BC/WMG

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.

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.