WASHINGTON • Forty years ago, hip-hop was little known outside its birthplace, New York, until the Sugarhill Gang decided to record their rhymes, launching the genre's rise as a dominant cultural and commercial force.

The result was the group's 1979 smash Rapper's Delight, which is credited as the commercial start of an unforgettable era in music.

Once an underground style centred on live performance in New York's Bronx borough, rap and hip-hop are the most influential styles in contemporary music today.

To mark hip-hop's big anniversary, a pop-up museum has been established in the American capital through the middle of this month.

"I never thought it would reach those proportions," said Grandmaster Caz, an icon of early hip-hop who wrote parts of Rapper's Delight. "Back in the day, we were discouraged from doing hip-hop - nobody respected it."

But the track found runaway success, becoming a global sensation and selling millions of copies.

In 2011, Rapper's Delight was even added to the prestigious National Registry at the Library of Congress - a nod to its role in ushering in a brand new genre.

"Putting it on record was the smartest thing that could happen to hip-hop," one of the three Sugarhill rappers, Master Gee, told Agence France-Presse at the opening night of the museum, which boasts more than 500 pieces of autographed memorabilia.

"Commercially we are the first of our kind," Master Gee said. "We landed on the moon, if you will."

At the beginning, making music was not necessarily Master Gee's end goal.

"I just wanted to get a date," he said.

The rap pioneer, now 57, was then in his last year of high school and performed at neighbourhood parties.

"All the lyrics I wrote were in the effort for people to know which one I was out of the three" in the band, he said.

In those days, hip-hop was a fledgling cultural movement that revolved around four elements: dance, graffiti, vocals - known as "MCing" - and controlling the turntables, or "DJing."

The MCs - short for "masters of ceremony" - were primarily on stage to support the DJs. But they ultimately became the true stars of the genre.

Hip-hop took some musical cues from disco and funk: Rapper's Delight features the famous bassline of Good Times by the band Chic, which was later sampled in 1980 by rock band Queen in Another One Bites The Dust.

Early rap and hip-hop were focused on the principles of "peace, love, unity and having fun" - a cocktail that blended a festive atmosphere with a medium used by black people to express themselves.

"It's a release; it's self-expression," said Grandmaster Caz, who at 57 still proudly wears his blingy chains. "It was a fun and new way to express yourself."

At the opening of the museum in Washington, Grandmaster Caz and the Sugarhill Gang performed an old-school concert with another hip-hop trailblazer, Melle Mel.

Part of the pioneering group Grandmaster Flash and the Furious Five, Mel co-wrote another cornerstone of the genre: The Message.

Released in 1982, it was the first recorded rap song to describe life in the ghettos, addressing socio-economic issues like poverty.

Taking on social ills became signature to rap - a musical revolution that, like hip-hop's rise to commercial fame, happened at first by accident.

Mel said he was just trying to "do something different, to write outside the box from the basics of hip-hop rhymes - when you rap about how good you can rap or about a girl."

"I was just trying to change my style and write a different concept," he said.

Today, Mel considers the song "the most important hip-hop record ever made", but at the time of its release, it was hardly a hit.

"Nobody really believed in the record," he said. "I didn't think it was going to be popular or a commercial success because it was serious."

But the record proved to be a sensation and in 2007, the group became the first rap artists inducted into the Rock and Roll Hall of Fame, the pantheon of popular American music.

"It put our group and hip-hop music on the level I thought it should be on, with all the other great genres," Mel said.

And though today's hip-hop is markedly different from what came out in those nascent days, its "parents" are optimistic about its future.

"You can't stop what can't be stopped and you can't kill what can't be killed," Master Gee said with a grin. "And that's hip-hop."

AGENCE FRANCE-PRESSE