How does this happen? How does "Year One," a movie with so much raw talent behind it, turn out so lousy? Look at the cast: Jack Black and Michael Cera are the stars, with supporting roles by David Cross, Paul Rudd, Kyle Gass, Oliver Platt, McLovin from "Superbad," Hank Azaria, Bill Hader, and Paul Scheer. It was directed by Harold Ramis, who made "National Lampoon's Vacation" and "Groundhog Day," and written by Ramis and two writers from "The Office." This should be one of the funniest films of the year. Instead, it's one of the dullest.

Like Mel Brooks' "History of the World Part I," Ramis' film takes an "anything goes" attitude, mixing Old Testament figures with pop-cultural views of ancient man, then stringing together a bunch of skits that sort of add up to a story. Black and Cera play themselves, basically, a lazy gasbag named Zed and a timid worrier named Oh, both hunter-gatherers in a small village. When Zed's buffoonery gets them exiled, they embark on a road trip to see if the world really does end on the other side of the mountain.

It does not. Instead, Zed and Oh encounter Cain and Abel, played by Cross and Rudd, just before the notorious incident for which the brothers are famous. Their third brother, Seth (Gabriel Sunday), is there, too, but he's a drooling moron who admits to having sex with sheep. Oh, and there's sister Lilith (Eden Riegel), the world's first lesbian, a predilection that completely baffles Zed.

Zed and Oh continue their travels, meeting Abraham (Azaria) and Isaac (Christopher Mintz-Plasse) before finding their way to Sodom, which they hear is a real party town. It's also where the women from their village, Eema (Juno Temple) and Maya (June Diane Raphael), have been taken as slaves, and the lads want to rescue them.

The film starts off well enough. Cera and Black have a comfortable chemistry together, and their familiar personas are entertaining. They get plenty of mileage simply by applying their modern attitudes and speech patterns to their characters' ancient situations. Cera's Oh describes himself not as a hunter or gatherer but a maker: "I made this loincloth, these arrows, a shelf unit for my hut...." And Maya, the woman Zed has a crush on, explains why she likes him: "When my parents got killed by that pack of wild dogs, you really helped me see the funny side."

But the first sign of trouble comes soon, in a scene where a large snake wraps itself around Oh as Zed watches. Oh panics as the snake coils around his neck. And then ... the scene ends. We cut to later that night, with Zed and Oh sitting at the campfire. The snake is not mentioned. Oh has no injuries. What happened? How did Oh get away? The scene appeared to be working toward a joke, a punch line, a payoff of some kind, and then nothing happened. Was it just a goofy throwaway thing, one of those spoof-movie bits where characters are injured for the sake of a joke and then back to normal in the next scene? OK -- but the snake bit wasn't funny. It didn't even seem like it was TRYING to be funny. Why is the scene in the movie? What was cut from it that would have made it make sense, and why was it cut?

I return to my original question: How does this happen?

The second sign that the film is in serious trouble is when Paul Rudd and David Cross appear as Abel and Cain. These men are funny. The premise of two bickering brothers, one rebellious and one sanctimonious, is funny. Yet the scene is not funny. It's thunderingly unfunny. What kind of devil magic do you have to employ to make David Cross and Paul Rudd as Cain and Abel NOT FUNNY?

Everything is downhill from there. As the film ambles from one plot point to the next (leaving too much sloppy evidence of excised subplots in its wake), you're lucky to find the occasional chuckle amidst all the desperate clowning. Oliver Platt scores now and then as an outrageously fey royal priest who takes a liking to Oh, and it's hard not to smile at Abraham's introduction to "the Hebrews, a righteous people, but not very good at sports." (Mel Brooks would be proud of that joke.) And given the film's fondness for obvious, lazy jokes, I'm impressed by its restraint when it comes to Sodom-related humor.

But then there's Zed eating poop, and Oh urinating on himself, and both of them vomiting, and Seth farting. Is that all you got, Ramis and friends? Bodily functions? If you can't take this premise and these actors and come up with anything funnier than Jack Black eating poop, then you have no business making comedies.

Grade: D+

Rated PG-13, a lot of vulgarity, profanity, and sexual innuendo, some gross humor, comic violence

1 hr., 40 min.