We hear it everywhere: in shopping malls, concert halls, carpools and cathedrals.
Even when there is none playing, we often hear it inside our heads. Because music occupies so much of our lives, could it have played an important role in the development of the species?
Some scientists have recently proposed that music may have been an evolutionary adaptation, like upright walking or spoken language, that arose early in human history and helped the species survive.
"Of course it's utter speculation,'' says David Huron, a professor of music at The Ohio State University in Columbus.
Most experts still assume music was a cultural invention, like cave painting or writing, that humans invented to make their lives easier or more pleasant.
Yet Huron and many of his colleagues wonder if music might have biological roots. The "music gene'' would have arisen tens or hundreds of thousands of years ago, and conferred an evolutionary advantage on those who possessed it. Natural selection would have nurtured the gift of music, favoring those who possessed it with more offspring who were themselves more likely to reproduce.
There are several things about music that suggest it has biological roots:
For one thing, music is ubiquitous. From the tribal dances of the Amazon to the frenetic raves of Amsterdam, every culture makes music an essential part of its rituals. You simply can't find people who don't sing, chant or beat on drums.
That music is everywhere suggests it arose early in the history of the species, before humans scattered across the globe and developed manifold cultures. In fact, concrete evidence of music's antiquity exists in the form of a carved bone flute found recently in a cave in Slovenia. The "Divje babe flute,'' as musicologists call it, is the oldest known musical instrument. It dates back 40,000 years, to a time when Europe and much of North America were mantled in ice, and humans lived side by side with Neanderthals.
If the oldest instruments existed 40,000 years ago, then vocal music probably goes back twice as far, Huron speculates -- perhaps even to the dawn of the species.
Another line of evidence to support music as an evolutionary adaptation:
Some people with brain damage to the right temporal lobe can't remember tunes. In one experiment, a man with right temporal lobe damage could not name a single tune played for him -- but when he was read the lyrics to the same songs he correctly identified 24 out of 25.
During a recent meeting at the New York Academy of Sciences, Isabel Peretz of the University of Montreal described several such people. Researchers have also shown with brain imaging studies that when most people hear music, the right temporal lobe is activated.
"Brain specialization is not enough to claim that a function is biologically determined, but I think it is necessary,'' Peretz says.
Finding one or more genes for music would settle the issue. If music is genetic, it if such genes exist.
"If there are genes for music I suspect that we'll find out about them within our lifetimes,'' Huron said.
Steven Pinker doubts that will ever happen.
"Music is auditory cheesecake,'' he says.
Music is one of those wonderful things that makes life worth living, Pinker says, but he doesn't believe it ever contributed to the propagation of the species. To a biologist, that is what counts.
"As far as biological cause and effect are concerned, music is useless,'' Pinker wrote in his 1997 book "How the Mind Works.'' "Compared with language, vision, social reasoning and physical know-how, music could vanish from our species and the rest of our lifestyle would be virtually unchanged.''
Biologists use the word "adaptive'' to describe a trait that is cultivated by evolution. Anything that increases an individual's chances of passing its genes along to the next generation is adaptive.
Music, Pinker argues, is not adaptive. He sees no evidence that having rhythm or being a good singer ever helped a person survive or generate more offspring.
Pinker believes that music is something humans invented and then cultivated because it tweaks our brains and bodies in a pleasurable way. In other words, humans invented music because they enjoyed it.
Maybe humans first made music simply because it makes us want to dance, tap our feet and clap our hands. Maybe it started as a way of painting an auditory picture of a pleasant environment -- birds singing, leaves rustling, brooks babbling and the like.
Perhaps we developed music because it evokes emotions. In language, emotional content is often tonal rather than verbal. We moan, sigh, shriek and giggle to express how we feel. So it's not too outlandish to suggest that we started making those sounds in rituals or performances to make ourselves feel better -- or worse. That practice then developed into music.
No evolution there.
"Cute,'' is how Sandra Trehub of the University of Toronto responds to Pinker's assessment. "Cute, but dead wrong.''
Trehub travels the globe, studying mothers as they sing to their children. No matter where she goes, people sing to their infants the same way, at a high pitch, in a slow tempo and in a distinctive tone. Every culture has lullabies. They are so similar that you could never mistake them for anything else.
"Even if you don't understand the language, even if you know nothing about the musical culture, they're recognizable,'' Trehub says.
That suggests to her that music is no human invention. If we all use music to communicate with infants, maybe it arose as an instinctual form of communication between mother and child, a way of forging an emotional connection.
Music would have been adaptive because mothers who were better musicians had an easier time calming their babies, Trehub suggests. A happy baby who fell asleep easily and rarely made a fuss was much more likely to survive to adulthood -- expecially in primitive societies. Their cries would not attract predators, they and their mothers would get more rest, they would be less likely to be mistreated.
So if a genetic predisposition to music appeared early in human history, those who had it would have produced more healthy offspring who themselves reproduced. The most musical of those children would have the same advantage, and they would pass the music genes to their children, and so on, each generation benefiting from the gift of music.
There are other evolutionary possibilities as well. Perhaps music was adaptive because it made us more attractive to members of the opposite sex. Certainly the allure of bands like Hanson and 'N Sync among girls of a certain age support that notion.
Darwin himself favored such an explanation for music, but many scholars dismiss the idea because most biological traits designed to attract mates -- the peacock's tail, the moose's antlers, the canary's song -- are displayed by the male of the species. Music is something that both men and women make.
OK then, perhaps music is something that pulls us together into groups. As individuals we are slow, clawless and hairless -- easy prey for all manner of vicious beast. But in groups, Homo sapiens has conquered the globe.
Music is all about groups -- choirs, symphonies, ensembles and bands. Maybe people with a biological penchant for music lived more effectively in societies.
"National hymns, military music, battle songs of fans and cheerleaders encouraging their favorite sports teams, or the strict musical preferences of youth gangs may serve as examples of this phenomenon, whose origin may go back to the very beginning of human evolution,'' Thomas Geissman of the Tieraerzlich Hochschule in Hannover, Germany, writes in "The Origins of Music.''
We will never know exactly how music arose, but maybe all this speculation does have something to tell us. We know that every culture has music and humans have made it since the dawn of the species. The brain dedicates valuable space to it. Academics have dreamed up countless possible reasons for its invention.
Because of these things, music must be a truly rare gift.