Against all odds, hip hop—the genre that seemed, for those who came of age in the 1980s or 90s, to have been handed down from the heavens to exasperate or frighten your Baby Boomer parents—has grown up.

That's not to remind you of Kanye's recent fatherhood or Jay Z's third decade of commercial reign, nor is it to quibble about Steven Hyden's theory of hip hop's "post-youth period." It's just to say that hip hop (if you trace the family tree to its most commonly accepted origin myth) has just turned 40.

The story begins in the South Bronx, a brick apartment building at 1520 Sedgewick Avenue, 40 years ago this week: on August 11, 1973, to be precise. In the midst of tremendous social upheaval and vicious gang wars, that was the night Cindy Campbell, a Jamaican teenager, decided to host a "Back to School Jam" in the recreation room of her building.

That's the flier above, styled on an index card: she charged $0.25 admission for women and $0.50 for men, hoping to fund a shopping trip, and she advertised it as a "DJ KOOL HERC PARTY." Kool Herc was the stage name of her brother, 16-year-old Clive Campbell. Well before Grandmaster Flash emerged with "The Message"—and around the same time fellow pioneer Afrika Bambaataa began hosting hip hop parties of his own—Campbell had been developing a DJ technique of his own. Here's New York magazine's recounting of it in 2008:

He’d ignore the majority of the record and play the frantic grooves at the beginning or in the middle of the song. Herc referred to this as “the get-down part,” because this section of the song was when the dancers got excited. Utilizing two turntables and a mixer, Herc used two copies of the same record (removing the labels so others couldn’t “steal” them) to isolate and extend the percussion and bass. This became known as “the break.” It wasn’t until he was spinning at his sister’s party that he showed it to an audience.

But "something else, unplanned, happened that night," Michael A. Gonzales explains: clad in Adidas pants and a suave new haircut, Coke La Rock, a friend of Herc's, seized the mike and began speaking over the beats. By his own admission, he didn't have much to say: 

“Our friends Pretty Tony, Easy Al, and Nookie Nook were all at the party,” he recalls. “At first I would just call out their names. Then I pretended dudes had double-parked cars; that was to impress the girls,” he continues. “Truthfully, I wasn’t there to rap, I was just playing around.”

But no matter. Attendees were thrilled, and thus a revolution was sparked. Given the supposed guest list at the party, it's no wonder one impromptu performance could pack such an impact:

“That first party was epic,” says Grandmaster Caz, leader of the Cold Crush Brothers, who was there. “Afterwards, everybody attempted to re-create the energy of that night.” The list of those who attended the party (or at least later claimed to) reads like a Who’s Who of old-schoolers, including Grandmaster Flash, Busy Bee, Afrika Bambaataa, Sheri Sher, Mean Gene, Red Alert, and KRS-One. 

Forty years later, there's no need to expound on how far hip hop has traveled from its makeshift break-beats in an overcrowded apartment building in the tumultuous Bronx of the 1970s. But it hasn't forgotten its roots. Or Kool Herc hasn't, at least. This past weekend, he and Cindy spoke at a panel discussion on the roots of hip hop hosted by the City Parks Foundation. He also got his own mural by artist Tats Cru on the corner of Grand Concourse and 166th Street in the Bronx.

Of course, other forces behind hip hop's emergence were  also in the air in 1973 (including the work of Pete DJ Jones and the formation of Afrika Bambaataa's Zulu Nation, as NPR points out), and this is just one moment. But for the hip hop genealogist, it is invariably a big one in the evolution of the genre.