Michael Cera, left, and Jack Black play prehistoric patsies in “Year One.”

Movie studios must be so strapped for ideas that they’re handing out deals like gym towels in middle-school locker rooms. That’s the only possible explanation for the joylessly juvenile, aggressively unfunny comedy “Year One” starring Jack Black and Michael Cera, who turn out to be the worst idea for a pairing since Jon met Kate.

Black and Cera are best buds Zed and Oh, two prehistoric patsies who end up leaving their hunter-gatherer village; Zed, who is good at neither hunting or gathering, is exiled and Oh follows him. Oh believes they’ll fall off the edge of the earth while Zed claims there’s a whole wide world out there they need to explore.

As if to prove Zed’s point, they soon run into Cain (David Cross) slaying Abel (Paul Rudd in an uncredited cameo) in a surprisingly violent, laugh-free scene that plays like one of those late-in-the-show, goes-on-too-long “Saturday Night Live” sketches that reek of desperation passing as humor.

What follows as they head off to what appears to be some ancient Roman outpost and then the Sodom-Gomorrah metroplex is a cascade of excrement jokes, fart jokes, urine jokes, sex jokes, and gay jokes that feel as if they were scrawled between takes.

The scrap of a story involves our heroes trying to rescue two women from their village, Maya (June Diane Raphael) and Eema (Juno Temple), who’ve been sold into slavery to the Romans. The movie’s scattershot, a-thousand-monkeys-typing approach to laughs means there are a couple of amusing moments (Zed trying to free himself from being chained to a wall) but no doubt these can be chalked up to a happy accident, not skillful design.

Black tries to get by on sheer force of will while Cera plays his patented geeky loser, but they can’t overcome the ineptitude and inertia of the script. Lazily directed and co-written by Harold Ramis — who should know better considering he gave us “Caddyshack” and “Groundhog Day” — “Year One” seems like a make-work project for Hollywood comic actors: Hank Azaria, Christopher Mintz-Plasse, Bill Hader and Horatio Sanz all show up for varying degrees of humiliation.

If “Year One” doesn’t turn out to be the worst movie of the year, we’ve got a cruel, hard six months ahead of us.