Salvation Through the Ages, Chap. 4, The Age of Authority
From the Flood to Abraham
After the Flood, God set up a rudimentary concept of government, to slow down the spread of evil and punish it. God gave humans as a group the right and responsibility of capital punishment— a right which was not granted to the individual. Yet even then people found a way to corrupt this institution and use its corporate power to defy God again. The Tower of Babel was to be mankind's triumph over God's authority, so God scrambled their language in judgment.
Little is said to be different in this age concerning salvation. This right of capital punishment was a matter of people killing other people, rather than failing to bring the right sacrifice to God. Murder was always wrong, but in this age it became the responsibility of people rather than God to take human life. God would of course make exceptions at times, but the norm was for people to govern themselves.
Keep in mind that this was about what God officially sanctioned, not what people might have been doing anyway before this time. There is evidence of civilizations before the Flood, of cities and great populations, and no one thinks they didn't have any group rules or punishments— which may well have been based upon oral histories passed down from Adam and Eve. The Bible is focused on history as it pertains to sin and salvation, through a line from Eve to Noah to Abraham to David to Mary to Jesus. So the absence of information about other civilizations or continents is not any attempt to hide things from us, as some may allege. Nor is it proof that the Bible only got its content by borrowing or plagiarizing the writings of other religions.