Well, regardless of the bible story, even if you believe in evolution, there probably was a common language at one point and geographical isolation mutated it into the various languages we have today. Basically, all current languages stemmed from like 5 or 6 languages (my bro knows what they all are, since he's got a linguistics degree), and those 5 or 6 had to stem from somewhere. Anyway, yeah, the bible says that because the people built the tower of babel and got all prideful about their accomplishments (thinking that if they built it tall enough they'd be equal to God), God confounded their languages so they'd not be unified enough to come up with another such idea. That's the story to the best of my recollection.
I figure even if the Bible isn't true, it's got some interesting ideas in it. Basically, the languages were split up at one point, and they explained it with the tower of babel story (obviously most of you consider it a myth, and at the moment I'm inclined to agree). It's like the flood; there is significant evidence that at one point most of the earth was flooded. Anyway, the point I'm making is if you take the religious lessons that are put in the Bible, and just use it for a historical reference, you get a basic gist of the history of the world, albeit a limited one.