Great Idea, but...
The plot for Year One, similar to that of The History of the World, Part I, is something that could be comedy gold. But this was far from golden. I chuckled a few times, and those were generally only the scenes involving David Cross as Cain.
It's surprising that this movie was so bad considering it was partly written by Harold Ramis, who is generally a sure thing. Ramis is at least partially responsible for Animal House, Caddyshack, Stripes, the Ghostbusters films, and Groundhog Day. That's a fairly impressive resume. However, I don't think they'll be including this movie in his list of successes when they write his obituary.