Throughout virtually all of its brief history, rap music has been obsessed with its own origin story, getting endless thematic mileage out of questions of authenticity, legitimacy, and defining the “old school.” Exactly what constitutes the old school has always been up for grabs: Is it the early-eighties South Bronx of block parties, shell-toed Adidas, and D.J. battles? Run-D.M.C.’s leather-coated machismo? The naïve materialism that gave us thick gold-chain necklaces? There’s no consensus (and, of course, no sanctioning body to supply definitions), which can make for strange historical revisionism: Defending the practice of sampling in 1988’s “Talkin’ All That Jazz,” Stetsasonic took a swipe at the genre’s patron saint, opining that “James Brown was old till Eric B and Rakim came out with ‘I Got Soul.’” And De La Soul, who made their mark parodying early-rap stereotypes, are currently held up as paragons of – you guessed it – old-school values.

Now, as rap enters its third decade, some of its past masters have resurfaced after spending varying numbers of years eclipsed by newer, slicker, often less worthy stars like Puff Daddy and his clique. Last month at the Knitting Factory, mid-eighties party-rapper Biz Markie made an unlikely cameo alongside avant-jazz clarinetist Don Byron and ended up stealing the show with a few freestyle lines. And last year, Rakim, widely regarded as the genre’s greatest lyricist, even after a few years’ absence from the scene, released the thoroughly satisfying The 18th Letter. Surprisingly, the fiercest champions of roots rap aren’t vets on the comeback trail; instead, a new generation of young turks seem to be the ones most intent on reviving the “old” ways.

Chicago rapper Common dedicates the bulk of his third album, One Day It’ll All Make Sense (Relativity), to exploring personal and artistic roots. Dozens of family photos adorn the CD booklet (including the cover’s arresting portrait of the rapper as an unself-conscious, prepubescent mama’s boy), and Common’s dad closes the album with a free-form “rap” on fatherhood. “Gettin’ Down at the Amphitheater,” which features De La Soul as guest rhymers, fondly evokes the film Wild Style, a 1982 tale of South Bronx graffiti, love, and rap. And at Tramps last month, Common looked farther back, updating the Last Poets’ sixties proto-rap “Niggers Are Scared of Revolution.” Introducing “1’2 Many … ,” he hit the audience with a series of challenges: “Fake niggas play basketball, football, baseball, Playstation. But when it comes to the revolution, they’re like, ‘Nah, I’m playin’.”

In “Retrospect for Life,” over the swelling, schmaltzy piano chords of Stevie Wonder’s “Never Dreamed You’d Leave in Summer,” Common earnestly agonizes over a girlfriend’s abortion. Technically, the song is one of the album’s weakest: The rapper all but abandons meter and rhyme so he can cram in as many emotional twists as possible. “I’m sorry for taking your first breath, first step, and first cry,” he tells the aborted fetus in one of the song’s most awkward moments. “But I wasn’t prepared mentally nor financially.” Still, Common actually believes in the power – revolutionary or redemptive – of hip-hop, and it is there, even more than in any stylistic aspect of his work, that his affinity to rap’s early days resides. Music as art, not as business. Now, that’s old-fashioned.

The X-ecutioners, who opened for common at the Tramps date, are exploring music history in a very different way. This quartet of D.J.’s in their early twenties creates dizzying soundscapes through virtuosic manipulation of vinyl. The practice itself harks back to rap’s pre-digital-sampler roots, when D.J.’s and rappers were accorded equal status (Grandmaster Flash was the D.J., his Furious Five the rappers). But the X-ecutioners also use plenty of vintage material in their sound: Retooled beats form the foundation; rhyme snippets from Boogie Down Productions and Rob Base become the D.J.’s’ own impressionistic lyrics. The compositions on their debut CD, X-pressions (Asphodel), are perfectly structured, intricate, and head-bobbingly funky – but the possibility of studio trickery lurks behind the deftly configured sounds. Live, though, their rubber-wristed chops dispel any such doubts.

At Tramps, the D.J.’s (Rob Swift, Roc Raida, Total Eclipse, and Mista Sinista) opened working in tandem, at four pairs of turntables set up across the front of the stage. The effect was mesmerizing, but it was during the performers’ solos that their favorite technique – stretching a few seconds of one song into something entirely new and barely recognizable – was on display. Using two copies of the record, Swift resequenced the guitar solo in James Brown’s “Get Up, Get Into It, Get Involved,” rendering it so novel that its very unfamiliarity became a virtue. Sinista gave LL Cool J’s “Rock the Bells” a similar treatment.

This obsessive approach to music – searching for new sounds, or new ways to mess with familiar ones – isn’t going to make the X-ecutioners rich, or even, probably, get them on the radio. But that’s not what they’re after: They banded together specifically to win D.J. competitions – to be the baddest, most skillful, most respected bunch of vinyl slingers on the scene. Kind of like the block-party boasters who started it all.