Miles Davis booked Columbia's Studio B in Manhattan for three days in August 1969 with a lineup that, on paper, was simply too crowded for jazz: a single soprano sax and a single electric guitar, three keyboard players, two drummers, two percussionists, electric bass, and a bass clarinet. The producer, Teo Macero, was instructed to roll tape and edit later. What came out of those tapes the following year was Bitches Brew, a double LP whose title track alone stretched to twenty-seven minutes. What was it — a doorway into a new generation, or the end of jazz? Depending on which jazz writer you read, opinion is still split.

The record sold at a pace unheard of for a jazz release in 1970, peaking at number 35 on the Billboard 200; in 1976 it became Miles's first record to be certified gold (roughly half a million copies). It also broke the implicit understanding Miles had been operating under since Kind of Blue (1959). The understanding ran like this: jazz was a music played by acoustic quintets — five-piece bands — on labels like Blue Note and Columbia. And it was sold to a college-educated audience who took it seriously enough to sit in silence through a long, unhurried track. But Bitches Brew had electric pianos. It had a wah pedal. It had, in stretches, no obvious key center at all. The shards thrown off by that single record would tear jazz apart in every direction — and, in the process, cut it loose from its own audience.

What followed inside three years was one of the densest family trees in twentieth-century music: from a single record, bands erupted in wildly different directions at once.

At the speed extreme stood McLaughlin. He took the high-velocity guitar idea, added a violinist and a drummer who could play in odd meters like 9/8 and 7/8, and launched Mahavishnu Orchestra. The 1971 debut The Inner Mounting Flame opened with Meeting of the Spirits — a free, drifting intro erupting into a terrifyingly fast 6/4 pulse under a distorted guitar — and that one record effectively defined what a guitar-led fusion band would sound like for the next decade.

At the opposite, hummable-pop extreme stood Weather Report — the band that proved jazz could still land a radio hit. Joe Zawinul and Wayne Shorter co-founded it and spent the seventies inching from open-form modal jazz toward something polished enough for FM radio. By 1977, Heavy Weather hit Birdland — a Zawinul tune carried by Jaco Pastorius's fretless bass — and the band had a song people could hum. From the same source, other branches reached toward Latin and toward funk. Chick Corea convened Return to Forever; its earliest, Latin-tinged lineup with Flora Purim produced Spain (1973), one of the most-covered fusion pieces in the repertoire, before Corea added Al Di Meola and steered the band toward rock. Herbie Hancock, who had left Miles earlier, watched Bitches Brew sell and responded with Head Hunters (1973). Its lead track, Chameleon, rode a clavinet — a funky electric keyboard with a sharp, percussive bite — over a two-bar synth-bass vamp that repeats and repeats. And it went platinum.

By the mid-seventies the trade press had a problem. The word "jazz" was being attached to Hancock's Chameleon and to Cecil Taylor's dense, dissonant tone clusters — notes jammed together and hammered down — in the same Downbeat issue. Wynton Marsalis, a teenager at the time, would build his entire career on a return to the pre-fusion canon and a vow never to play anything driven by a rock backbeat instead of a swing feel. In the eighties the critic Stanley Crouch, writing in the Village Voice, branded fusion a betrayal, and in 1990, in The New Republic, called Miles "the most spectacular sellout in the history of jazz."

The receipts were unambiguous. The new audience came in: kids who already owned Led Zeppelin records found Mahavishnu, took it home, and never went back to bebop. The old audience walked out. A Hard Bop quintet that had packed a club every night in the sixties found, by the seventies, that the crowd no longer came and the bandleader was teaching at a university just to get by. The jazz club circuit contracted hard through the decade, and the survivors — the storied Village Vanguard among a bare handful — narrowed their bookings to the acoustic mainstream. A whole generation of straight-ahead players found themselves making most of their income teaching at the new university jazz programs, the conservatory cushion that has kept post-bop alive ever since.

The fusion-purist fight was already over by the time the children of that audience came of age in the late nineties. In the UK club scene, a jazz record sitting on the same shelf as a downtempo electronica record was simply normal; the nu jazz acts who lived there — the early Cinematic Orchestra among them — never treated the boundary as a question in the first place. Robert Glasper's Black Radio (2012) put hip-hop vocalists in front of an acoustic jazz trio and won a Grammy. By the time Kamasi Washington's The Epic (2015) showed up, the fusion question — is this betrayal, or extension? — wasn't being asked anymore.

What Bitches Brew actually broke was not jazz. It was the assumption that a single audience could be expected to follow a music wherever it went. Fusion produced an enormous legacy — McLaughlin's chord-melody playing, fretting the harmony and the tune at once, is required learning in university jazz programs; Hancock's clavinet riff has been sampled hundreds of times; and Weather Report's Birdland, born of the avant-garde, is now in every wedding band's set list. But it cost jazz its casual listener — there since the Swing Era, gone for good. That listener went to rock, then to pop, and never came back. The conservatory survives in that listener's absence.