In a movie season where giant robots jostle one another for elbowroom, the comedy “Year One” shuffles backwards in time to mine some broad laughs from mankind’s early history.

The final result is a less schizophrenic version of Mel Brooks’ “History of the World Part I” or Monty Python’s “Life of Brian,” minus the sacrilege.

Writer and director Harold Ramis crafts a predictable plot full of physical comedy. There are plenty of sight gags and the sometimes clever dialogue has a loose, improvised feel to much of it, though the jokes rarely push too aggressively against the movie’s PG-13 rating.

Jack Black and Michael Cera play Zed and Oh, two hunter-gatherer losers who can’t excel, can’t fit in and can’t get women to “lay with them,” especially their two romantic interests, Eema and Maya. After being cast out for eating forbidden fruit from the tree of knowledge (which has no effect), the pair stumbles haphazardly through the book of Genesis, encountering mishaps and Bible character celebrity cameos at a rapid rate.

When the guys leave the tribe, however, the laughs gained from contrasting their modern attitudes with their primal surroundings disappear. Soon enough, every character in the movie is sillier than they are, and they’re reduced to playing the straight characters in several ridiculous interactions.

Around the halfway point, the plot settles in Sodom, where Zed and Oh scheme to free their prospective girlfriends from slavery. Fifty minutes of hijinks later, everything’s peachy and the two friends have learned a little bit about life, namely that sometimes a man must urinate himself in the course of saving the woman he loves.

“Year One” is definitely a worthy light-hearted diversion. The performances are what you’d expect from Black and Cera playing the same versions of themselves in loincloths. It’s an amusing 97 minutes of fun, but wait for the DVD.