Life as we know it is over, and late-night television will never be the same.  Chelsea Handler made it known last weekend that she would stop hosting her talk show, “Chelsea Lately,” on E! before the end of the year.  David Letterman announced on Thursday that he would step down from his CBS talk show sometime in 2015. And while that is a bigger milestone, the end of “Late Show With David Letterman” won’t necessarily spell the end of the late show; one of the sadder rules of show business is that nobody stays original forever.  When he first became a star, hosting “Late Night With David Letterman” on NBC in 1982, Mr. Letterman was a thinking man’s Everyman, dryly mocking all the clichés of television talk shows and conventional comedy. His is still a distinctive, often cranky voice, but so many latter-day hosts, including Jimmy Fallon, Seth Meyers and Jimmy Kimmel, were influenced by Mr. Letterman that his once innovative comic style is well scattered across the late-night landscape. For younger viewers, Jon Stewart and Stephen Colbert are the Johnny Carson and David Letterman of their day.  Ms. Handler sticks out because she is a woman, of course, but more important, in a field that rewards boyish charm and accessibility, she is not the least bit lovable. Mr. Letterman has an acerbic side, but he has been welcome in people’s homes for more than 30 years because he balances his misanthropic moods with Midwestern self-deprecation and a winning, gaptoothed “what, me worry?” smile.  Ms. Handler’s onstage persona is brutal, without a softening smile (hers is predatory) or mitigating gesture. She takes that persona with her on other people’s shows. Last month, when the soon-to-be canceled CNN anchor Piers Morgan told her she tweeted amusingly, she coldly replied, “I wish you did.” He laughed a little too heartily, but she didn’t relent, telling him he was a terrible interviewer and suggesting that that’s why he was fired by CNN. (He signed off on his last show on March 28.)  Ms. Handler sounds different, she looks different, and her show doesn’t look like anything else on late-night television, not that many people have noticed — her audience is only a fraction of what “The Tonight Show” gets.  One of the mysteries about the late-night landscape, now that Jay Leno is gone, and Mr. Letterman is preparing to leave, is how little talk shows have changed in format, tone and casting since the days when those rivals were on their way up.  Mr. Fallon brings “Saturday Night Live” versatility and a cool band, the Roots, to “The Tonight Show,” but it is remarkable how old-fashioned that program still is. Even the huge blue curtain that Mr. Fallon steps out from seems retro, a throwback to the days when Johnny Carson had the job.  Sometimes even his monologue seems like a throwback. Pivoting off an announcement that Vladimir Putin’s divorce had been finalized, Mr. Fallon imagined pickup lines for the newly single Russian president. “Is your father baker?” he said in a heavy Slavic accent on Thursday’s show. “Because I would wait in bread line for three days for you.”  Mr. Letterman, who turns 67 this month, also made jokes about the dissolution of the Putin marriage, but with a more modern spin: “Is it too soon to hit on Mrs. Putin?”  Mr. Letterman is witty even on his off days, and his announcement on Thursday’s show was a classic mix of candor and self-awareness. He told a long story about how he enlisted his staff to help him identify an eagle that he and his son had photographed on a fishing trip. “If you spend most of your day trying to ID birds,” he said, “should you really be running a network television program?”  Ms. Handler is often offensive and not always funny, but when she leaves, there will be a noticeable gap. She is the only unfriendly face of late-night television.  A version of this article appears in print on April 5, 2014, on page C1 of the New York edition with the headline: Why That Sharp Wit Stood Out on Late Night. Order Reprints|Today's Paper|Subscribe
David Letterman’s retirement announcement and Chelsea Handler’s coming exit highlight the scarcity of caustic humor in the late-night talk landscape.