I took a music appreciation class at Brevard College 15 years ago, and the instructor spent some time explaining the evolution of music in ancient China. He said, (and I'm fairly certain that our textbook corroborated it), that a certain emperor outlawed the 4th note of the diatonic scale. And then another emperor came along shortly after and outlawed the 7th note. So it left composers of the day with just the 1, 2, 3, 5 and 6, or the pentetonic scale that is characteristic of traditional Chinese music. ... Now I can't find that old textbook, and I have Googled extensively, but have not been able to confirm this. Can anyone help me?
I've never heard of this, and would be very surprised to find it's true. Everything from sub-Saharan African music, to bagpipe music, to Bulgarian folk songs are based on the pentatonic.
You're operating on the assumption that the diatonic scale is a universal constant, and it's not. Many cultures have developed scales totally different, or even just slightly different from the diatonic. Take Georgian folk music - their scale operates with a perfect 5th, and then 3 even spaced notes between the 5th and the tonic. Makes for an odd sound: https://www.youtube.com/watch?v=p9m6blsUzmI
Ooh, I might actually be able to contribute to this! Background: Bachelor's in Music Performance, Master's in Music Composition, Master's in Music and Technology.
The diatonic scale as you're thinking of it is a rather European thing. And this is largely based on the subdivision of an octave into twelve distinct pitches. The reality is there is an infinite number of pitches between an octave (what you can realistically hear is not infinite, however). The creation of the diatonic scale is a separate issue, but essentially all standard scales come from the overtone series.
The overtone series is a natural phenomenon. If you picture a vibrating string, you should know that you'll hear a fundamental pitch. If you touch the string exactly halfway through the length and vibrate it again, the pitch will be an octave higher. Cut the length in half again and it will be a fifth above that. If you do this enough times, you'll have every note in theory. But it just so happens that the notes that are in the pentatonic scale are the first notes that are in the overtone series. This is why you find the pentatonic scale in the music of many cultures that have never had contact with each other.
A good graphic is here.