Music probably started with people clapping stones together or banging sticks on a log. It was about rhythm at that time, and it probably appealed to our desire to detect patterns. In other words, it speaks to us.
It spoke not just to our brains, but to our bodies. It urged us to move, to dance. It became a social lubricant. It enabled people to express their most urgent needs without actually having to say them.
When combined with voice—and I imagine that could have happened before, simultaneously, or after the first rhythm; actually, now that I think about it, I’m leaning towards before—you got melody and rhythm, and you developed a sound that people responded to on a very deep level—some might call it a soul level. It just felt right, perhaps as @marinelife suggests, because of some inclination towards mathematical expression, but I prefer to think of it as pattern recognition. Math came much later.
Pattern recognition is important to us because it proffers a survival advantage. Music provides training or practice in pattern recognition. It lays the groundwork for speech. To my knowledge, no one knows because no one alive was there, but I think music came before speech and laid the groundwork for speech. Rhythm and tone are very important elements of speech.
Of course, the complexity of music, speech and dance grew over the millenniums. We developed many forms and structures for music. The music of each people around the world is widely varied. The difference between folk music and highly structured forms such as classical and jazz music grows ever more varied and complex.
We’ll never run out of combinations because there are an infinite number of combinations of pitches and chordal structures and melodies and rhythms. Rock music generally uses very simple and basic forms, so it might seem like it isn’t very complex, but the layers and vocals and styles of play offer it the possibility of infinite variation all on its own. Every band has it’s own sound, and they are all unique. Even the cover bands are different from original bands.
Jazz is designed to offer opportunities for uniqueness. It is all about improvisation. Perhaps because I get bored easily, I practically never play the same riff twice. There is something in my brain constantly pushing me to do something new and different. It’s as if I actually hate doing the same thing twice. I could send you some recordings and you’d see. It’s both a challenge and a natural thing, and for me, that creates an obsession right there.
Music is all around us all the time. The pattern of your clicking keyboard is music. The hum of the heater is music. The sound of the fire engine siren is music. You can’t escape it. Humans live music.