What is interesting to me is that the last we hear of Cain in the Bible (after he had been banned form 'among the Godly' for murdering his brother) is that he "built the world's first city." The varying estimates of the age of Earth that have been calculated from the Bible (c. 5,000 to c. 8,000 years) are absurd as such, but are excellent estimates for how long cities have existed on Earth.