John Belushi was there. So was Chevy Chase. To be honest, though, the sketch comedy grandee I remember most vividly was Christopher Guest doing the finest impression of Bob Dylan there ever was or ever will be.

The name of the off-Broadway show was “National Lampoon’s Lemmings” or just “Lemmings” for short. You filed into Art D’Lugoff’s venerable Village Gate in Greenwich Village in 1973 for one of its 350 performances. For the entire second half, you were treated to a savage and glorious popular music festival parody called “Woodshuck: Three Days of Peace, Love and Death.” Belushi did his Joe Cocker bit, but the gospel truth is I don’t remember it at all. More memorable, by far, was Chevy Chase with a dopey scarf around his neck, a flashlight in his hand and a doofus, simpleton grin on his face singing the parody number “Colorado” in his best John Denver voice. But then Chase’s memorability may have had something to do with the fact that at that early stage, he was perfecting his wild, sprawling comedy pratfalls and one of them, I swear, came no more than 4 inches away from spilling every drop of cider, iced coffee and cream on our nightclub table.

It was Christopher Guest, though, who won my allegiance and my biggest belly laughs with his cunning pseudo-Dylan bit singing for us all “Positively Wall Street.” So good, in fact, was Guest’s Dylan impression that I copied Guest’s vocal inflections feebly for years. What happened to the “Lemmings” principals in the next 40 years is a matter of universally acknowledged showbiz history. Belushi rocketed through “Saturday Night Live” and movie blockbusters and died sloppily and tragically young. Chase flourished crazily on his own “SNL” rocket ride and in his own blockbuster movie comedies and then aged badly – badly enough that most of us were pleased as can be that he was Chevy Chase and we weren’t.

Guest found a much quieter and more brilliant way to keep going for four more decades – right up, in fact, until this coming Sunday when his half-hour sitcom, “Family Tree,” debuts at 10:30 p.m. on HBO. Sure, Guest had his high-profile moments on “Saturday Night Live” when he and Billy Crystal would recount baroque horrors from their experience and then acknowledge blandly “I hate when that happens.”

But the eternal contribution of Guest to the culture-forming world of post-“National Lampoon” comedy was to become the key figure in the world of movie “mockumentaries.” Who is ever likely to forget Guest as Nigel Tufnel dialing it up to 11 in Rob Reiner’s “This Is Spinal Tap?”

More importantly came Guest as writer-director with what became a stock company giving us those truly great semi-improv beauties “Best in Show,” in which he and his bunch pretended to be major dog show functionaries and, my favorite of them all, “Waiting for Guffman,” a satire on community theater profoundly distinguished by the big-hearted generosity of absolutely everyone involved in the project while they were being so cruelly accurate and hilarious about the aspirations and eccentricities of community theater.

Which brings us to Sunday’s newest exploration of the comic mind of Christopher Guest.

You need to indulge Guest in the first installment of “Family Tree.” Not only is it in a clumsy time slot that will make it an almost-certainty on DVRs for those wanting to keep up with it, the first episode establishes the show’s basic premise without getting into mammoth guffaws.

For that, you’ll have to wait until an episode called “Treading the Boards,” wherein our schlemiel hero (played by Chris O’Dowd) goes on the second of his excruciating blind dates with a buxom woman with deep décolletage who confesses, in all candor, “Some girls are really into shoes. I’m into bones,” and then proceeds to tell him many of the fascinating bones she’s encountered in her life.

The premise of “Family Tree” goes like this: Our boy has discovered on his latest visit to his Dad (Michael McKean, honored senior member of the Guest stock company) that his great aunt Victoria has bequeathed him a trunk full of family memorabilia in her will.

Which, thereby, sets him on a search through a family that has, so far, offered him a divorced father who watches brilliantly inane sitcoms (especially created by Guest) and a sister (Nina Conti), whose teenage vision of a masturbating puffin has set her on a life course where she communicates through a monkey puppet at the end of her arm.

If you know that Guest is, in life, so authentically related to British peerage that he actually had a seat in the House of Lords, it is that much funnier when our hero, in search of his relatives, discovers that his grandfather was well-known in his time for being the back-end of a comedy horse act. (“They were very well respected,” offers an amateur show business historian, by way of consolation.)

It’s also that much funnier that his research into his boxer grandfather leaves him in episode three with an old boxing glove from the 1948 London games and granddad’s 1948 jockstrap.

And to think, his travels to the New World for his family’s American branch are still to come.

“Family Tree” sneaks up on you in the first episode, begins to offer you mammoth belly laughs in the second and, by the third, leaves you thanking the world that Guest is still in it with you.