WMG / B.C.

B.C. is not set in prehistoric times, but in a post-apocalyptic second Stone Age.
This would explain the anachronisms, such as the fact that all the cavemen are Christians.
  • Uh...this one's true. They even have a library of pre-apocalyptic literature.
  • This also explains all the hints of modern society that appear, which real prehistoric cavemen would have no idea about.
  • Where was this Library of pre-apocalyptic literature?
    • In the late nineties, B.C. went on a trip around the world and visited the library. He also met a man named Anno Domini who explained everything. Those strips are hard to track down; the first strips in the online archive start about eleven months after B.C.'s journey ended (although he meets Anno Domini again), and the storyline took place after B.C. ceased being popular enough for regular collections.
  • This has also been suggested about The Flintstones. Speaking of which...

B.C. and The Flintstones share a universe.
The differences in tone can be explained easily — Bedrock is a respectable-size city for the modern Stone Age. BC is set in a considerably more rural area.

B.C. takes place in the same universe as Teenage Caveman and/or Fallout
In the case of the former, they're both "prehistoric cavemen" whose worlds turn out to be After the End. In the case of the latter, it would take place somewhat closer to the Great War than the rest of the series. But hey, this is WMG.

The dinosaurs and other prehistoric creatures that appear in B.C. were cloned ala Jurassic Park
Well, if the comic sets in the future, then the extinct animals should have been resurrected at some point by some way. It may also explain why the Velociraptor that appear in later strips are big and featherless.