Gary Marcus: We both love music and think it's important in modern human life, but we have different views about how music came to be. In Guitar Zero, I argued that music is a cultural technology, something that human beings have crafted over the millennia, rather than something directly wired into our genomes. Why do you think music is a biological adaptation?

Geoffrey Miller: Music's got some key features of an evolved adaptation: It's universal across cultures, it's ancient in prehistory, and kids learn it early and spontaneously.

Marcus: "Ancient" seems like a bit of stretch to me. The oldest known musical artifacts are some bone flutes that are only 35,000 years old, a blink in an evolutionary time. And although kids are drawn to music early, they still prefer language when given a choice, and it takes years before children learn something as basic as the fact that minor chords are sad. Of course, music is universal now, but so are mobile phones, and we know that mobile phones aren't evolved adaptations. When we think about music, it's important to remember that an awful lot of features that we take for granted in Western music—like harmony and 12-bar blues structure, to say nothing of pianos or synthesizers, simply didn't exist 1,000 years ago.

Miller: Sure, and other things like the pentatonic scale and the verse-chorus-bridge structure of pop songs aren't as universal as most people think.

Marcus: I think it's deeper than that. Pentatonic scales are fairly common, but what we think of as music isn't what our ancestors thought of as music. Virtually every modern song revolves around harmony, but harmony is an invention that is only a thousand years old. Even if you ignore electric guitars and synthesizers, there is still some fairly significant difference between virtually all contemporary music and the music that people listened to a few thousand years ago.

Miller: A lot of music's features vary across cultures, especially in elite/pretentious music like Arnold Schoenberg, Indian ragas, or Chinese opera. But every culture includes singing, drumming, and dancing, and many aspects of folk music look fairly universal, such as rhythm, dance, pitch contours, scales, structural repetition, and timbre changes to express emotion

Marcus: When ethnomusicologists have traded notes to try figure out what's universal about music, there's been surprisingly little consensus. Some forms of music are all about rhythm, with little pitch, for example. Another thing to consider is the music is not quite universal even with cultures. At least 10 percent of our population is "tone deaf," unable to reproduce the pitch contours even for familiar songs. Everybody learns to talk, but not everybody learns to sing, let alone play an instrument. Some people, like Sigmund Freud, have no interest in music at all. Music is surely common, but not quite as universal as language.