There’s something rather poignant about hearing the MC counting down the seconds to 1977 at the beginning of the Grateful Dead’s second set of this historic New Year’s Eve gig, ushering in a new era when the great psychedelic jam-band’s music suddenly ceased to be at the vanguard of hip, underground culture, swept aside by the year-zero nihilism of punk.

In fact, listening now to this three-hour marathon it’s occasionally hard to grasp fully just how this rock ‘n’ roll institution ever managed to achieve such iconic and beloved status in the first place. For the first 40 minutes of the opening set, they sound like little more than a fairly unremarkable, pedestrian yet good-natured bar-band, cranking out inoffensive and technically undemanding country-rock, blues-rock, boogie-rock, and just plain adult-oriented rock.

And then, suddenly, something happens. Three minutes into “Playing in the Band” they simply take off. Time seems to stand still as the band rockets into a collective improvisation that challenges all the notions of song form they’ve spent the preceding 40 minutes laying straight. It’s jazzy, sure, and exploratory but most importantly it’s utterly transcendent: a meandering yet constantly evolving investigation of melody and timbre that goes everywhere at once while still moving forward with an irresistible linear narrative. Oh, and it’s psychedelic as hell. Twenty minutes in, we realise we’re headed straight back to the infamous regions of the “Dark Star”, on a journey both inwards and outwards: the term mind-blowing could have been invented for this. Deep in the famous “Phil Zone”, bassist Phil Lesh is pouring out impossibly rugged yet sensitive variations; the twin drums of Mickey Hart and Bill Kreutzmann push things along with a free-flowing, liquid propulsion; Keith Godchaux’s piano is like delicately inter-laced fingers holding everyone up: and, centre-stage, Bob Weir and Jerry Garcia weave cramped curlicues of guitar notes around each other, chasing the moment into spiraling wormholes of expression. This, you realize, is precisely what the army of Deadheads gave their lives to. This is unfettered, spontaneous, selfless beauty and it’s no surprise at all that a multitude of young souls decided this was all the perfection they ever needed. This is the Grateful Dead, exactly where they belong, in their native element, live on stage, hammering out truth in the crucible where all their vital alchemy took place.

After that, inevitably, the band’s all warmed up and just wants to play. What follows is a showcase of the various different aesthetic touchstones that the Dead made their own over the years: the sunny jazz-funk of “Eyes of the World” with mellifluous, dancing bass and guitars; the"La Cucaracha”-style, Tex-Mex, good-times rock and roll of “Good Lovin’”; the slick, west-coast funk of Blues for Allah‘s “Help on the Way”, coming on like a bearded Steely Dan on their way back from an all-night party; and the slinking, slow-hand ooze of Buddy Holly’s “Not Fade Away”, picking up the methadoned Bo Diddley vibe that their buddies Quicksilver Messenger Service first smeared all over “Mona” and “Who Do You Love?”

And through it all, there’s Garcia’s guitar - tumbling into vertiginous, intuitive, hit-and-miss runs of pure freedom, taking every opportunity to dive into extended, joyous improvising, letting loose great cascading peals of notes like a thousand churches ringing out celebrations all at once. It sounds like a celebration, it is a celebration—never mind that it’s New Year’s Eve and everyone’s getting loaded—this here is a Dead gig and we’ve got spontaneity and love on our side.

But you know this party has to end. To cap it all, there’s the ultimate psych-ballad, “Morning Dew”—a genuine outpouring of melancholic hippie idealism that scales great heights of emotion without once resorting to overblown rock theatrics. It’s not just the sighing resignation of the lyrics, or the swaying lighters-in-the-air anthemic quality that makes this so moving and so quintessentially Dead: it’s something about the whole sound, the way there’s no distortion, no overdriven effects, no heaviness, no empty swaggering or flashy technique for its own sake. What we have here is the basic, ground-level sound of American music: the clean guitars, the piano, the bass and drums, the self-same honest instrumentation that birthed rock ‘n’ roll, the lingua franca of modern music.

And, suddenly, it all makes sense, you’re overwhelmed by a wave of context, you understand how this band of freaks got away with playing Chuck Berry and Merle Haggard tunes, cranking out unadventurous white-boy boogies and still somehow sounding truly “great” in every sense of the word. You realize, at last, that they pretty much helped invent this music. You can draw a straight line through Hank Williams, Elvis, Johnny Cash, and the Dead straight on up into rock ‘n’ roll immortality. This is as American as it gets. There’s just no need for excuses. Let ‘em play whatever they damn well please.