How far can you stretch a joke? In today's film comedies, from here to eternity.

Or in the case of Year One, backwards, from here to the dawn of man.

The premise for this dopey but resourceful yukfest is simple: Take primitive Mesolithic tribesmen and load them up with modern vernacular and a standup's fast-twitch delivery.

So you have guys in loincloths talking about "downtime" and "pooching the dog."

The fall guys in this slapstick saga are Zed (Jack Black) and Oh (Michael Cera). They're total losers with the ladies and at their jobs - respectively, hunter and gatherer.

The actors make a surprisingly effective comic team, with Black's manic energy trampolining off Cera's deflated passivity.

If all they did was hang around the village, you'd have the makings of a Saturday Night Live skit. A short one. Hunt boar, end up getting chased by boar.

But producer/director/writer Harold Ramis, another Hollywood smart guy mining dumb humor, has some tricks up his sleeve.

Zed and Oh are exiled, turning Year One into a raucous road comedy.

They step out of the primordial forest into a large and elastic world. Run out of gags? No problem. Simply move our hapless wanderers on to the next civilization. Historical eras be damned.

In short order, Zed and Oh drift from early agrarians to Israelites marooned in the desert to a Phoenician outpost to the glory that was Sodom.

Zed and Oh join the military garrison in that degenerate city. Strapped into his oversized helmet, Cera looks like Marvin the Martian.

Ramis has assembled an all-star supporting cast to play the Old Testament figures they meet along the way. David Cross and Paul Rudd, for instance, are Cain and Abel. Hank Azaria is Abraham. ("We are the Hebrews," he proclaims, "A righteous people. Not very good at sports.")

Abraham is intent on introducing the ritual of circumcision to our horrified heroes. Year One never drifts far from penis and poop jokes. Its other go-to gag: Bible-era double entendres.

Oh, who shrinks from any physical contact, is thrown repeatedly into homoerotic situations, as with the flamboyant head priest (Oliver Platt) who wants oil rubbed all over his outrageously hairy body.

The itinerants are constantly being forced into bondage. At one point Zed, explaining his reluctance to trust Cain, says to him, "You did sell us into bondage." "Hold a grudge much?" responds the king of fratricide. "That was like a fortnight ago."

OK, it's a thin concept. Maybe that's why its antic silliness spreads so evenly over its hour- and-a-half duration.