ROB BECKER'S POPULAR one-man show, "Defending the Caveman," returns to the Washington area this week at the Rosslyn Spectrum Theatre. This time, actor Chris Sullivan fills "Caveman" creator Becker's shoes -- do cavemen even wear shoes? -- with Becker's full blessing.

"Becker wrote the show, and he directs it," Sullivan explains. "I have his personal stamp of approval."

The show, which set a record in New York as the longest-running solo play in Broadway's history (702 performances over 21/2 years, from 1995 to mid-1997), is a comedic look at the differences between men and women.

It all boils down to evolution, contends Becker's cavemanifesto. Blame it on our prehistoric ancestors. Men were hunters, women gatherers. And so it went. For survival, hunters needed to maintain a single-minded goal, while gatherers were able to see the bigger picture, to take in an entire landscape and process a lot of information visually. Today, according to Becker's wry observations, these long-honed old-brain traits are most evident in actions such as dominating the television remote (male) and shopping at the mall (female).

Part stand-up comedy, part marriage counseling, the allure of "Caveman," Sullivan says, is in the sharp detail of its observations.

For Sullivan, there have been times when these details have hit a little too close to home.

"Much of what Becker writes has happened to me, word for word, line for line, in my own relationship. I really do think it's universal. Anyone who is in a relationship, or has been in a relationship, can relate to this show."

While Becker may have schooled Sullivan on his cavemannerisms and comedic timing, there was no way he could have prepared him for the power of performing it in front of a live audience.

"The first time the audience laughed really hard it completely knocked the wind out of my brain," Sullivan says. "When you're doing a one-man show, the rehearsal process is just you saying all the lines. You keep wondering: Is this funny? After a while, it's hard to believe how funny it is. I wasn't ready for those really big laughs; when they hit the first time, I forgot where I was."

Sullivan is younger than Becker and has what he describes as a more animated and physical approach. He concedes that it is a challenge to take on a role that many associate so closely with its originator, but he hopes to bring his own personalized style and imbue the part with a renewed youthfulness.

One of the things that has surprised Sullivan most since starting the tour in August of 2003 is those couples who come back again and again.

"I think we get a lot of return viewers because the show reintroduces laughter and humor into relationships. The show is really about how our problems are not unique. I see couples elbowing each other all the time and laughing about the same things that we all go through."

For the record, this modern caveman is no Fred Flintstone, he's capable of deep insights, warmth and understanding, as long as it doesn't get in the way of his television viewing. "Caveman's" appeal may lie in its sense of humor, but its underlying message is one that Sullivan fully endorses: Recognizing and sharing a laugh about our differences only makes our relationships stronger.

Chris Sullivan has taken over the heavy lifting in Rob Becker's "Defending the Caveman."