Looking backward, broadcast TV history takes a step into the future Monday.

When Jay Leno starts his monologue at 10 p.m., he will initiate the first five-nights-a-week prime-time broadcast program ever by digging into the past with the first hosted broadcast TV comedy/variety show in more than 20 years. The last hit series in the genre, The Carol Burnett Show and Donny and Marie, fell off the ratings charts more than 30 years ago.

The Jay Leno Show will be a success by one measure, if NBC sticks to its promise of a two-year test drive. But in other ways, it is a glaring admission of failure. NBC, once the pinnacle of television drama, is acknowledging that in the age of DVRs, computer playback, and vital cable networks, it can't get a robust audience at 10 p.m.

And the problem is not just with NBC. If the Leno experiment succeeds financially, the other networks could follow suit, choosing inexpensive, topical entertainment, produced for $3 million a week, over finely crafted drama costing about the same amount per episode.

Just as digital technology has fragmented audiences and cut into circulation at mass-interest magazines and newspapers, it has hurt 10 p.m. viewership at the Big Three broadcasters.

Ten o'clock is DVR prime time, when, by some estimates, the live network audience is only 50 percent larger than the number of people watching recordings.

It's also prime time for sophisticated drama on cable, such as AMC's Mad Men and FX's Sons of Anarchy, whose second-season premiere Tuesday set cable records and attracted a big chunk of young adults.

The Jay Leno Show doesn't seem primed to do that. With his broad humor, Leno "skews old," in TV talk.

"Good food at sensible prices," the host says over and over - exactly what NBC has struggled for years to avoid, as it grasped for youngsters, while pooh-poohing the "big-tent" strategy at ABC and, especially, CBS.

Leno says he doesn't care about NBC's strategies. Nonetheless, the new show may have some of the pizzazz that attracts young viewers to the topical comedy of cable offerings like The Daily Show.

Leno has signed a stable of "correspondents," most of them known for edgy material, to deliver out-of-studio comedy experiences. They range from King of Comedy D.L. Hughley and HBO veteran (Down and Dirty) Jim Norton, a regular on the Opie and Anthony satellite radio show, to Rachel Harris, last seen in the feature Hangover. NBC anchor Brian Williams "will hone his comedy chops" on the show, Leno said.

But many Tonight Show remnants survive.

Monologue? Check. Kevin Eubanks? Check. Beloved Leno bits like "Headlines"? Check. Big guests? Check. (Jerry Seinfeld Monday, Tom Cruise, Miley Cyrus, Robin Williams, and Halle Berry the rest of the week.) Music? Check. (Jay-Z, Kanye West, and Rihanna Monday. Eric Clapton and Bruce Hornsby on Thursday.)

Tonight Show desk 2.0 will be used only when Leno sits to hold props for his bits. Rather than doing straight-ahead interviews, he says he hopes to involve the stars in comedy, which will be the heart of the show. Music will be sporadic, averaging two performances a week, most combining entertainers who rarely work together.

Rick Ludwin, NBC executive vice president for late night and prime-time series, told TV critics at their summer meeting that viewers aren't interested in music.

The Leno show "will have more comedy and will have bigger comedy, more stunts," Ludwin said.

That's great, says TV researcher and historian Tim Brooks, who worked at NBC in the '80s, because "NBC's problem is: How do you take this comfortable old shoe and make it into a new Nike with a swoosh on it?"

"Leno is not a fresh personality," Brooks says. "If the show is very Leno-centric, I think they've got a problem. If he can be the centerpiece around whom this stuff swirls - stunt casting, hot people - they'll do better."

Away from cable, only ABC and CBS will remain in the 10 p.m. drama race. But many top actors and writers, and viewers, don't stay away from cable at that time.

Mad Men, created by Sopranos veteran Matthew Weiner, won the best-drama Emmy last year. Glenn Close, star of FX's Damages, won a best dramatic actress Emmy.

"I think cable started to take risks that broadcast couldn't," said John Landgraf, FX president. "The shows are more bracingly challenging or intelligent or serialized."

ABC has introduced three bona fide hits at 9 p.m. in the last few years with Lost, Desperate Housewives, and Grey's Anatomy, but has had mediocre results, at best, with new dramas to follow them at 10.

CBS makes money with two CSI spin-offs, but last season canceled the number-three 10 p.m. show in all of television, Without a Trace, after six years, because the return did not justify the expense of making it.

"We are still in the 10 o'clock drama business," CBS entertainment president Nina Tassler said in August at the TV critics' summer gathering, "and it is still a very profitable business for us."

Will The Jay Leno Show find similar success?

"The odds are always against taking a big leap like this," says Brooks, "but that's where the gains are, too. If it does succeed, it would be one of the true breakthroughs in network television in this decade."

Contact television critic Jonathan Storm at 215-854-5618 or jstorm@phillynews.com.