"It should obviously have said 'instruments were evolved' or perhaps 'developed' would be a better word."

Developed from what? From nature. These ancient peoples didn't envision human-like gods. To them, animals embodied the gods. The storm god was a fierce bull or bison, ram or other such creature rampaging, stampeding, crashing into things. So to speak or commune with this god, they donned the horned or antlered head and formalized the stampeding run into dance steps and the sounds of the hooves into drumbeats to communicate to this god. They didn't envision a human god with a big hammer because they didn't have hammers at that time. Their own potential was as yet unrealized. As they watched this animal in the rutting season, they started to associate it with male sexual virility and saw it as a fertility god and the dance steps might change to symbolize sex or the mating ritual. Hell, even male birds flash their plumage and dance to impress females. Of course, we observed this and then adorned ourselves with those feathers--they meant something besides fashion.

The ancient Greek myth that cicadas were once humans who sang so much that they forgot to eat and drink is certainly a throwback to a much more ancient time. You can see the shamanistic connections in the myth. So when humans began to sing, they didn't bellow out arias, they learned their own potential from what the creatures around them did--bird calls and dog howls. Certainly chanting only occurred to them because they heard the crickets, the cicadas, the frogs and the like showing them how to do it.

"Why couldn't it have been simple 'chance' in the extremely long period involved? It doesn't take much leisure time or ingenuity to hold a bit of cord up to something hollow and twang it!"

A lot of leisure time is implied when you way they were just trying things out haphazardly rather than with a specific purpose in mind. WHY would ancient man hold up a string and twang it? In an effort to do what? You can't make a leap from that to music without something in between. He didn't just think, "Say, that string twangs! Let's make music with it!" It would have to remind him of something he already has experience of--animal and insect song and sound not to mention wind and thunder and fire and rain and a raging river.

Do you really think when humans first heard crickets chirruping rhythmically throughout the night that it had no bearing on the development of our concept of rhythm? That we never associated the sound of the flute with the sound of a bird? You must have never listened to "Peter and the Wolf." It's a NATURAL association that we still carry to this day.