thorough (and sourced) explanation when I get home, but the first instruments to have music played for them were percussion and voice, although not necessarily determinable rhythms and pitch in the sense that we think of today (indigenous/native cultures are an example of this).
However, there is no instrument that was the "Big Bang" of our modern system of pitches (called a temperament). In the medieval age there were monks that sang chants as part of their daily mass. These monks learned everything aurally, so there was no need for a visual representation of pitch or rhythm. As time went on, however, there arose the origins of modern notation called "neumes" that didn't give a definite pitch but gave a relative idea of how the motion of the line should be, e.g. go higher or lower from the starting pitch. Over time, this system became more sophisticated, but the problem was that there was no absolute reference for pitches -- if you sing an A 432 and a friend sings A 445 you can reference a piano or a tuner to figure out who's right, but back then, they didn't have an be-all-end-all reference like that, so there were many different temperaments of the same scales going around at the time. Add in the fact that there were also many different neumic and notational systems all going around at once, and there's just no single instrument/voice/person that did this; rather it's a culmination of hundreds of years of musical development.
So, to answer your question, there simply was no single progenitor of written music, instead it's evolved and developed with our instruments and our music throughout history. Again, I'll flesh this out more thoroughly when I sit down and get some time.
Source: Wright, Simms, Music in Western Civilization
From my studies, the first written Western music was for voice, likely Christian Plainchant. At that time, most instruments were not designed to be intermingled. Everything worked in different modes or keys. Occasionally, one could find a match and be able to play a similar tune, but the diatonic system (major and minor, using any note in an octave, with that note having a set value) did not arrive until the 18th Century.
This also provided instrument makers with easier to make instruments, led to amplification via multiple instruments playing the same or harmonic notes, and ushered in the great composers.
As a luthier (guitar maker) I would think the first instrument would be a stringed instrument of some kind. Each string plays one note, then has "hidden" harmonics. They can be sounded by lightly touching the string at various points along it's length. Those harmonic notes were then used to form basic chords. There is also an argument for the pipe organ being the first to set notes, as those were physically built into the instrument, and could be duplicated across instruments and manufacturers.
Source - "How Music Works", Howard Goodall